Testing Instrument Accuracy
Diagram of a master reference manometer connected to an aneroid with a “Y” connector
Do not attempt to test for accuracy by making a comparison of actual blood pressure readings taken with different instruments and inflation systems on the same patient, by one or more practitioners. Your goal is to test the instrument for accuracy, not the entire process of blood pressure measurement. In short, you will have introduced a very large number of variables to the test. *See “The Clinical Measurement of Blood Pressure” for information on correct technique.
You will need a Baumanometer® instrument (mercury-gravity standard) and a “Y” connector with an inflation bulb and valve attached. Connect the Baumanometer® instrument and the other instrument to be tested as shown below. Cuffs and bags are not used in this test.
The Pressure Standard
A Baumanometer® instrument is to be used as the pressure standard if:
- The mercury meniscus is at zero with no pressure applied to the instrument.
- The instrument is in a vertical position.
- The instrument responds promptly to pressure changes. Any two Baumanometer® instruments, regardless of age, will provide accurate, linear pressure readings at every pressure level if they meet the stated criteria for a correctly functioning manometer.
Check each instrument to be sure that it is at zero. Slowly inflate the instruments to 250 mm Hg and compare the readings. They should be the same, however, a deviation of ± 3 mm Hg is acceptable. Repeat this procedure at 200 mm Hg, 150 mm Hg, 100 mm Hg, 50 mm Hg, 10 mm Hg and 0 mm Hg. If the deviation is greater than ± 3 mm Hg at any of these points, the instrument being tested is inaccurate and needs adjustment or repair.
Notes on Aneroid Devices
“Remember, the mere fact that the needle points to zero on the dial of the aneroid manometer when the compression cuff is deflated does not necessarily mean that the instrument is accurate over the entire pressure range”, Primer Of Clinical Measurement Of Blood Pressure by George E. Burch,M.D., and Nicholas P. DePasquale, M.D.
“The readings on the dial at different pressures should check with those of a properly constructed and perfectly functioning mercury manometer. The fact that the pointer indicates zero may be no guarantee of accuracy over the whole pressure range”, American Heart Association, Recommendations for Human Blood Pressure Determinations by Sphygmomanometers.
“Aneroid devices use metal bellows that expand with the application of pressure. A mechanical amplifier transmits this motion to the pointer. They are subject to inaccuracies caused by hysteresis, drift, temperature differentials, offset and friction errors”. ASME B40.1-1991 for Gauges – Pressure Indicating Dial Type- Elastic Element.