In order to make blood pressure measurements with maximum accuracy, a pressure sensor should be calibrated prior to implantation. This is best accomplished by mimicking the conditions which the gauge will be exposed to in-vivo. This is done by placing the pressure gauge under water in the partially filled flask. The water is kept at body temperature 37 C (or as required). The airspace in the flask is pressurized to several values and each of the output values is recorded. The following procedure should be used as a guideline in establishing the user's own calibration procedure.
Pressure Gauge to be calibrated
1 to 2 liter glass flask plus Rubber Stopper with 2 holes (see below)
Mercury Pressure Manometer with bulb and hose
Pressure Amplifier with appropriate cables and manual
Thermometer (Water Bath optional)
Barometer (optional)
Output Measurement or Recording Device (Chart recorder, Data Acquisition System, Digital
voltmeter, etc.)
The rubber stopper should have two holes passing through it. One hole should have a diameter which is the same as the cable on the pressure gauge. The side of the rubber stopper should be split with a scalpel or razor blade in order to place the stopper on the pressure gauge cable without air leaks. The stopper should be positioned such that the sensor portion of the gauge will be under the surface of the water in the flask, when the stopper is sealing the mouth of the flask.
The second hole in the stopper should be large enough to pass a short length of stiff plastic tube without any air leaking. It is best if the plastic tube is a stepped-taper type, designed to accept a flexible hose. The hose from the Mercury Manometer will connect to the tapered plastic tube fitting and be used to increase and decrease the pressure in the flask.
The water level in the flask need only be deep enough to cover the sensor of the gauge (2-3 cm). The water in the flask should be at body temperature. This is best accomplished by placing the flask in a temperature controlled water bath. (This step may be eliminated by working quickly so that the temperature of the water in the flask does not change significantly during the calibration.) The gauge should be allowed to temperature equilibrate for several minutes in the water prior to making any measurements. The volume of air in the flask acts as a filter and dampens out rapid pressure changes and the effects of small leaks.
Calibration is accomplished by pressurizing the flask to various pressure levels and recording the output voltage of the pressure amplifier for each level. The Pressure Amplifier should be adjusted per the manual to compensate for gauge offset and gain sensitivity. Typical output levels might be 0.00 volts at 0 mmHg and 2.50 volts at 150 mmHg. Common calibration levels are 0, 25, 50, 75, 100, 125 and 150 mm Hg.
Most pressure gauges are sealed, and measure pressure relative to the internal pressure. As atmospheric pressure changes, the measured pressure will vary. It is good practice to measure and record atmospheric pressure while calibrating a pressure gauge and during any subsequent in-vivo measurements. This will make it possible to normalize all measurements, which is particularly important when measuring end diastolic pressure (EDP).
Copyright 1995 Triton Technology, Inc.
Updated May 29, 1998