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Voltage Difference between ordinary Voltmeter and Agilent 34420 (Ground problem perhaps)

drehman

Hello Engineers. image

 

We are working with pressure  and temperature measurements for a flow through a pipe. All the data acquisition system is regulated using Labview.

 

1 In our laboratory we are using Agilent  34970A34970A as a Switch Unit(Scanner with sensors attached to  34902A34902A 16 channel multiplexer and Agilent  34420A34420A as DIMM

 

2- Switch Unit and DIMM are connected with each other using HP10833B cable. A GPIB-USB-HS connects them both to comuter via USB port

 

3 High and Low of Common from multiplexer board are connected at Channel-1 of the Agilent  34420A34420A

 

4- Probelm: For a specific instrument Voltage measured and sent to Labview by Agilent  34420A34420A is higher than real value if measured by an ordinary voltmeter at the output pins of sensor However the moment when Common terminals coming from Agilent 34970A(switch board connected at Channel 1 of  34420A34420A are joined together by a big resistance e.g placing fingers on the connections voltage measured and sent by Agilent  34420A34420A becomes equal to what an ordinary voltmeter shows And with previous experiments conducted we know that this voltage is correct for given experimental conditions

 

5- I am assuming there might be a problem of ground with computer and instruments, however I do not know how to proceed to rectify this issue.

 

6- Any help in this regards will be highly appreciated.

  • Hi Danish,

    Your ordinary voltmeter is likely to have a lower input resistance than the Agilent. This is specified as Ohms per Volt. When you put a high value resistance across the sensor you are adding a resistance that would mimic the lower resistance of the ordinary meter. Check the specifications of the two meters to see how their input resistances compare. The highest input resistance should give you the most accurate voltage reading.

    John

  • in reply to jw0752

    Hello Mr. Wiltrout,

     

    Much thanks for your prompt reply and it is highly appreciated.

     

    The problem turned out to be of using wrong strategy of data acquisition in Labview. And now agilent34420A is measuring and sending same voltage as the other ordinary voltmeter.

     

    Regards.