Monitoring system

Transducer excitation, amplifier, and filter

The sensing elements of most pressure transducers are four resistive elements connected as a Wheatstone bridge. Resistive bridges require the application of an excitation voltage (typically 3-5 V). Modern monitors provide very accurate (±0.1 per cent) excitation voltages to the transducers. These monitors also contain amplifiers which take the small voltages supplied by the transducer, typically 30 pV/mmHg, and 'magnify' them by about 1000 times. In amplifiers, 'magnification' is called 'gain'. With modern amplifiers, the gain is very stable and is calibrated with an accuracy to about ±1 per cent. Because of the accuracy, reliability, and stability of both the transducers and amplifiers, it is no longer necessary to calibrate pressure monitors ( Cooper..and...Pa.ulsen 1994; Gardner..1996). Most pressure amplifiers include a low-pass filter to filter out unwanted high-frequency signals.

Zero control

The zero control on a pressure monitor permits the adjustment of the output signal from the monitor to be zero when the fluid-air interface of a designated 'zeroing stopcock' is placed at the mid-axillary position. Once the zeroing process has been completed it is important that the vertical relationship between the patient and the pressure transducer remains fixed. If there is a vertical movement of the transducer or of the patient relative to the transducer, the system must be 're-zeroed.' However, the vertical position of the zeroing stopcock is unimportant once it is closed.

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