Disposable pressure transducers commonly used in hospitals have been sanitized and reused to reduce costs.
TruWave is a disposable pressure transducer distributed by Edwards Lifesciences® that is widely used in operative rooms and intensive care units. Despite its main specifications being available on the producer website, the construction details are patent-protected, and some assumptions have been made and tested to use it for our purposes.
It was assumed that TruWave is based on a Wheatstone bridge as many others pressure sensors, and the red and black wires that compose its connector line are the power wires while the others are the sensor outputs. Based on that hypothesis, the sensor was supplied by 12 V DC and the output wires were checked using a multimeter by connecting the sensor to a water column: the output voltage of two wires (green and white) was proportional to the pressure, while the yellow one output showed a stable voltage value..
The order of magnitude of the TruWave sensor output is μV (10^-6 volts) and it must be amplified to mV (10^-3 volts) in order to be clearly detectable by the microcontroller board since its resolution is around 4,9 mV (5V / 1024).
For this purpose, a simple amplifier was built, based on the OP90 integrated circuit.
Two single-supply instrumentation amplifiers were assembled, to achieve the desired pressure range amplification : 0-45 mmHg for venous pressure, and 0-145 mmHg for arterial pressure.
Electronic schematic of two amplifiers
Testing the amplifier circuit on a bread-board
Schematic on PCB
The actual amplifiers
Testing the pressure trasducers
Calibration was carried out using a water column as standard for venous pressure and a Hg column for arterial pressure.
Several measurements for each pressure value were recorded.
Statistical analysis shows a linear correlation.
Hacking the pressure sensor and building the amplifiers were the most challenging and time-consuming aspects of the project, primarily because of my lack of knowledge and experience. However, going through this process deepened my understanding of the fundamentals of electronics. Since there are many pressure transducers and instrumental amplifier boards available on the market, these steps can be simplified significantly.
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