Calibrating and Servicing a Sphygmomanometer

An aneroid sphygmomanometer measures pressure through a mechanical linkage of bellows, levers and a spring. Those parts age, and the gauge drifts — usually reading low, which means under-treated hypertension.

Nothing on the device announces this. A drifted gauge looks and feels exactly like an accurate one.

How often to check

At least annually for any device informing treatment, and immediately after a drop. Devices in heavy use, or moved between rooms on a trolley, deserve checking more often.

The zero check — do this monthly

With the cuff deflated and disconnected, the needle should rest exactly on zero, inside the marked zone. A needle sitting off zero at rest is a gauge that needs servicing. This takes two seconds and catches a good proportion of problems.

Checking against a reference

Connect the device and a calibrated reference gauge to a common cuff or volume with a Y-connector. Inflate and compare at several points across the range — typically 60, 100, 150, 200, 250 mmHg — on the way up and on the way down.

A difference of more than about 3 mmHg at any point means the device should be serviced or retired. Check both directions: hysteresis, where the reading differs going up versus coming down, is a common failure the single-direction check misses.

The rest of the set

• Cuff and bladder. Split seams and perished bladders fail to hold pressure.
• Tubing. Cracks at the connector ends are the usual leak.
• Bulb and valve. A valve that will not hold, or releases in jumps rather than smoothly, makes accurate reading impossible — you cannot control the 2–3 mmHg per second deflation rate proper technique needs.

Spares are in sphygmomanometer accessories.

A simple leak test

Wrap the cuff around a rigid cylinder, inflate to 200 mmHg and close the valve. The reading should fall no more than about 1 mmHg per second. Faster than that and you have a leak in the cuff, tubing or valve.

Keep a record

Log each check with date, result and who did it. If a reading is ever queried, a maintenance record is the difference between a defensible measurement and a guess.

Replacement sets are in sphygmomanometers. See also the full monitoring guide.

Browse the range in our blood pressure monitors collection.