Problem with Pressure Transducer and NI 9205 with NI 9263

[MAF101]

I have a bridge based pressure transducer with a range of 0 - 5000 psi and requires a 10 V excitation. The output is 3 mV/V. I have connected it to a cDAQ with a NI 9263 providing the 10 V excitation and a NI 9205 to read the output values. I should expect a 0 - 30 mV output corresponding to 0 - 5000 psi, correct? (As an aside, the NI 9205 should be accurate to 0.1 mV and this should lead to an accuracy of about 16 psi, right?).

However, when I actually tested the transducer out running a test going from 0 to 1000 and back to 0 psi (holding for some time at the pressure states) the values I am getting from the NI 9205 are about 204.614 mV for 0 psi and then going to 204.867 mV for 1000 psi (the actual results are noisy, so these are the averages). I have attached the SignalExpress file used for the test as well as the Excel file containing the resutls. The test was performed with a number of other sensors active (load and displacement gauges, LVDTs, and strain gagues) in addition to the pressure as I will ultimately be using them for my experiment. In the SignalExpress file, the pressure transducer is the only sensor receiving excitation from the NI 9263 (ao_0) and is ai_3 on the NI 9205.

So this time I tried to run the test again, but modified the SignalExpress file (second attached .seproj file) to only include the pressure transducer. But now I didn't get a response at all from the transducer (it simply stayed flat).

As to the hardware connection for the transducer, it is a 4 wire connection (+ Excitation, - Exciation, + Signal, - Signal). I connected the signal wires to pins 4 and 23 (AI3 and AI11) for a differential pair. I then connected the excitation wires to pins 0 and 1 (AO0 and COM). Finally, I took a jumper wire and connected pin 23 on the NI 9205 to pin 1 on the NI 9263. I performed this last step at the suggestion of NI support regarding a previous problem with a string potentiometer. This step connected the grounds between the two modules (the sensor did not provide a response with a change in pressure when I did not have the jumper wire).

I appreciate any assisstance as to why I'm not getting a 0 - 30 mV output.

[Catherine-B]

Hi MAF101,

Does your 9205 module have a spring terminal or a DSUB connect? I want to verify the pins for the differential measurement are correct, since for the spring terminal, the pair is pins 4 and 22, while for the DSUB connector, the pins are pins 4 and 23. Can you double check the wiring for your system?

Also, what transducer are you using? Do you have a datasheet for the transducer? 

[MAF101]

My 9205 module is a DSUB, so pins 4 and 23 should be correct. I have attached the datasheet for the transducer. It is the "mv/V output standard" model (no internal amplifier).

[Catherine-B]

What cDAQ chassis are you using?  Do you have another power source you can try using for the 10V excitation besides the output of the 9263?

Do your other sensors give expected results? Are you able to scope the output of the pressure sensor to confirm what voltage is being output?

You said that when testing the pressure sensor by itself in a SignalExpress project, you didn’t get any signal, is that correct? If you scope the voltage being output when running the first project (with all of the sensors) versus the second project (pressure transducer only), how to the voltages compare?

[MAF101]

I am using a 4-slot USB chassis (cDAQ 9174).The other two remaining slots are taken up by two NI 9237s, used for exciting and measuring the strain gauges. I have tried using the 9237 to excite and measure the V/V response from the pressure transducer, and I do get the proper 6 mV output at 1000 psi (which I have attached).

The only problem with using the 9237 is that I need to measure 6 strain gauges, so I need both 9237s to have enough channels (four gauges on one module, and two on the other). I did try in the past to connect both the two strain gauges and the pressure transducer on the second 9237 module, but I got an error message saying that I could not excite the strain gauges to 2.12 V and the pressure transducer to 10 V at the same time.

I have attached the Excel file containing the pressure transducer's response when using the second SignalExpress file, where all other sensors have been removed. The voltages simply stayed around the 205 mV level, without the plateau corresponding to the applied 1000 psi as in the previous test.

[Catherine-B]

It seems like resolving the excitation issue is one of the first things that needs to be investigated. Do you have a power supply, other than a C Series module, that you can use to excite the pressure transducer?

[JÞB · Accepted solution]

its not going to work like that.

Your transducer needs @30mA at 10VDC.  there is only 1mA available from the 9263 AO channel.  you need another supply.

[MAF101]

All right, thanks for both of your assistance.