Pressure transmitters not working

[JParedes]

Can someone tell me what is wrong with my VI attached below? I am attempting to obtain temperatures from some type K thermocouples and pressures from some pressure transmitters. The VI worked fine when I just had the code for the thermocouples but when I tried to write the code for the pressure transmitters it is displaying NAN for all values on the front panel. So the VI named K_Project_VI_Thermocouples below is working fine, the other VI is the one that is displaying NAN for all values on the front panel. I would appreciate any help!

[JParedes]

This is the e-mail I sent a NI employee asking for help:

"Thanks for your help. Our project consists of modifying an air conditioning system's evaporator coil in order to make the system extract more humidity from the air. Because the A/C would provide more humidity extraction the consumer would then be able to set their thermostats at higher temperature settings and still feel comfortable. This would save people electricity because the A/C would run for a shorter amount of time. In theory.

We will be using thermocouples to measure air temperatures and 4 pressure transmitters to measure R-22 refrigerant pressures at the compressors. This is where the NI equipment and LabVIEW come in to play. We have two DAQs which we will use to acquire the signals from these sensors. The equipment we possess and plan to use is:

• National Instruments cDAQ-9172 LegacyNI CompactDAQ Chassis
  • National Instruments 9203 8-Ch ±20mA, 200 kS/s, 16-Bit Analog Current Input Module
  • National Instruments 9263 4-Channel, 100kS/s, 16-bit, ±10 V, Analog Output Module
    
• NI USB-6251 Screw Term
  • NI SCC-68

The reason we want to use two DAQs is because the NI SCC-68 has thermocouple inputs so it is best suited for those sensors. The pressure transmitters output current which would be best measured using the cDAQ-9171 chassis with the 9203 current input module. The 9263 analog voltage output module is required to power the pressure transmitters since they require a voltage input to operate.

LabVIEW is being used to write the VI which will acquire (and create) all of these raw signals and turn them into useful data such as temperatures and pressures. It will also display graphically this data on a front panel. It will also output all data to a text file/spreadsheet. We have already created a working VI which acquires the temperatures from the thermocouples. This VI is working well with no problems. It is attached to this e-mail.

The problem we are facing is that when we tried to incorporate the stuff to acquire pressure data from the transmitters it is not working. LabVIEW is not showing an error which causes the program to not run. Specifically, the problem we are facing is that all values on the front panel display NAN. The non-working VI which incorporates the pressure stuff is also attached to this e-mail. We believe there might be an issue with LabVIEW trying to use two different DAQs simultaneously."

[JParedes]

Also, just in case you were wondering I am using these pressure transmitters (0-300 psi range). The two wire cable connection type.

http://www.dwyer-inst.com/Products/Product.cfm?Group_ID=20134&Product_ID=1577&sPageName=Specs

[Dennis_Knutson]

Try it with a separate read task for each device.

[JParedes]

Thanks for the tip Dennis. I did make seperate read tasks for the thermocouples and pressure transmitters. It seems to be working now EXCEPT that the pressure transmitters always produce the same current (smaller than 4mA which corresponds to 0psi). I think I either have the transmitters wired incorrectly or I am using the wrong module to apply voltage to the pressure transmitters.

I am currently using the NI 9263 analog voltage output module. I also own a 9472 digital voltage output module. Do you think that this would be better suited for the transmitters since the transmitter spec sheet says it requires a VDC power supply?

I have attached the most up to date VI.

Thanks for your help guys!

[Mr.F · Accepted solution]

Can you probe the output lines while they are running your code? They should read 10 V, but it’s possible that you are trying to drawing too much current and that voltage level is dropping. If they read 10 V, then your external connections/wiring is most likely to blame.

[JParedes]

Matt,

Thanks for your reply. You were right on target. I was underpowering the transmitters because the current input module's inherent resistant was too large. I checked the voltage output leads on the analog voltage out modules and sure enough they were only reading about 2.5 volts. So I decided to not use the voltage out modules and instead I am powering the pressure transmitters with a seperate variable DC Power supply which can supply up to 28 V. Sure enough the pressure transmitters started outputting 4mA (current output for 0psi). Thanks for the help!