pressure measurement

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Chapter 5 Measurement of Atmospheric Pressure

CONTENTS 5.1 Definition and Units ·············································································································· 1

5.1.1 Definition ························································································································· 1

5.1.2 Units ································································································································· 1

5.2 Principles of Atmospheric Pressure Measurement ·························································· 1

5.2.1 Mercury Barometers ······································································································ 1

5.2.1.1 Principle of Mercury Barometers ·········································································· 1

5.2.1.2 Structure of a Fortin Barometer ············································································· 2

5.2.1.3 Handling Precautions for Mercury ········································································ 3

5.2.1.4 Correction of Barometer Readings ······································································· 3

5.2.2 Aneroid Instruments ······································································································ 4

5.2.2.1 Aneroid Barometers ································································································ 4

5.2.2.2 Aneroid Barographs ······························································································· 5

5.2.3 Electronic Barometers ··································································································· 6

5.2.3.1 Cylindrical Resonator Barometers ········································································ 6

5.2.3.2 Electrostatic Capacity Barometers ········································································ 7

5.2.4 Reduction to Mean Sea Level ······················································································· 7

5.3 Maintenance ·························································································································· 9

5.3.1 Maintenance of Mercury Barometers ·········································································· 9

5.3.2 Aneroid Instruments ···································································································· 10

5.3.2.1 Aneroid Barometers ······························································································ 10

5.3.2.2 Aneroid Barographs ····························································································· 10

5.3.3 Electronic Barometers ································································································· 10

5.3.3.1 Cylindrical Resonators ························································································· 10

5.3.3.2 Electrostatic Capacity Barometers ······································································ 10

5.4 Calibration ··························································································································· 10

5.4.1 Mercury Barometers ···································································································· 10

5.4.2 Aneroid Barometers ···································································································· 10


5.4.3.1 Cylindrical Resonator Barometers ······································································ 10

5.4.3.2 Electrostatic Capacity Barometers ······································································ 11

5.5 Repair ··································································································································· 11



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5.6 Transport ····························································································································· 12


5.6.1.1 Method of Transport ······························································································ 12

5.6.1.2 Precautions for Transport ···················································································· 12


5.7 Installation ··························································································································· 13






5.8 Practical Training ················································································································ 15

5.8.1 Aneroid Barometers ···································································································· 15

5.8.2 Aneroid Barographs ···································································································· 15

5.8.3 Disassembling and Cleaning a Mercury Barometer ················································ 15

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Chapter 5 Measurement of Atmospheric Pressure

5.1 Definition and Units 5.1.1 Definition

Atmospheric pressure is the force exerted by the weight of the Earth's atmosphere expressed per unit

area over a given horizontal cross section. It is thus equivalent to the weight of a vertical column of air

above the Earth's surface extending to the outer limits of the atmosphere.

5.1.2 Units

In meteorology, atmospheric pressure is reported in hectopascals (hPa). A unit of 1 hPa is equal to 100

Pa, the pascal being the basic unit under SI (International System of Units) definition, and 1 Pa is equal to 1

newton per square meter (N/m2). Additionally, 1 hPa is equal to the previously used unit of 1 mb.

The scales of all barometers used for meteorological purposes should be graduated in hPa. Some

barometers are graduated in units of inHg or mmHg. Under standard conditions, the pressure exerted by a

760-mm-high pure mercury column is 1013.250 hPa, so the conversion factors are represented as follows:

1 hPa = 0.750062 mmHg;

1 mmHg = 1.333224 hPa.

In line with the relationship between inches and millimeters (1 inch = 25.4 mm), the following

conversion coefficients are provided:

1 hPa = 0.029530 inHg;

1 inHg = 33.8639 hPa;

1 mmHg = 0.03937008 inHg.

Pressure data measured with a barometer should ideally be expressed in hectopascals (hPa).

5.2 Principles of Atmospheric Pressure Measurement 5.2.1 Mercury Barometers

5.2.1.1 Principle of Mercury Barometers

When a one-meter-long open-ended glass tube is filled with mercury

and turned upside down into a mercury-filled container, part of the mercury

flows out of the tube into the container. A Torricellian vacuum is then

produced at the top of the tube, and the mercury level stabilizes at a height

of approximately 76 cm from that in the container (see Figure 5.1).

Torricelli's experiment revealed that this height indicates the ambient

atmospheric pressure. A mercury barometer works on the principle that

atmospheric pressure can be ascertained from precise measurement of this

height.

Figure 5.1 Torricelli’s experiment

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5.2.1.2 Structure of a Fortin Barometer

As shown in Figure 5.2, a mercury barometer consists of three main parts: a mercury cistern (right), a

glass barometer tube (center) and a scale (left). The bottom of the mercury cistern is a wash-leather bag

(sheepskin), and the mercury level can be changed by rotating an adjustment screw. The barometer tube is

secured to the wash-leather bag in the upper part of the mercury cistern so that atmospheric pressure can be

lead from the point at the bound leather. The tip of an ivory pointer at the top of the mercury cistern

indicates the zero point of the scale. When the mercury level reaches this point, the atmospheric pressure

is read at the top of the mercury column. The precise height of the column is measured on a vernier scale.

The main body has a hanger hook at its top to allow it to be hung from a latch on a hanger plate. The

bottom is secured to the screw bridge with a vertical-axis pivoting link using three screws. Both the

hanger hook and the screw bridge can be rotated when the barometer is set on the hanger plate to allow

verticality checking at all times.

