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7/30/2019 Beschreibung Low Cost Final

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Stephan Mieke, Eberhard Seiler

Low cost, self-made pressure standards for checking blood pressure

instruments

Checking the pressure indication of blood pressure instruments (sphygmomanometers)

Blood pressure plays an important role in health care. Its value, together with the body

temperature is usually measured by physicians in order to get a first indication about the

health status of a patient. For historical reasons the unit of blood pressure is the millimetre

Mercury, the mmHg. A mercury column was and is still very often used in blood pressure

instruments as reference pressure and as indication of the actual value. Nowadays, other

pressure sensing elements (aneroid manometers, piezo-resistant transducers) are used in

combination with electronic components which calculate and indicate the blood pressure

values. Since Mercury is banned for environmental reasons in more and more countries

alternatives become more important. Commercial blood pressure instruments usually have a

measuring range from 50 to 300 mmHg. The performance of aneroid manometers which often

replace Hg manometers depend extremely on the technical design and the way they are

manufactured, e.g. pre-aging is required to avoid drift over time. Additionally, most of these

manometers are sensitive to mechanical shocks, thus rough handling can affect the accuracy.

Consequently, it is necessary to check the accuracy of the pressure measurement periodically,

It is recommended to check periodically every 6 to 24 month and after exposure of the

instrument to mechanical shock or in case of doubtful indications.

In the following a method is described which allows to check the accuracy of the pressureindication by a simple, low cost pressure standard. This kind of standard is intended for use

mainly by hospitals, clinics and medical services which have no access to professional

calibration or verification services. Nevertheless, it is highly recommended that verification

services will promote and support the idea of metrological controls in order to contribute to

accurate measurements in the very sensitive area of health care.

The principle

Pressure is defined as force per area, in SI units: newton per square meter. Pressure can be

realized by a column of liquid which, under the local gravity, produces a force proportional to

the height of the column on a given surface. In the case of blood pressure the unit of pressureis either the kPa (kilo pascal) or the more often used millimetre of mercury (mmHg), a unit

outside the SI but recognized and accepted. If mercury with a density of 13,6 g/cm3 is

replaced by water (H2O) with the density of 1,0 g/cm3 the column must be higher by a factor

of 13,6 to produce the same pressure. In contrary to mercury water is cheap, available

everywhere and non toxic. For these reasons the standard described here uses water as

reference liquid. Figure 1 shows the principle of the test arrangement.

Tolerable error limits for the verification of blood pressure instruments for re-verification are

4 mmHg (see OIML R16-1) corresponding to 5,4 cm of water.


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Figure 1: Test arrangement for checking the cuff pressure indication of blood pressure

instruments. The cuff can be used as air reservoir to reduce the effect of leakage, if there is

any. Before starting the test the instrument must be switched to the test mode.

Design details

Necessary parts for the pressure standards are easily available and cheap. A glass bottle with

screw cap as used to preserve food stuff (e.g. pickles) serves as water container. Two holes

must be drilled into the cap, the diameter depends on the pipes used as ducts. One simple

solution is to use copper pipes of 10 mm diameter which are airtight fixed by two-component

epoxy glue to the cap. The air pressure produced by the pump of the blood pressure

instrument is fed into the bottle through the shorter pipe which does not reach down to the

water level inside the glass bottle. The other pipe should be long enough to reach down intothe water. The upper end is connected to a transparent plastic hose of at least 2,5 m length.

Usually, connectors are necessary which enable the tight connection of hoses with different

diameters. Examples are shown in figure 3. These connectors are available at shops selling

laboratory equipment. The essential point is that all ducts and connectors must be airtight.

This is the case if, after pumping air into the system, the water level in the hose does not drop.

Otherwise the leak must be detected by putting the glass and all connectors into a water

container. Air bubbles will indicate the leak.

Tolerable error limits for the verification of blood pressure instruments are 4 mmHg

corresponding to 5,4 cmH2O. For calibration/verification purposes the accuracy of the

standard should be at least three times better. Even 10 times better accuracy (0,5 cmH 2O) canbe achieved with the water standard. Ambient temperature and pressure effects are much

smaller and can be neglected. Figure 2 shows one example of such a pressure standard.

Schematic set up of the standard(Example for a pressure of 125 mmHg corresponding to 170 cmH2O)

125 mmHg

Pump

Water

(125 mmHg)

Instrumentunder test

Water

170 cm water


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Test procedure

The instrument under test should be set into the verification mode as indicated in the

instruction manual. If the verification (or test) mode is not described in the manual, the

(original) manufacturer should be contacted, many of them have special information for

verification bodies.

If all connections are made according to figure 1, the long hose should be positioned

vertically in parallel to a meter stick. The zero point for the measurement is the surface of the

water inside the glass. For a better reading some drops of ink can be put into the water. As

soon as the system is pressurized the water will climb up into the hose. Readings should be

taken at different pressures (low, medium, high). The results are then compared using the

conversion table mmHg-cmH2O given in the Excel sheet. Measurements should be taken at

least up to 200 cmH2O in order to cover the most significant part.

Due to the compression and decompression of air during the test the thermodynamic

equilibrium in air is disturbed and it takes some time until it is restored again and the reading

should be taken.

C:\Dokumenteund Einstellungen\

Fig.3: Examples of

hose connectors

Fig.2: Example of the

realization of a

pressure standard

using water.Pressure generated by

the pump is fed into

the cuff and the water

container.


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Corrections

The level of the water reservoir decreases with increasing pressure due to the water that goes

into the hose. That means the zero level changes and either corrections have to be calculated

or the meter stick must be adjusted to the water level every time a measurement is taken. Thisprocedure can be avoided if the ratio: glass bottle diameter/ hose diameter is equal to or larger

than 20. If this is the case the deviation will be 0,5 cm or less at 200 cm which is not more

than 1/10 of the permissible error limit.

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