measure incorrect current with scxi-1102 and scxi-1308

[PansinoEngineer]

hi,
When i used scxi-1102 and terminal block scxi-1308, i couldn't get correct current.For examples,4mA can be measured as 3.55mA.But if i measure the current with DMM(common,not modular instrument),i can get correct result.
I tried another channels, the result was same.
Now ,I can make sure that SCXI-1308 and SCXI-1102 are good.
I also tried another way: Firstly, convert current to voltage with an known register,then measure the voltage with SCXI-1125,the result is correct.
If you know the answer,please tell me! Thanks.

[Andy F.]

Kevin,

I have a few questions for you: How are you checking the current measurement on the computer? What are you using for the 4 mA source? You should try to use the same outlet for the SCXI chassis as the computer to prevent referencing a different ground. Another thing to try would be to short the channel, configure the range to include 0 mA, and see if the offset still remains.

Regards,
Andy

[PansinoEngineer]

hi,
I measured the current with an common DMM.The source is a pressure transducer.
Now the outlet of the computer and SCXI chasis are the same.
Should the earth of source be the same as the SCXI chasis?
The experiment have many equipment,some equipment's signal is right.But one equipment's signal with the same type transducer was what I said before.
Thanks.

[Andy F.]

To answer your question, the same ground should be used for the measurements to prevent faulty readings. How are you measuring the signal in software? Is it in Measurement and Automation Explorer (MAX) or in a program? You mentioned that you have other transducers that you are using, and that they are reading correctly. Are they current readings as well? Are they using the SCXI 1102 and 1308? If so, it might be that the transducer is faulty. If not, try a known current source and see if you get the correct readings using test panels in MAX.

Regards,
Andy F.
Applications Engineering
National Instruments

[ShawnHu]

One of my customer has met the same problem.  I don't know how to explain that, either.

The program should be OK since in MAX, the reading is the same, 4mA to be read as 1mA.  With DMM, the reading is 4mA.  I am thinking of the termination.  Maybe the transducer cannot afford such a load, that is 249 ohms.  DMM uses different method to measure the current, so the result is not the same as using 1308.

As for the transducer, in my case, it is a 3 wires one, one VCC pin, one ground pin and one signal pin.  ch0 of 1308 is connected to the signal pin and the ground pin of the transducer.  A DC voltage source is connected to the VCC and the ground pin.  I have tried to connect the ground pin of the transducer to the ground pin of SCXI 1308. 

Another phenomenon is that the reading of the 1308 is greatly vulnerable to the environment change, say,  walking around the SCXI will affect the readings so much, say, 16.2mA to 16.8mA.  It is not noise since the reading is changing slowly without noise, according to the move of me.

[PansinoEngineer]

hi,

      You should assure the ground of the sensor and the ground of the measurement system is same. For example,the outlet of the power should have three lines.

[ShawnHu]

What I cannot understand is that the incorrect reading happens between times.  In some cases, it does not happen.  My customer tried 1303 with 1102 to measure the current on a resistor, the reading was smaller than that of DMM, too.

[PansinoEngineer]

hi,

   Try to make sure that the ground of the sensor and the ground of the measurement system is the same. That's key point! 

[ShawnHu]

Thanks.  I will try.

[ShawnHu]

Problem solved.  Thanks again.