A mica plate is wound inside the brass cylinder to prevent direct contact between the brass and the

hPa

1050

90

80

70

60

40

30

20

10

1000

90

80

60

40

30

20

10

70

950

90

80

70

900

650

70

60

80

90

700

10

20

30

40

5

10

5

10

No.1234

40

30

20

10

0

10

(1)

(4)

(2)

(3)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

(14)

(15)

(16)

(17) (18)

(19)

(20)

(21)

(22)

Figure 5.2 Fortin barometer structure (1) Hanger ring (2) Slot (3) Vernier (4) Top of mercury column (5) Knob

(6) Pin-face wrench (7) Attached thermometer (8) Barometer tube (9) Wash- leather vent (10) Three screws (11) Ivory pointer (12) Glass cylinder (13) External-thread wooden frame (14) Internal-thread wooden frame (15) Wash-leather bag (16) Undercover (17) Screw bridge (18) Adjustment screw (19) Wooden base for leather washer (20) Metal frame (21) Brass cover (22) Mica plate

3

wash-leather bag. The plate serves as a heat insulator as well as preventing contamination, discoloration

and wear.

5.2.1.3 Handling Precautions for Mercury

High-purity distilled refined mercury is used in mercury barometers. As the interface between its

surface after oxidization and the ivory pointer becomes unclear, heavy contamination on the mercury

surface necessitates cleaning. Since mercury is a toxic substance, the following points should be noted in

handling:

(1) Mercury containers must be sealed tightly to prevent leakage and breakage. Do not place mercury in

a metal container as it reacts and amalgamates with almost all metals except iron.

(2) The floor of rooms where mercury is stored or used in large amounts should be shielded and laid with

an impervious covering. It must not be stored together with other chemicals, especially with

ammonia or acetylene.

(3) Mercury has a relatively low boiling point of 357°C, and produces dangerous poisonous gas upon

combustion. It must not be stored close to heat sources.

(4) Regular inspections of the room and staff should be carried out when mercury is handled to catch

hazardous levels of mercury concentration. (Environmental regulations on water contamination

affecting personal health set a 0.0005 mg/l limit for the total amount of mercury.)

5.2.1.4 Correction of Barometer Readings

Mercury barometer readings should be corrected to the standard condition, which is defined as a

temperature of 0°C with a mercury density of 13.5951 g/cm3 and a gravity acceleration of 980.665 cm/s2.

In actual observation, readings should be corrected for index error, temperature and gravity acceleration

as follows:

(1) Correction for Index Error

Individual mercury barometers are affected by index errors (the difference between the value

indicated by an individual instrument and that of the standard). The index error is found by

comparison with the standard, and the value is stated on a comparison certificate.

(2) Correction for Temperature

Temperature correction involves the adjustment of a barometric reading obtained at a certain

temperature to a value for mercury and graduation temperatures of 0°C. The temperature indicated

by the attached thermometer is used for this purpose.

The height of the mercury column varies with temperature even when atmospheric pressure

remains unchanged. The graduation of the barometer is engraved so that the correct pressure is

indicated at a temperature of 0°C. When the temperature is above 0°C, the graduation expands and

the measurement indication will be smaller than the true value. This temperature effect must be

corrected in regard to these two aspects. Correction for mercury expansion and contraction is much

larger than that for the expansion and contraction of the graduation.

The correction value for temperature Ct is expressed as follows:

4

where H hPa is the barometric reading after index error correction, t°C is the temperature indicated by

the attached thermometer, is the volume expansion coefficient of mercury, and is the linear

expansion coefficient of the tube.

There is a small difference in absolute values for correction between temperatures below and above

0°C. Those for correction at temperatures above 0°C are negative, and those below 0°C are positive.

(3) Correction for Gravity

Gravity affects the height of the mercury column. After correction for index error and

temperature, a reading made under local gravity acceleration has to be reduced to give a value under

standard gravity acceleration – a process known as correction for gravity.

The gravity value for correction Cg is derived by:

where g0 is the standard gravity acceleration, g is the gravity acceleration at the observation point, H is

the barometric reading after index error and temperature correction, and H0 is the value already

corrected for gravitation.

The gravity acceleration value used in correction for gravity is calculated to five decimal places in

m/s2. When the gravity acceleration at the observation point is larger than the standard gravity

acceleration, the gravity value for correction is positive. Otherwise, it is negative.

When a barometer is used for regular observation at a particular location, a synthesis correction

table summarizing the correction values for index error, temperature and gravity should be provided.

5.2.2 Aneroid Instruments 5.2.2.1 Aneroid Barometers

Aneroid barometers have lower accuracy than mercury barometers, but their compact and portable

configuration means that they are easier to handle and use, and are suitable for self-recording.

An aneroid barometer measures the distortion of an evacuated, sealed elastic capsule with changes in

atmospheric pressure. It consists of a barometer capsule, a spring to prevent the capsule from being

crushed by atmospheric pressure, and gears and levers that intensify and transmit small variations.

The elasticity of the barometer capsule varies with temperature, and a bimetallic plate is used for

temperature compensation. When an elastic body is distorted by pressure, it does not return to its exact

original shape even after the pressure is relieved. Due to this characteristic (called hysteresis), errors arise

from sharp changes in atmospheric pressure, and such errors are subject to secular change. To prevent this,

special materials are used for the elastic body, and stacked thin capsules or bellows are adopted in a number

of barometers.

The appearance of an aneroid barometer is shown in Figure 5.3, and its structure is shown in Figure 5.4.

Two barometer capsules facing each other are balanced around the pointer rotation axis. Two gears are

used symmetrically to reduce vibration, and two hair springs are attached to the gears to prevent backlash

from the pointer rotation gear.

