biometrix overview of temperature measurement and calibration

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Overview of Temperature Measurement and Calibration


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Objectives

History of Temperature Measurement Principle Operation of Temperature

Measurement Devices The Construction and Materials used Advantages and Disadvantages Applications Calibration Indications of Instrument Failure


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History

Galileo is credited with Inventing the first thermometer in 1592.

In an open container filled with colored alcohol he suspended a long narrow-throated glass tube, at the upper end of which there was a hollow sphere.

When heated the air in the sphere would bubble out thru the tube.


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History Continued

When cooled the air would contract and water fluid would move up the tube.

Fluctuations could then be noted by the position of the liquid in the tube.

This “upside-down” thermometer was a poor indicator because fluctuations could be caused by barometric pressure and there was no scale.


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History Continued

Vast improvements where made in the ensuing years using the Florentine thermometer which incorporated sealed construction and a graduated scale.

Over from the next few decades many temperature scales where used based on two or more fixed points.


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History Continued

One scale was not universally recognized until the early 1700’s, when Gabriel Fahrenheit, a Dutch instrument maker, produced accurate and repeatable mercury thermometers.

The low fixed point that he used was a mixture of Ice and salt (ammonium Chloride). The lowest temperature he could produce was 0 Deg F.


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History Continued

The high fixed point was 96 Deg F, the temperature of the Human Body.

Fahrenheit chose 96 Deg because in his earlier scales he had started with a resolution of 12 and then later increased his resolution to 24,48 and then 96.

His scale increased in popularity because of the consistent repeatability and quality of thermometers he built.


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History Continued

Around 1742 Anders Celsius proposed that the melting point of ice and the boiling point of water be the two benchmarks.

Celsius proposed that 0 Deg C be the boiling point and that 100 Deg C be the freezing point. Later the end points where reversed and the centigrade scale was born.


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History Continued

In 1948 the name was officially changed to the celsius scale.

In the early 1800’s William Thompson (Lord Kelvin) developed a universal thermodynamic scale based on the coefficient of an expansion of an ideal gas.

Kelvin established the concept of absolute zero and his scale remains the standard for modern thermometry.


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Temperature conversions

Celsius=5/9(F-32) Fahrenheit=9/5(C+32) Kelvin= C+273.15 Rankine= F+ 459.67

Rankine is the Fahrenheit equivalent of the Kelvin Scale


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Thermocouple

Theory of Operation When two wires of dissimilar metals are joined at both ends

and one end is heated, a continuous current is induced, which flows in the thermoelectric circuit.

If this circuit is broken, the net open circuit voltage is a function of the junction temperature and the composition of the two metals.

All dissimilar metals exhibit this effect. For small changes in temperature this voltage effect is

linearly proportional to temperature.


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Measuring Thermocouple Voltage We can not directly measure thermocouple

voltage with a voltmeter. When the voltmeter is attached to the thermocouple, the voltmeter leads themselves cause a thermoelectric circuit.

One way to eliminate this issue is to implement a reference junction at 0 Deg C so the we know the voltage created at that point.


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Measuring Thermocouple Voltage Using an Ice Bath at the junction is not a convenient method for

field calibrations, athough it is highly accurate. The two most common methods are:

Software Compensation: Using an RTD measure the temperature at the junction to compensate for reference voltage an give an accurate thermocouple voltage.

Hardware Compensation: Using a battery inside the voltmeter to electronically compensate for the reference voltage.

In modern voltmeters software adjustment is generally used. This is because a separate battery would be required for each type of thermocouple.


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Converting Thermocouple Voltage to Temperature Conversion of the thermocouple voltage to

temperature is achieved with the software of the thermocouple reader. Multiple equations are required depending on the relevant voltage as the Temperature vs. Voltage curve is not completely linear. Thermocouple voltage can also be converted using the National Bureau of Standards Thermocouple Tables.


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Materials Used in Thermocouples As discussed previously, any two dis-similar metals

joined together will create a thermoelectric effect. Common material construction Types are:

J type-Iron vs. Copper-Nickel K type-Nickel Chromium vs. Nickel-Aluminum T type-Copper vs. Copper-nickel There are many more types of thermocouples used J,K & T

are three of the more frequently seen in Biotech Industries.


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Advantages of Thermocouples

Self Powered Simple Rugged Inexpensive Wide configuration variety Wide temperature range


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Disadvantages of thermocouples

Non-linear Low voltage Reference required Minimal stability Minimal sensitivity


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Thermocouple Problems Poor Junction Connections

There are a number of acceptable ways to connect two thermocouple wires together. Soldering- Even though a third metal is introduced to the junction it

will not effect the reading as long as the temperature is consistent on both sides of the junction. However, solder does limit the temperature range of the thermocouple. Therefore welding must be used at higher temperatures.

