Wrong measurements with the Pressure transmitter PX309-100 mV - ni9237

[dav78]

I am measuring with a Load Cell, ratiometric, full bridge capability, and not problems.

However, unexpectedly the pressure transmitter has been giving problems. I have contacted to the supplier for any advice and recalibration, and apparently  there is nothing wrong with the pressure transmitter. They have replade the pressure transmitter and we get the same readings  

This is a ratiometric sensor, same as our load cell, so using the same wiring should work perfectly   The issues are:

-          I am varying the excitation (3.3 V, 5 V and 10 V) in order to check that it is working properly, and the reading are always the same. I.e with 3.3 V Vex, 0.0012 V reading for 0.48 bar, and it should be 0.0046 V. The reading is 0.0012 V the same, using 5 VDC excitation. Why?????????

-          Measuring with the multi-meter  on pin 2 and 3, I have 3.9 mV, which is more closer to the expected reading (4.6 mV), and the voltage varies on pin 2 and 3 when we vary the Vex, so the sensor seems working properly.

-          Initially, I thought that was our scaling mistakes, and we multiply by 4 (factor) to get the value shown on our pressure gauges . However we need to multiply by 4 only when is 0.3 bar or 0.2 bar, and 3.85 when is 0.5 bar, and 3.45 when is 1 bar, so this is not a linear behaviour.

At the moment we do not have any clue of how to sort it out this problem. In addition the pressure tranmitter resistance is 5000 Ohms, which indicates that there is no saturation with power of the NI9237. 

Someone can help, please???

[Henrik_Volkers]

As far as I have heard, piezo resistive bridges are not fully ratiometric.

That is why some specs don't give a mV/V sensivity , just mV, and the nominal supply voltage .

Or maybe you have a sensor with an active electronic??

And for the DAQ: A bridge simulationr/calibrator is a nice tool to check the card. Output voltage stable?  

[dav78]

Hi Henrik,

Thanks for your answer.

There is not active electronics. Supplier has guaranteed basically 100% ratiometric, however having a fixed pressure, we vary the excitation from 5 VDC to 10 VDC, and the output is the same. So it is a lot frustraiting.

Someone has used the NI9237 with a ratiometric pressure transmitter???

Thanks,

David

[Henrik_Volkers]

How do you read the value?

If the channel is configured for mV/V (ratiometric) reading it would be the expected output 😉

What sensor do you exactly have? Link to Spec?

[dav78]

Hi Henrik,

Thanks for your response,

I read using the NI9237, connecting pin 6 and 7 to excitation, and 2 and 3 output of the sensor. I am using Matlab code and the configuration is 'Bridge' and 'Full', this works correctly with the Load Cell, which is ratiometric 2mV/V. Do you think , I should use reading 'voltage'; the thing is that using the multi meter I have similar to expected voltage at pin 2-3, and going through the NI9237, my readings are around 4 times lower than the shown on the pressure gauge.

I have linked the spec,

Any advice, highly appreciated,

Kind Regards

David

[Henrik_Volkers]

First the manual stated that you must connect the sense wires....  See http://www.ni.com/pdf/manuals/374186e.pdf

Next: Check the driver help files ... If you read bridge, you should get already mV/V  so every change in the bridge supply is already compensated. (assuming the sense (RX) lines are connected correctly 😉  )

[dav78]

Hi Henrik,

 Many thanks for your advice.

Yes I had tried to used the RS previously, I found unneccesary. However, I will try again and see if this works. The thing is that the bridge is not accesible, and assume that the Rlead is 0 due to the .lead is very short. The only thing I can do is to link the pins (Vex and Rs) with a very short lead.

I am surprised that the load cell with the same wiring connections and software is working properly and not the pressure sensor. The reading does not change if I change the Vex, Do you think that is because I am not connecting the RS?

Kind Regards,

David,