Ct Ht

t

( ) 1

C H H Hg g

gg

00

0

5

The hole for adjusting the pointer should be covered with a screw cap if one is available or with thin

paper to keep out dust and insects.

When kept in good condition, an aneroid barometer reading has a difference of about 0.2 hPa from that

of a mercury barometer. Its readings should be adjusted using the relevant correction value, which is

obtained at each observation using a mercury barometer. When recording a measurement, tap the glass

surface lightly and read the value to the nearest 0.1 hPa while paying close attention to parallax error.

5.2.2.2 Aneroid Barographs

The principle of an aneroid barograph is the same as that of an aneroid barometer, except a recording

pen is used instead of an index needle. The structure of an aneroid barograph is shown in Figure 5.5.

The displacement of the barometer capsule (1) caused by changes in atmospheric pressure is transmitted

to the recording pen (4) through the reed (2) and the lever (3). The recording pen (4) moves up and down

on the side of the clock-driven drum (5) to record variations in atmospheric pressure.

The barometer capsule (1) is vacuumed and balanced with atmospheric pressure through an internal

helical spring. As the elastic modulus varies with temperature, temperature changes at the equilibrium

point are corrected using a bimetallic compensator (6) on the mounting part of the barometer capsule to

compensate for the effect of temperature.

The indicator can be adjusted by rotating the indicator adjustment knob (7) in the upper part of the

barometer capsule (1) and by moving the pen arm (8) up and down.

Knob of index

Figure 5.3 Appearance of an aneroid barometer

Index needle

Index Barometer capsule

Scale

Index needle

Gear Bimetallic compensator

Lever Hair spring

Figure 5.4 Mechanism of an aneroid barometer

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5.2.3 Electronic Barometers

A stable and continuous power supply is required to measure atmospheric pressure with an electronic

barometer.

5.2.3.1 Cylindrical Resonator Barometers

Cylindrical resonator barometers measure atmospheric pressure by resonating a thin cylinder and

reading changes in resonance frequency caused by variations in atmospheric pressure.

The sensor is a metallic double cylinder with one end closed and the space between the outer and inner

cylinders vacuumed (see Figure 5.6). The natural frequency of the inner cylinder (known as a cylindrical

oscillator) changes with the pressure applied to its inside, and atmospheric pressure can be ascertained by

measuring this frequency.

This cylindrical oscillator is equipped with four piezo-electric elements – two for driving and the other

two for detecting the resonance frequency (see Figure 5.7).

Figure 5.6 Cylindrical resonator barometer cylinder

Cylinder case

Vacuum

Resonator

Pressure intake hole

Piezo-electric element

Detection elements

Driving elements

Second mode

Fourth mode

Figure 5.7 Resonance mode of detection and driving elements

Figure 5.5 Structure of an aneroid barograph (1) Barometer capsule (2) Reed (3) Lever (4) Recording pen (5) Clock-driven

drum (6) Bimetallic compensator (7) Indicator adjustment knob (8) Pen arm (9) Pin with ring (10) Holding screw of the clock-driven drum (11) Gate suspension arm

7

To eliminate the influence of temperature changes, the cylindrical oscillator is provided with a

temperature sensor to allow correction.

5.2.3.2 Electrostatic Capacity Barometers

This type of barometer has a pressure sensor with dimensions of a few millimeters. It consists of a

silicon wafer to detect pressure, a substrate silicon chip and an insulating glass plate (see Figure 5.8). The

silicon wafer to detect pressure is etched to make an electrode and a diaphragm, and a vacuum gap between

the silicon and the glass plate is created.

The substrate silicon chip is also etched to make the other electrode. The electrodes formed by the

silicon wafer and the silicon chip separated by a vacuum gap create a kind of capacitor.

The shape of the diaphragm changes with atmospheric pressure, causing the vacuum gap to expand or

contract. Such deformation causes a change in the electrostatic capacity of the gap and the electrodes; this

slight change is detected as an electric signal and converted to an atmospheric pressure value.

Electrostatic capacity digital barometers feature high precision and long-term stability.

5.2.4 Reduction to Mean Sea Level

To compare the atmospheric pressure value at a certain location to a value at another location, it is

necessary to convert the values to the same referential altitude. The internationally accepted method for

this is to use mean sea level as the reference, and the conversion is referred to as reduction to mean sea

Vacuum gap

Electrostatic

capacity

: large

Atmospheric pressure : small Atmospheric pressure : Large

Electrostatic

capacity

: small

Silicon substrate

ElectrodeⅡ

Glass substrate

Vacuum gap

Electrode interval

ElectrodeⅠMeasurement of

electrostatic capacity

Atmospheric pressure

Silicon

diaphragm

Figure 5.8 Pressure sensor of an electrostatic capacity barometer

8

level.

Various methods for this reduction are used in individual countries. For international comparison,

however, methods need to be standardized to ensure data interchangeability. Two basic equations – a

hydrostatic equation and a state of ideal gas equation – are used in each country. Differences among the

methods used are found only in how gravity acceleration and mean virtual temperatures are calculated.

When the vertical distribution of air temperature and humidity between mean sea level and an

observation point are known, reduction to mean sea level can be made accurately. However, the air

temperature and humidity exactly at the observation point itself are generally known. The atmospheric

pressure at mean sea level is therefore obtained assuming a standard vertical distribution of air temperature

and humidity

Suppose there is a vertical column of air from the observation point to mean sea level. The

relationship between atmospheric pressure P at the observation point in hPa and atmospheric pressure P0 at

mean sea level in hPa is given by:

where Tv is the virtual temperature of the vertical air column in K, R is the gas constant of dry air in Jkg-1K-1,

and Z is the height from mean sea level to the barometer in meters.

