Data Acquisition

I’ve been long overdue for a data acquisition system and finally got around to building something. Knowing the chamber pressure in a rocket motor is a very helpful tool in understanding the motor’s overall state of health, so for the time being I’m only going to measure the chamber pressure. To do this, a minimum of an analog to digital converter (ADC) and pressure transducer will be required. I suppose a regulated voltage supply to the pressure transducer would also be necessary.

For my ADC I purchased a DATAQ Instrument DI-145 which has a +/-10v input with 10 bit resolution. I found some cheap 2500 PSI pressure transducers on Ebay that output .5v – 4.5v from a 5v input which means an amplifier is not required. However, by utilizing the (near) full input range of my ADC I could achieve higher resolution. Using a simple 741 operational amplifier I could have the pressure signal amplified with a gain of 4 with a -9v offset to change my input range from .5v – 4.5v to -7v – +9v , four times the resolution.

Amp Schematic


I went through my stash and found some project boards, resistors and LM317′s for voltage regulators. The rest of the components required a run to Fry’s (yuck). I used a 3D-printer from work to make a project box to house all the amplifier electronics; it’s a bit bulky and if I get bored I’ll print something smaller.  Part of the problem was that I had five small circuit boards that I wanted to use rather than one that was large enough to mount all the components.


Lastly, I needed to calibrate the system using a nitrogen tank and accurate analog pressure gauge.



With the system calibrated I can now accurately measure the chamber pressure of my rocket motors to 3 PSI per count. Once the pressure is recorded I can calculate the motor’s thrust with an estimated thrust coefficient using the formula:

F  = Cf*At*Pe

Where: Cf = thrust coefficient

At =  throat area

Pe =  chamber pressure

Without getting into the details, a good starting point for Cf would be approximately 1.5. More to come.


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