I ordered some “biomedical” pressure sensors (Freescale’s MPX2300DT1) which have a differential output of 5µV/V/mmHg, and will need an instrumentation amp to get usable signals. I also designed a breakout board to mount the pressure sensor, but had not had the spare time to solder and glue the board to test the whole lab.
I finally got the board soldered last night. The surface mount soldering was not to difficult, nor was supergluing the board in place (which I did after soldering). I think I managed to do the gluing without fouling the back of the sensor, but testing will be needed to confirm that.
I now need to make the standoffs (2.4 mm thick), and drill a 1/8″ hole in a ¾” PVC male threaded plug and glue everything together (again without plugging the port). Luckily I’m not planning to use this pressure gauge at high pressure, so the weakness of a superglue joint as a seal is not too important.
The standoffs should be 2.4 mm (including glue) to match the height of the flat part of the sensor. The thickness matching has to be pretty good, since we want the standoffs to relieve any stress on the glue joint for the sensor. I think we could fill in a shortfall of 0.1–0.2mm with extra glue, but we don’t want the standoffs to be too thick, as that would require extra glue at the port, and the port only extends 1.3mm above the flat part of the sensor.
I going to try using material from plastic cable ties as the standoff, since cutting a uniform 2.4mm thick piece of plastic would challenge my shop skills. The cable tie material is easy to cut to length, but is it a suitable thickness? At 1.3mm, one layer is definitely too thin, but 2 layers may be a bit too fat.
After the superglue has had a chance to fully harden, I’ll try some tests of the pressure sensor. I can think of several I want to try: leak test, static pressure calibration, vibration test without water, and vibration test with water.
The leak test is the simplest: just put the plug into a pipe and put some water in it (only a few inches of water pressure). Does it leak?
The static pressure calibration will require providing power to the sensor. At a nominal 5µV/V/mmHg, I should get 9.34 µV/V/in H2O. With a 5V power supply, I should get 46.7µV/in H2O, which is too small to measure with my multimeter or oscilloscope, so I’ll need wire up an instrumentation amp to see anything. I’ll want a gain of at least 1000.
Once I can detect pressure, I’ll want to see if shaking the sensor on my homemade shaker table produces a signal when there is no water present, and when there is water present. If the sensor is too sensitive to vibration, I may need to rethink this lab—perhaps going back to the idea of recording the step-response from a bursting membrane.