Assuming that g is constant and equal to gravity acceleration at the observation point, the mean virtual

temperature is given by:

This produces:

The reduction to the mean sea level value P is therefore given by:

Now TVm is expressed as TVm ＝ 273.15 + tm + m (K), where tm is the average temperature of the air

column and m is the effect of air humidity. Assuming the lapse rate of air temperature as 0.5°C/100m

results in:

lnP

P R

z gdz

TV

0

0

1

TZz dz

T

Vm

V

0

P P P PgZ

RTVm

0 1exp

P PgZ

RTVm0

exp

9

where t is the air temperature at the observation

point.

The value of m is statistically determined as a

function of the average air temperature. The

relationship between tm and m is graphically shown

in Figure 5.9, and is statistically derived from surface

observation data obtained at eight meteorological

observatories in Japan. This relationship is almost

the same as that in the lower atmosphere under

average conditions in Japan. The value of R is

287.05 Jkg-1K-1. This formula can be used to

calculate the reduction to mean sea level value P to

one decimal place as a function of air temperature t

and atmospheric pressure P at the observation point. It is convenient to tabulate these reductions in

advance. Although the air temperature at the observation point t should be that at the height of the

barometer, the value at the observation site is used instead as the difference is negligible. Similarly, g

should be the average value down to mean sea level, but its influence is also negligible.

m = (Atm + B) tm + C

tm < -30.0 ﾟ C : m = 0.09

-30.0 tm < 0.0 : A = 0.000489, B = 0.0300, C = 0.550

0.0 tm < 20.0 : A = 0.002850, B = 0.0165, C = 0.550

20.0 tm < 33.8 : A = -0.006933, B = 0.4687, C = -4.580

33.8 tm : m = 3.34

5.3 Maintenance 5.3.1 Maintenance of Mercury Barometers

Maintenance for mercury barometers is carried out in the following ways:

(1) Once a month, brush dust off the outer surface with a soft brush and wipe metal and glass parts with a

soft cloth. Check the barometer for flaws and cracks.

(2) If dirt has accumulated on the mercury level where the mercury comes into contact with the ivory

pointer, turn the adjustment screw as shown in Figure 5.2 to lower the level by approximately 3 mm.

Restore the adjustment screw, and any dirt present will be removed. At this time, refrain from

shaking the main body to remove dirt, as the inside of the glass tube may become soiled above the

mercury level, resulting in unclear readings.

(3) The degree of vacuum present should not be checked unless absolutely necessary.

t t Zm 0 005 2.

Figure 5.9 Relationship between m and tm

m

tm

10

5.3.2 Aneroid Instruments

5.3.2.1 Aneroid Barometers

Clean the surface or the glass part of an aneroid barometer with a soft cloth, or brush every week (see

Figures 5.3 and 5.4.).


Check aneroid barographs as indicated in Chapter 2: Measurement of Temperature. If there is a

difference of 0.3 hPa or more between an aneroid barograph reading and that of a mercury barometer, turn

the indicator adjustment knob (7) to adjust the indicator as shown in Figure 5.5.


5.3.3.1 Cylindrical Resonators

As humid air in the sensor of a cylindrical resonator barometer results in an error of approximately 0.1

hPa in the pressure reading, replace the desiccant enclosed near the sensor every month.


Electrostatic capacity barometers have high levels of performance and stability, and require no daily

maintenance.

5.4 Calibration 5.4.1 Mercury Barometers

In Japan, mercury barometers have not been used since 2007. Until then, they were subjected to

inspection and compared with standard precision mercury barometers at district meteorological

observatories every five years in order to obtain new correction values. At each district meteorological

observatory, three standards were compared to each other every six months to ensure high precision.

5.4.2 Aneroid Barometers

When the reading of an aneroid barometer differs from that of a mercury barometer by 0.3 hPa or more,

adjust the pointer as shown in Figure 5.4.


5.4.3.1 Cylindrical Resonator Barometers

Cylindrical resonator barometers have high levels of precision and stability, and do not require frequent

calibration. However, they should be inspected and compared with standard precision mercury barometers

at regular intervals.

In Japan, when a new cylindrical resonator barometer is installed, the initial correction value should be

obtained by making at least 20 comparisons between the new device and a standard barometer and

calculating the average difference between them. Readings of the cylindrical resonator barometer and

those of a standard barometer at a fixed time every day should be compared to enable correction once a

month.

11


Electrostatic capacity barometers also have high levels of precision and stability, and do not require

frequent calibration. However, they should be inspected and compared with standard precision barometers

at regular intervals.

In Japan, when a new electrostatic capacity barometer is installed, the initial correction value should be

obtained by making at least 20 comparisons between the new device and a standard barometer and

calculating the average difference between them.

5.5 Repair 5.5.1 Mercury Barometers

When the difference in observation values increases between mercury and aneroid barometer readings

and the mercury barometer appears to be defective, it should be repaired as follows (see Figure 5.2):

(1) The difference increase is probably caused by an impaired vacuum or loose mounting of the ivory

pointer. If the vacuum is impaired, drain the unit of its mercury and refill it. If the ivory pointer

mounting part is loose, disassemble and screw it tightly.

(2) If the knob (5) used to move the vernier (3) comes loose and causes a large backlash when the

graduation is adjusted, tighten the two nuts on the knob using a special tool (a pin-face wrench) (6).

