TROUBLE measure 4-20 ma tbx and SCB-68A

[spouk]

Hello,

i'm begginer in NI and labsview. I try to read 4 20 ma on SCB-68a and TBX but i have some trouble. I

can measure the 4-20 ma in my Data logger and it's works very well. But when i want use NI object i can't have a good measure with same cable and resistor shunt.

I have a detector of hydrogene with 4-20 ma output to measure 0 to 5000 ppm .  I use different resistor shunt 50 ohm to 200 ohm. 

But when i go in NI with labsview this doesn't works properly. I'm in mode DIFF on AI/0 the 4 - 20 ma. The measure isn't stable and i have the same trouble when i use TBX or SCB with 2 different pc (6353).  When i have 8 ppm in my detector i measure 3,75 ma to 4,3 ma or in my data logger i have 8 ppm after scale the data. So in NIMAX  when i scale is 0 to 200 ppm.

i link Ai - to AI GND and the shunt resistor is beetwen AI + and AI -.

What i make wrong to have a good signal ?  the pcie is 6353

[santo_13]

To read a 4-20mA signal you need,

1. A power source in the 4-20mA loop or separate power supply for the sensor
2. A load resistance to convert the current signal into voltage
3. A DAQ to measure the voltage signal

In the case of 6353, you're missing #2 and depending on your sensor you're missing #1 too.

[spouk]

I dont need #1  and i use already a load résistance of 50ohm +/- 0.1 ohms or 4 of this résistance.

I Have put this output in à data logger aoip and i read correcly this 4-20 ma with the résistance load beetween + and -

So i dont know what i miss you think to ohter think ?

[spouk]

I have already a source in my detector for the #1.

[santo_13]

I overlooked that you already had a shunt resistor.

Please confirm if you're connecting as per the below diagram,

Please share the configuration you used in DAQmx to check the functionality.

[spouk]

I have try different too a cable beetwen 64 (AI gnd) and 37 (AI -), without.  

You can see in my last photo the signal is noise, 5 mA is good because i have 200 ppm.When i have 0 ppm i'm at 4 mA with noise too.

My sensor works good because when i put this  in a data logger calibrate, i have a good signal with the same resistor load and instrument cable.

I don't know what i miss because i have try all configuration in other AI and other card (TBX) it's the same.