I have a 4-20 mA signal coming in for the SCXI-1102 which reads incorrect

[ed200]

Since yesterday the VI logger displays a different value then the actual value in mA.
I use a TBX-96 connected to a SCXI-1102 with a 249 Ohm resistor. The measured value from the device is 6.7mA while the VI logger signal tester shows 6.3mA. What went wrong?

[e-Cash-Money]

Hello ed200,

It sounds like the Vi logger used to display values that matched up with the measured value from the device. What changes were made in the system right before VI Logger started displaying different values?

You can try configuring the 1102 to have a range that include 0 mA and then short the inputs and see if you can measure a stable 0 mA reading.

Which DAQ board are you using?

Thanks,
E.Lee

[ed200]

I HAVE FOUND THE CAUSE OF THE ERROR. THE COMPUTER WITH THE SCXI MODULE WAS CONNECTED TO A DIFFERENT POWER OUTLET THAN THE INSTRUMENT THAT GIVES THE 4-20mA SIGNAL. BECAUSE OF THE DIFFERENT GROUND THE COMPUTER IS READING WRONG VALUES. I MEASURED A SMALL CURRENT DIFFERENCE BETWEEN THE TWO GROUND WIRES. I RECONNECT THE COMPUTER TO THE SAME POWER OUTLET AS THE INSTRUMENT AND EVERYTHING WORKED FINE AGAIN.

[nlin]

I feel I have the similiar issue like you(mid-voltage measurement on SCXI-1104C) but still have a few questions about your solution before I will try it on my case:

Before you change the power outlet
1. Does the incorrect reading happend on specific channel or each one?

After you change the power outlet

1. Have you checked all channel readings correctly?

Any idea about the value of error related to the difference of two power oulets? How to exlain or calculate it?

[ed200]

To answer your question #1

All channels for reading voltages were affected, the other module with the channels for the RTD's worked fine.

Question #2
I re-checked all channels and they displaying the correct values now.

When the power outlets were different: It seems the value of error increases as the equipment is running over a period of time. When I just start up the equipment the readings are correct, but over time the error increases. It probably takes some time to build a ground voltage difference between the two outlets.

regards, ed

[nlin]

Hi, ed,

1. Can you confirm what measurement (current or voltage) do you use for analyzing the different power outlet?

2. Based on Answer #1, can you tell more details about how to do such measurement? For example, when you measure the ground of two power outlets, any power cord plugged in the outlet? Any process I should know in order to solve this issue.

Thank you.

Nelson