(3) Do not lubricate the adjustment screw (18) or the knob (5) excessively. Excess oil will spread to melt

paint varnish and cause sticky threads, which will stiffen the screw even more. It will also soak into

the wash-leather bag (15) and the wooden part and contaminate the mercury. If the adjustment screw

(18) is stiff, it is probably bent or the thread is soiled. In such cases, replace the screw or remove and

clean it with a brush and cloth.

(4) When the level of a mercury barometer seems to be improper due to influences such as earthquakes,

loosen the three screws of the vertical-axis pivoting link. Check the level and tighten the screws

again.



Aneroid barometers are very precise instruments, and cannot be easily dismantled or repaired on site.


Repair aneroid barographs according to the repair instructions in Chapter 2: Measurement of

Temperature. When irregular movements of the pen arm are noticeable, perform repair as follows (see

Figure 5.5):

(1) Pull out the pin with ring (9) and the connecting pin. Clean the pinholes on the barometer capsule (1),

the reed (2) and the lever (3) with volatile oil or benzine and remove old oil. Polish the inside of the

holes with an oil-absorbing toothpick and apply a thin film of high-quality clock oil inside the holes

before assembly.

(2) Feel how the pivots rattle using your hands. Remove one pivot at a time. Clear out old oil and dust.

12

If the pivot is rusty, polish it evenly with a lathe or oilstone and lubricate it with clock oil as outlined

above in (1) before assembly.

(3) The reed (2) must be centered on the crack of the lever (3). If it is not, check the pin and the crack for

distortion and repair any defective parts before reassembly.

(4) To repair the clock-driven drum (5), refer to the relevant section in Chapter 2: Measurement of

Temperature.


Cylindrical resonator and electrostatic capacity barometers mainly consist of electronic components, and

rarely have mechanical parts. It is therefore rarely possible to repair such devices on site.

5.6 Transport 5.6.1 Mercury Barometers

5.6.1.1 Method of transport

When transporting a mercury barometer, fill the vacuum part with mercury and turn the barometer

upside down to prevent air from entering. This also applies to indoor transport, regardless of distance.

For long-distance transport, use a leather carrying case and keep the barometer in the upside-down position.

(a) Removing a mercury barometer

To remove a mercury barometer from its hanger plate, gently turn the adjustment screw (18) (see

Figure 5.2) until the mercury column reaches the top of the tube. As this may be difficult to do only

by feeling the adjustment screw or listening to its metallic sound, pay careful attention to the mercury

column’s movement while rotating the adjustment screw. If the cistern has an air vent, it must be

closed tightly at this stage.

After turning the adjustment screw, loosen the three screws of the vertical-axis pivoting link and

remove the screw in the upper part of the hanger plate. Hold the main body firmly with both hands

and remove it from the hanger plate.

(b) Turning the mercury barometer upside down

After removing the mercury barometer from the hanger plate, tilt it slowly and turn it upside down.

(c) Storing the mercury barometer in a leather carrying case

Check the leather carrying case to ensure that the barometer will not fall out, that the shoulder belt is

not worn, and that the cap can be tightened securely. After these checks, carefully put the barometer

(as turned upside down in step (b)) into its case. When the mercury cistern is about to enter the case,

grab the adjustment screw securely with one hand and lift up the case with the other hand so that the top

of the mercury barometer reaches the end of it.

After putting the barometer into the case, pad the area around the mercury cistern with cushioning

material for support.

5.6.1.2 Precautions for transport

If the temperature of the barometer rises during transport, the mercury will expand and may break the

glass tube or leak out. To prevent this, loosen the adjustment screw (18) by one or two turns (see Figure

13

5.2) in advance.

Carry the leather case over the shoulder and do not swing it.

For long-time transport by rail or boat, place the case upright in a safe place so that the barometer is

always upside down. If it is impossible to keep the case upright, be careful not to allow it to tilt more than

30 degrees. For temporary placement during transport, ensure it is stable so that it does not fall down

accidentally.

Air transport of mercury and associated instruments is regulated by the International Air Transport

Association (IATA).


Generally, aneroid instruments have a measuring range from 900 hPa to 1,050 hPa. This type of

barometer should not be transported by air, as barometer capsules may break when the measurement range

is exceeded.

For details on the transportation of aneroid barograph clock-driven drums, refer to Chapter 2:

Measurement of Temperature.

5.7 Installation 5.7.1 Mercury Barometers

(1) Checking a Mercury Barometer

After taking the barometer out of its leather carrying case, check it for damage, distortion and

mercury leakage . After confirming that there are no problems, tighten the adjustment screw (18)

(Figure 5.2) until it stops and all air is removed.

(2) Returning the Mercury Barometer to its Vertical Setting

After removing the air by tightening the adjustment screw (18), hold the barometer with both hands

and turn it slowly back to a vertical setting. Then, turn the vernier knob (5) and check that it is not too

stiff or too loose.

(3) Testing for the Presence of Gas in the Barometer Tube

Holding the barometer firmly with one hand, tap the brass cover (21) of the mercury cistern with

the fingers of the other hand a few times and loosen the adjustment screw (18) a little. When the top

of the mercury column (4) appears in the upper part of the slot (2), tighten the screw by half a turn so

that the top of the mercury column (4) is slightly hidden in the upper part of the slot (2). Tilt the

barometer slowly holding it with both hands. At an angle of about 30 degrees, the mercury will reach

the top of the barometer tube (8) and emit a clicking sound. If the click is sharp and metallic, the

mercury column has reached the top without coming up against any gas. When performing this test,

the operator should be aware of the risk of breaking the barometer tube by tilting the unit too quickly. (4) Checking the Attached Thermometer

Check the attached thermometer (7) for breakage or disconnection of the mercury column.

