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 manometersdepend 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 sensitiveto 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 pressure indication 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 theunit of pressure iseither the kPa (kilo pascal) or the more often usedmillimetre 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 1,0g/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.
Figure 1: Test arrangement for checking the cuff pressure indication of blood pressure instruments. The cuff can be used as air reservoirto 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 into the 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 cmH2O) can be 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.
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.
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. This procedure can be avoided if the ratio: glass bottle diameter/ hose diameteris 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.