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[Vshu]

Hi Everyone,

                    I am trying to read 4-20ma signal frpm a Rockwin flowmeter using scxi1102b and scxi 1308 connector, when i read 4ma signal I am not getting a constant dc 4ma signal instead its fluctuating with a constant period as shown in image below, but when i read it using a multimeter there is no fluctuation and the reading is constant, how do I read a constant dc 4-20ma signal 

Thanks and Regards,

Vishwas

[Henrik_Volkers]

What you see is a classical 100Hz ripple most probably due to a  (classical diode bridge) AC-DC power supply that is interfering with your signal.

The reason why you don't see it with a multimeter is the build in low pass filter of the meter 🙂

Easy solution : capture 20ms of data and take the mean of it.
(for a 50Hz system.. if the step response of your sensor is slow anyway, take 300ms of data, that will fit for 50Hz and 60Hz systems)

If you want to learn more about your system, try to get rid of that ripple, by finding the source(s). Not so clean power sources and sinks, ground loops, magnetic and capacitive couplings..  wellcome in the world of EMC 😄

[Vshu]

Hi Henrick,

                       Thank you very much for your quick reply I am reading so many 4-20 signals using same card and connector but why is only that signal getting affected, is there any problem from device side, becoz i remember reading somewhere that in a signal current wont get affected by noise or interfaces that is why 4-20ma is so popular.

Thanks and Regards,

Vishwas

[Henrik_Volkers]

Swap the channel with a 'good' one to check if it is the source or the DAQ.

Check the power supply of the flowmeter.

How does the connection of the flowmeter looks like in detail? (Every wire !)

[Vshu]

Hi Hnrick,

               Sorry for late reply here is the detailed drawing of the flow meter,  

Thanks,

Vishwas

[Henrik_Volkers]

1. Usually I don't open docx from the internet and drawings could be posted as png or jpg... 😉

2. Shure there is no PE or ground connection?

Have you testet the input on a different input?

If you can't change the cabling you still can use a mean filter over (at least) one periode of the 230V line frequency

edit: I saw the flowmeter has a 230V line input.  How old is that flowmeter?  ( Old equipment sometimes have dried capacitors in the powersupply, the thing migth still work but the analog DC they use is out of spec, so the analog current output migth be affected) 

[Vshu]

Hi Henrik,

                    1. Its a brand new flow meter and the mean filter over 50hz period is working fine for me.

                         But i want to know why the SC module is not doing its job here. 

                    2. I dint get what you are trying to tell here  "Have you testet the input on a different input?" 

Thanks,

Vishwas,

[Henrik_Volkers]

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@Vshu wrote:

                    2. I dint get what you are trying to tell here  "Have you testet the input on a different input?" 

Thanks,

Vishwas,

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Sorry, typo ...

Have you tested the flowmeter output on another input channel?

and (if you want) you can mark my first post as solution.