(5) Checking the Hanger Plate

Check the integrity of the upper and lower milky white glass, the hanger hook, the vertical-axis

pivoting link and the wall hanger hook of the hanger plate. Check the latch screw and the three

14

vertical-axis pivoting link screws to ensure they are not distorted or missing.

(6) Installing a Mercury Barometer

Mercury barometers must be installed as close to the vertical as possible to minimize reading errors.

Before installation, remove the attached thermometer and the latch screw and loosen the centripetal

screw. With the glass cylinder (12) filled with mercury, stand the barometer upright and insert the

adjustment screw (18) into the center of the vertical-axis pivoting link.

Next, hang the hanger ring (1) from the hanger hook and attach the screw of the hanger hook.

Using the three screws, secure the vertical-axis pivoting link to keep it in an upright position.

Turn the adjustment screw (18) slowly until the mercury level in the cistern is 1 mm below the

ivory pointer. Do not lower the mercury level abruptly, as the air inside the brass cover (21) is

compressed and will leak through the wash-leather bag to the mercury level, causing bubbles in the

mercury tube (8). Make sure that no bubbles appear in the upper part of the barometer tube (8) during

this process. If the wash-leather bag is too hard, the mercury level may not go down smoothly when

the screw is loosened. In such cases, pay close attention to any sudden fall in the mercury level. If

the level does not go down spontaneously, tap the adjustment screw (18) from below with a finger.

After installing the barometer, re-attach the thermometer. Leave the unit as is for at least a day for

conditioning at room temperature.



(1) Pre-installation Inspection

Before installing a barometer, check it for glass breakage. Make sure that the index (Figure

5.3) moves smoothly and can stop at any point. Shake the barometer slightly and listen to its internal

sound to check for loose screws and nuts.

(2) Pre-installation Adjustment

Rotate the indicator adjustment knob to set the indicator to atmospheric pressure measured with a

mercury barometer on site.

(3) Installing an Aneroid Barometer

The barometer should be installed in a barometer room. If this is not possible, place it away from

direct sunlight and extreme temperature changes. The unit should also be positioned in a place free

from vibration and strong impacts. When installing a barometer on a pillar or a wall, secure it tightly

with wood screws to prevent it from falling.

A barometer specifically intended for horizontal installation should be used in its accessory case or

wooden box for protection.


(1) Pre-installation Inspection

Before installing an aneroid barograph, check the main body (Figure 5.5) and the clock-driven

drum (5) for breakage, distortion, loose or missing screws and other disorders. If there are no

problems, attach the clock-driven drum (5) to the main body. With the pin with ring (9) removed,

15

make sure that the tip of the recording pen (4) aligns with the graduation line for time (curvature) on

the recording chart. Make sure that the pen pressure is appropriate.

Insert the pin with ring (9) into the lever (3) and the reed (2). Turn the indicator adjustment knob

(7) to adjust the reading to atmospheric pressure measured with a mercury barometer on site. At this

time, apply slight vibration to ensure that the pen tip stays at the same point.

Finally, wind the spring of the clock-driven drum and check that it operates properly.

(2) Pre-installation Adjustment

Avoid changing the magnification by mistake on site, as this will necessitate re-inspection. Also

be careful not to change the temperature correction bimetallic mounting position, as this will affect

precision.

If the tip of the recording pen (4) does not align with the graduation line for time on the recording

chart, the clock-driven drum may be offset. Correct it with reference to Chapter 2: Measurement of

Temperature.

(3) Installation of the Aneroid Barograph

As a general rule, aneroid barographs should be positioned on a solid desk or table in a barometer

room. Lay a rubber sheet or similar cushioning under the barograph to absorb any vibration from the

building.


Cylindrical resonator and electrostatic capacity barometers should be used according to the operating

instructions. Since they contain precise electronic parts and circuits, they should be installed away from

humidity, direct sunlight and vibration.

5.8 Practical Training 5.8.1 Aneroid Barometers

It is not necessary to perform aneroid barometer repair at the station. For practice here, open the cover

and check the mechanism. Observe the barometer capsules, gears, lever and hair springs (see Figures 5.3

and 5.4).

5.8.2 Aneroid Barographs

Examine the aneroid barograph to understand its mechanism for maintenance. An aneroid barograph

consists of a barometer capsule, a clock-driven drum and a pen system. The indicator can be adjusted by

rotating the pointer adjustment knob. Repair the clock-driven drum with reference to the repair

instructions in Chapter 2: Measurement of Temperature. It is advisable not to shift the pen arm or the

bimetallic compensator.

5.8.3 Disassembling and Cleaning a Mercury Barometer

(1) Preparation and Precautions

This section covers the practice of disassembling and cleaning a Fortin barometer. Cleaning

should ideally be carried out on a fine, dry day as high-temperature air with high humidity contains

16

more dust. Cleaning should be performed slowly and steadily.

The barometer must be turned upside down for disassembly and cleaning work in the same way as

for transportation. With the barometer in a vertical setting, tighten the adjustment screw (Figure 5.2

(18)) and fill the glass tube with mercury.

When the mercury comes into contact with the top of the glass tube, it will emit a metallic click.

This sound should be kept in mind to check for the entry of air bubbles after cleaning.

After filling the glass tube with mercury, remove the barometer from the hanger plate and turn it

upside down.

Ensure that cleaning tools are free of oil, moisture and acid to prevent amalgamation.

During disassembly and cleaning, it is important to remember the feeling of the screws’ tightness

so that the components can be reassembled as they were before.