Welding- To reach the higher temperature range the connection must be welded. Welding however can cause damage to the thermocouple by damaging the wire or causing it to overheat and reacted with the air thereby creating a different substance with different thermoelectric properties. Industrial thermocouples are welded under exacting conditions using expensive equipment thereby minimizing these effects.


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Thermocouple Problems cont. Decalibration

Decalibration can be caused by the result of the diffusion of atmospheric particles into the metal at extreme temperatures.

It can be caused by high temperature annealing or by cold working the metal. This effect can be caused when the wire is drawn through a conduit or strained through by rough handling or vibration.


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Thermocouple Problems cont.

These problems can be avoided by careful handling of the wire. Proper placement and protective metal sheathing if necessary.


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Thermocouple Problems cont. Shunt Impedance- High temperatures can also take

there toll on thermocouple insulation. If a breakdown in the insulation occurs, a junction could be created in a place were temperature is not meant to be measured.

Galvanic Actions- The dyes that are used in some insulations can form an electrolyte that can produce voltages much greater than the normal thermoelectric effect. Care should be taken to ensure that the insulation is not subject to chemical contamination.


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Thermocouple Applications

Thermocouples are used in a variety of different Biotech applications. Lyophilizers- Due to broad operating temperature

range. Ultra Low Temperature Freezers-Due to broad

operating temperature range Validation Test Equipment- Small size allows

easy placement.


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Resistance Temperature Detector Resistance temperature elements are available in

many different types, conforming to different standards, and capable of different temperature ranges with various sizes and accuracies.

All Resistance temperature elements operate in the same manner with a given resistance value for a specific temperature.


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Resistance Temperature Detector cont. The resistance temperature elements are the heart

of the Resistance Temperature Detector. Generally a bare resistance element is to fragile and sensitive to be used in its raw form therefore in must be placed in a protective casing.

Resistance Temperature Detector (RTD)- Is a general term for a temperature measuring device that consists of a temperature element, a sheath, lead wire and a termination or connection.


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Resistance Temperature Detector cont.

The Sheath, a closed end tube, immobilizes the element, protecting it against moisture and the environment which it measures

The sheath also provides protection and stability to the transition lead wires from the sensing element.


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Resistance Temperature Detector cont. Some RTD’s may also be enclosed in thermo

wells that protect the RTD but also keep the system closed. (ie. A tank or boiler)

This becomes of great importance when removing an RTD for replacement or calibration.


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Resistance Element Characteristics Material of Resistance Element

Several types of metal are common in resistance elements. Platinum- Most common due to its linearity with

temperature. Nickel & Copper- although most are being replaced by

platinum Balco- an Iron-Nickel Alloy (very rare) Tungsten and Iridium- Also very rare


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Resistance Element Characteristics Temperature Coefficient

The temperature coefficient is a physical and electrical property of the material.

This changes the average resistance change per unit of temperature from the 0 to 100 Deg C.

Different Organizations have developed different temperature coefficients as there standards.

In 1983 the International Electrotechnical Commission developed the standard of a Platinum Element 100 Ohm at 0 Deg C with a 0.385 Ohm change per Deg C.

This is now the most widely used and accepted standard.


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Resistance Element Characteristics

This standard is derived by taking the resistance at 100 Deg C 138.50 Ohms and subtracting 0 Deg C 100.00 ohms and dividing by 100 temperature units.

Even though the coefficient is 0.385 Ohms/Deg. It does not assume that the slope is perfectly linear.

Some Other Less common standards are .390 US Industrial Standard .392 Old US Industrial Standard


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Resistance Element Characteristics Nominal Resistance

Most standards use 0 Deg C and 100 Ohms. However, there are others that use 50, 200, 400,

500, 1000 and 2000 Ohm.


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Resistance Element Characteristics Temperature Range of Application

Depending on the mechanical configuration, construction and manufacturing, RTD’s maybe used from-270 to 850 Deg C.


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Resistance Element Characteristics Physical Dimensions or Size Restrictions

The most critical dimension is the Outer Diameter which must be small enough to fit into a an outer sheath.


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Resistance Element Characteristics Accuracy

RTE’s have two standard accuracies Class A and Class B. With Class A having a tighter tolerance.


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Pt 100 tolerancesLimiting Deviations

Classification A Classification B

Temperature ºC ºC Ohm ºC Ohm

-200 ±0.55 ±0.24 ±1.3 ±0.56

-100 ±0.35 ±0.14 ±0.8 ±0.32

0 ±0.15 ±0.06 ±0.3 ±0.12

100 ±0.35 ±0.13 ±0.08 ±0.30

200 ±0.55 ±0.20 ±1.3 ±0.48

300 ±0.75 ±0.27 ±1.8 ±0.64

400 ±0.95 ±0.33 ±2.3 ±0.79

500 ±1.15 ±0.38 ±2.8 ±0.93

600 ±1.35 ±0.43 ±3.3 ±1.06

650 - - ±3.6 ±1.13

700 - - ±3.8 ±1.17

800 - - ±4.3 ±1.28

850 - - ±4.6 ±1.34


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Resistance Element Characteristics Response Time

These times are based on the time it takes to reach a given value in a steady stream of flowing water or air.


