Gas station without pumps

2012 October 23

Rethinking the pressure sensor lab

I’ve had several posts now relating to building a shaker table and putting together a pressure sensor lab project for the circuits course:

  1. Pressure sensing lab possibilities
  2. PC board for pressure sensor
  3. Characterizing tactile transducer
  4. Characterizing tactile transducer again
  5. New amplifier and shaker table
  6. Good and bad news for circuit course
  7. Pressure sensor assembly
  8. Pressure sensor miswired
  9. Pressure sensor noise problems

I’m beginning to think that this lab, as I originally envisioned it, is both too much work to set up and too much work for the students. It was also beginning to look like a major spill hazard (much more so than the thermistor lab or the electrode characterization lab).

I want to back off now and see whether there is a lab that fits better into the course and is less trouble both for me and for the students.  Let’s look at the different parts of the lab, and see which are the most important—discarding the parts that are more trouble than they are worth.

  • Building an audio amplifier (op amp plus one discrete transistor) to drive shaker table.
  • Building an instrumentation amplifier with gain in the range 500–2000 to read strain-gauge bridge pressure sensor.
  • Calibrating pressure sensor with a water column.
  • Inducing pressure waves in water with shaker table, and measuring with pressure sensor.
  • Making measurements at two ends of a flexible hose to try to characterize water in hose using the hydraulic analogy.

I like the idea of having students build an audio amplifier.  In fact, we were planning a simple amplifier in an earlier lab, so extending it to drive more current than the op amp chip can source is a good one.  But we don’t need to build a shaker table for that—we can buy cheap 4Ω or 8Ω speakers and have them build amplifiers for the speakers.

I definitely like the idea of having the students learn about strain gauges and build an instrumentation amplifier for them.  The $5 MPX2300DT1 pressure sensor is a good example of a strain-gauge bridge (with temperature compensation).  We could go with the uncompensated MPX53DP for $7.80, the $8 MPXV53GC7U or the $11 temperature compensated MPX2053DP.  I rather like the sturdier “unibody” packaging for the differential pressure sensors (the DP suffix), and we could attach a hose to them directly, since they have barbed ports (which look like they are designed for 3/16″ ID tubing).  I’d still want a breakout board with screw terminals for the sensor, but assembling it would be easier, since the sensor can be soldered as a through-hole component and  screwed to the PC board, eliminating the gluing I needed for the MPX2300DT1.

I’m currently leaning towards a simpler (and cheaper) setup—eliminating the shaker table, the ¾” PVC plug, and the PVC water reservoir, and just having an MPX2053DP (or even MPX53DP) pressure sensor on a breakout board.  This would discard the hydraulic analogy part of the lab, but students would still build an instrumentation amplifier, characterize the pressure with a water column (easily measured as the height of water in clear tubing), and use the pressure sensor to measure breath pressure (inhalation and exhalation).

The maximum pressure of human breath is about 25kPa or 100″ H2O, so the ±50kPa range of the differential sensor should be plenty. The MPX2053 sensor is spec’ed at 800µV/kPa with a 10V power supply, so with a 5V supply it would provide 400µV/kPa.  We probably want a 0–5V output for a -25kPa to +25kPa input, so an amplifier gain of 250 is called for.  That’s a bit less touchy than the gain of 1000 I  used with the MPX2300DT1, but will still be good warmup for the EKG amplifier (which needs higher gain and has to use two stages to avoid saturating from small DC offsets in the first stage).

The uncompensated MPX53DP is spec’ed at 1.2mV/kPa at 3v (2mV/kPa at 5V), so less gain would be needed for the uncompensated part.  If you don’t need temperature correction, then the cheaper part gives you greater sensitivity. I’ll have to think about which would be pedagogically more useful—currently I lean towards the temperature-compensated part, as a concept that they should learn and because it forces them to make a higher gain amplifier.

Building the instrumentation amp and making breath pressure measurements should only take one 3-hour lab period, rather than two, so if I go with this design, I’ll need to come up with another lab.  Perhaps a second audio amplifier lab, with an output transistor and some filtering would be a good lab to insert I have to decide whether that should be a soldering lab or a breadboard lab.  I think that the two instrumentation labs (pressure sensor and EKG) should be done by soldering on a PC board, but I’m not sure the instrumentation amps should be their first soldering projects.

2 Comments »

  1. […] to a pressure sensor with a built-in barbed port, so that hoses can be directly connected (see Rethinking the pressure sensor lab). The new choice of pressure sensor has a different […]

    Pingback by Thinking about PC boards and parts kits for circuits lab « Gas station without pumps — 2012 October 24 @ 22:52 | Reply

  2. […] showed them the tiny medical-grade pressure sensor that I had considered using, but ultimately rejected as too fragile, switching to a sturdier sensor with an easier-to-assemble breakout board. I also showed them my […]

    Pingback by Twenty-first day of circuits class | Gas station without pumps — 2013 February 28 @ 20:52 | Reply


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