The items required for disassembly and cleaning are listed in the attached table.

(2) Disassembling the Mercury Cistern

In the same way as for barometer transportation, turn the unit upside down after the mercury

reaches the top of the glass tube. Install it in a repair

frame (Figure 5.10). It is advisable to pack the

barometer with a polyethylene bag from the scale to the

barometer tube. While pressing the brass cylinder, turn

the screw under the cover (Figure 5.11 (b)) to the left to

remove it.

Pick up the wash-leather bag to check for mercury

leakage. If mercury is leaking, press the wooden base

screw bridge with a finger, turn the barometer back to the

vertical setting and run the leaking mercury into a beaker.

This leaked mercury will be amalgamated, and must not

be mixed with that in the cistern.

Pressing the upper part of the three screws (Figure

5.11 (d)) with one hand, turn the brass cylinder (Figure

5.11 (c)) to the left to remove it. If the screw is stiff,

tighten it slightly and apply a small amount of oil or tap it

lightly before loosening it again.

Tap the wash-leather bag to remove the mercury, and

turn the boxwood (Figure 5.12) to which the bag is attached counterclockwise. If the screw is stiff,

tighten it with a thickish hemp thread (Figures 5.13 (a), (b)) with the other end wrapped around your

hand (Figure 5.13 (c)) and turn the boxwood counterclockwise with the thread. This should be done

carefully to avoid leakage from the mercury-filled glass cylinder (Figure 5.12).

(3) Draining Mercury from the Mercury Cistern

Syringe a small amount of mercury into a beaker being careful to avoid splashing. Insert a finger

into the mercury cistern to feel for the opening of the tube (Figure 5.14 ). Plug the opening with a

finger to prevent mercury from escaping, and bear in mind that excess force will break the tapered part

Figure 5.10 Repair frame and polyethylene tub

Repair frame

Polyethylene tub

17

of the tube. While plugging the opening of the tube with the middle finger, lift the barometer with the

other hand and pour the mercury into a beaker slowly without spilling any outside it. After pouring

all the mercury, turn the barometer upside down as before and unplug the opening of the glass tube.

(4) Disassembling the Mercury Cistern Glass Cylinder

Loosen and remove the three screws (Figure 5.15 (a)) one by one while feeling their tightness for

future reference. Next, remove the brass metal frame, the boxwood and the glass cylinder (Figure

5.16).

It is advisable to mark the screws and holes to prevent mismatching during reassembly. The glass

tube seam and gasket positions should also be marked to prevent mercury leakage after reassembly.

The leather gaskets of the upper and lower parts of the glass tube should not be removed. If they

become heavily soiled, however, remove and soften them well. Pinch each gasket with a pair of

tweezers and immerse it in a beaker filled with filtered mercury to adsorb dirt, then attach the gaskets

during reassembly.

(5) Filtering Mercury and Cleaning Components

To filter mercury, pour it through a strainer over a beaker covered with thick paper (cross-grained

paper) (see Figure 5.17).

First, filter the mercury with a coarse strainer, then again with a fine one. Repeat twice or more

until no dust remains. Since this process takes a long time, filtering work should be started as soon as

the mercury is drained into the beaker.

Wipe the glass cylinder using cleaning paper moistened with alcohol while being careful not to rub

off the marks. Taking care not to damage the glass cylinder, scrape off excess dirt with a cutter then

polish the surface using a toothbrush and toothpaste. Wipe the glass cylinder sufficiently to remove

moisture.

Wipe the boxwood part (Figure 5.16) and the inside of the wash-leather bag (Figure 5.12) with

cleaning paper. Then, fill the bag up to about 1/3 with single-filtered mercury, hold the bag with one

hand to prevent the mercury from splashing, and rub it with the other hand to allow the mercury to

adsorb the dirt. Repeat this process until no dirt remains in the bag.

Filter the contaminated mercury repeatedly it until it is clean.

Gently wipe dust off the ivory pointer using a brush and paper taking care not to move or damage

the pointer, as this will affect the level of index error. Wipe dust off the wooden frame and glass tube

using a brush and cleaning paper.

ome barometers contain mica plates (Figure 5.2 (22)) inside the brass cover (Figure 5.2 (21)).

Remove these plates carefully and wipe off dust with cleaning paper or a brush.

Bubbles often cling to the inlet of the glass tube. Remove them by inserting a well-dried iron wire,

a needle or a similar object.

(6) Assembling the Mercury Cistern Glass Cylinder

Reassemble the glass cylinder in the reverse order of disassembly. Before reassembly, wipe any

dust off with clean paper or brush very carefully.

Holding the upper and lower parts of the glass tube, assemble it to the main body. Be careful not

to mix up the upper and lower ends and their orientation or leave fingerprints. While the gasket is off,

18

soften and wash it well with mercury before setting it into the groove.

Place the boxwood (Figure 5.16) on the main body and cover it with the brass metal frame (Figure

5.16), paying careful attention to the marks and gaskets.

Attach the three screws (Figure 5.16) in their original positions. Tighten them while rotating the

main body and adjusting their balance in relation to each other. Unbalanced tightening may result in

mercury leakage or glass tube breakage.

To clean the assembled mercury cistern (Figure 5.15), pour mercury through the clearance of the

wooden base while filtering so that the mercury adsorbs dirt. To drain the mercury, plug the tube

opening (Figure 5.15 (b)) with a fingertip covered with a fingerstall and turn the main body upside

down. If this is done when the mercury is concave at the opening of the tube, bubbles will enter the

tube. To prevent this, heat the tube’s main body with a dryer to allow the mercury to expand before

plugging the opening.