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Resistance Element Characteristics Measurement Current and Self Heating

Since there is a electrical current flowing through the element this causes heat. There is a temperature error that is created by this.


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Lead Wire Configurations

2 Wire RTD A two wire RTD is inherently inaccurate due to the

fact that the two lead wires resistances are added. R1 + R2 + Re =Rt This will produce a temperature readout that is

higher than the measured value.


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3 Wire RTD The 3 wire RTD is the most commonly used. It provides a way to compensate for lead resistance. It provides one connection to one side of the element and

two to the other. The 3 wires that are used have identical resistances.

R1=R2=R3 If the resistance is measured though R1 +Re + R2 then the

Resistance measured through R2 + R3 is subtracted. Thus, we are left with only the value of Re.


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4 Wire configuration Two connections are provided to both sides of the element. This configuration is used for the most accurate readings

requiring the most precision. A constant current is passed through the outer wires 1 & 4. A voltage drop is measured across the inner leads. So from

V=IR we learn the resistance of element from the element alone.


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RTD Advantages

very stable higher accuracy than thermocouples better linearity than thermocouples


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RTD disadvantages

higher cost power supply is required low resistance value self-heating effect


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ITS-90

Although temperature was one of the first variables to be measured in industry the first formal international temperature scale was not adopted until 1927. The scale started at -200°C and went beyond the freezing point of gold (1063°C) and was based on fixed points such as the boiling point of oxygen and sulfur. Today, the internationally recognized temperature scale is referred to as ITS90 and is defined by the following table:-


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ITS-90 PointsTemperature deg C Substance State

-270.15 to -268.15 Helium Vapor pressure point

-259.3467 Hydrogen Triple Point

~-256.15 Hydrogen at 10192 Pa Vapor pressure point

-252.85 Hydrogen at 33321.3 Pa Vapor pressure point

-248.5939 Neon Triple Point

-218.7916 Oxygen Triple Point

-189.3442 Argon Triple Point

-38.8344 Mercury Triple Point

0.01 Water Triple Point

29.7646 Gallium Melting Point

156.5985 Indium Freezing Point

231.928 Tin Freezing Point

419.527 Zinc Freezing Point

660.323 Aluminum Freezing Point

961.78 Silver Freezing Point

1064.18 Gold Freezing Point

1084.62 Copper Freezing Point


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Non Contact temperature Measurement These devices are particularly useful for measuring temperatures

of moving objects or rotating surfaces where contact is not possible or is undesirable.

Radiation pyrometers measure the temperature of an object by measuring the amount of radiation that the object emits. All objects emit radiant energy but the intensity and wavelength of this energy depends on the object’s temperature and its emissivity (its ability to send out radiant energy). The wavelength of this radiant energy is from the visible light area, 0.35 to 0.75 microns, to the infrared area of 0.75 to 20 microns.


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Non Contact temperature Measurement Therefore some hot objects will emit visible radiation e.g. red-hot

steel, tungsten filament etc., but there is much more radiation emitted in the infrared area of the spectrum. The emissivity of an object is measured relative to a perfect emitter or a blackbody, which is an object that absorbs all heat to which it is exposed and emits that heat as radiant energy.


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Advantages

non-contact temperature measurement wide temperature range 20 to 3500ºC fast response adjustable for long term drift


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Disadvantages

expensive bulky needs to be adjusted for precise emissivity


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Thermistors

Thermistors, derived from Thermally sensitive resistors, are solid state devices that exhibit a high coefficient of resistivity. They are used to measure temperature typically in the range 50 to 200ºC, as well as being incorporated into electrical circuits as protection and compensation devices.


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Thermistors

Thermistors are manufactured from complex metal oxides such as cobalt, magnesium, manganese or nickel.


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Thermistors

There are two types of Thermistor; an NTC (negative temperature coefficient), whose resistance changes inversely with temperature, and a PTC (positive temperature coefficient) whose resistance changes proportionally to temperature. There are no industry standard Thermistors consequently each manufacturer produces devices with unique characteristics in various formats such as bead, disc, washer or chip.


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Thermistors

Because of the limited temperature range of Thermistor devices, the temperature probes are predominantly used in applications such as HVAC and refrigeration where they are connected directly to the measurement or control instruments or panel meters.


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Thermistor Advantages

cost effective in volume stable better accuracy than thermocouples


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Thermistor Disadvantages

linear only over a limited temperature range no industry standard Not always interchangeable

biometrix overview of temperature measurement and calibration

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