(7) Filling with Mercury

Pour filtered mercury into the mercury cistern while filtering it again through a fine strainer. Keep

the mercury covered (Figure 5.17) to prevent dust from entering.

The mercury will be concave at the opening of the finger-plugged tube. If it is poured while this

concavity remains, air bubbles will appear at the top of the tube when the barometer is turned back to

its vertical setting, thereby deteriorating the degree of vacuum. Expand the mercury by heating it

with a drier until it becomes convex at the opening of the tube. As this convexity is hardly visible

from above, it must be viewed carefully from other angles.

When the mercury becomes convex at the tube opening, filter and pour it again until the liquid rises

with surface tension just before overflowing. Add refined mercury if required.

(8) Assembling the Mercury Cistern

Wipe any dust off the wash-leather bag and secure the wooden frame tightly to avoid spillage of the

rising mercury. Make sure that the gaskets are properly attached inside the wooden frame.

Push the screw bridge wood of the wash-leather bag into the mercury cistern with a fingertip to

ensure that no mercury is leaking from the mercury cistern glass cylinder or the wooden frame (Figure

5.12). Stop any leakage by tightening the three screws of the brass metal frame. If mercury is still

leaking, disassemble the gasket and soften it again.

Take care not to break the screws of the brass cylinder when tightening them. If a screw has been

amalgamated due to mercury leakage, contact the relevant service personnel for repair.

Tighten the adjustment screw to complete the assembly of the glass cylinder.

At this stage, remove the polyethylene bag attached to the barometer.

(9) Cleaning the Graduation Protective Glass Tube

If a mercury barometer is left for an extended period without use, the graduation protective glass tube

(Figure 5.18 (b)) may become so soiled that the vernier cannot be read clearly. The tube should be

cleaned when the mercury cistern is checked.

To prevent the entry of bubbles, tighten the adjustment screw and turn the barometer back to its

vertical setting. Loosen the adjustment screw by two or three turns.

Loosen the glass catch ring-holding screw (Figure 5.18 (a)) under the graduation protective glass

19

tube to bring it down together with the protective tube. Remove the latch-holding screw (Figure 5.18

(c)) when it comes out of the graduation protective glass tube. The metal hanger fixture should now

come off.

The graduation protective glass tube will also come off. Clean the inside with paper and lightly

dust off the graduation and vernier. If there is any amalgamation on the graduation, the instrument

should be repaired by service personnel.

After cleaning the glass tube, attach the latch-holding screw. Lift the protective tube catch ring

slightly and tighten the holding screw. Assemble the glass tube with enough clearance to allow

rattling in order to accommodate expansion.

(10) Inspecting the Barometer

After disassembling, cleaning and reassembling the barometer, check the outer tube for mercury

drops. Stand the barometer upside down and leave it overnight to check for mercury leakage. If any

leakage is found, readjust the screws.

If there is no leakage, turn the barometer back to its vertical setting and inspect the degree of

vacuum. This can be judged by listening for a metallic click when the mercury comes into contact

with the top of the glass tube. When this sound is heard in the same way as before disassembly, the

degree of vacuum is satisfactory.

If two barometers are available, compare their index errors before cleaning. Compare them again

after cleaning to ensure that there is no change in the index error.

20

Tools for practice

Figure 5.11 Mercury cistern

Figure 5.12 Boxwood with wash-leather bag

Figure 5.13 Method of turning the boxwood if the screw is stiff

Figure 5.14 Pouring mercury into a beaker

(b) Tightening with thread

Figure 5.15 Glass cylinder

(c) Turning the boxwood counter clockwise with thread

(b) Tube opening

(a) Adjustment screw

(b) Screw under cover

(c) Brass cylinder

(d) Screw

21

Tube opening

Strainer

(c) Latch-holding screw

(b) Graduation protective glass tube

(a) Glass catch Ring-holding screw

Figure 5.16 Parts of a mercury cistern glass cylinder

22

ATTACHED TABLE

Tool and supplies for disassembly and cleaning of a mercury barometer

Tools or supplies Number of tools or

supplies Purpose

Repair frame 1 set Setting barometer upside down

Beaker 4 Pouring mercury from barometer

Mercury strainer 1 coarse, 1 fine Filtering barometer mercury

Squirter 1 Syringing mercury into a beaker

Tweezers 1 pair Pinching gaskets

Cutter 1 Scraping off excess dirt on glass cylinder

Drier 1 Heating barometer tube to prevent air-bubble entry

Soft brush and cloth 1 of each Cleaning barometer metal and glass parts

Tool for adjustment of 3 screws

1 Attaching 3 screws in original positions

Dust pan 1 Picking up splashed mercury

Polyethylene tub 1 Setting under repair frame and gathering leaked mercury

Well-dried iron wire 1 Removing air bubbles by insertion into glass tube

Alcohol 500 cc Wiping glass cylinder with moistened paper

Toothbrush and toothpaste

1 Polishing glass cylinder surface

Cleaning paper (fluff-free)

100 sheets Wiping glass cylinder, ivory pointer, etc.

Cross-grained paper (thick paper)

1 sheet Supporting strainer and covering beaker

Polyethylene bag 10 sheets Storing dust

Refined mercury 2 kg Adding to mercury cistern if required

Leather gaskets (attached to the glass cylinder)

2 sheets Spare

Gaskets (attached to the wash-leather bag)

1 sheet Spare

pressure measurement

Documents

axis pivoting

glass catch

leather carrying

atmospheric

thread wooden

standard gravity

indicator

2aneroid barographs