reading 4 - 20mA pressure signal

[johnnybond]

Hi,

I am using a usb 6211 multifunction DAQ and trying to continuously read 4 - 20mA signals from a Pressure Transmitter.

My problem is that I do not get a steady signal out.

The signal jumps about and wildly varies almost looking like it reads out of phase but i thought that the 4 - 20mA signal would be constant?

Does anyone have any ideas?

Cheers

Johnny

[K C]

Hi Johnny,

Can you tell me how you connect the 4-20mA signal to your DAQ and which DAQ input do you use ?

Before I tell you things you already might know.  

[johnnybond]

Hi,

I am using ai2 putting the -ve of the transmitter to the +v terminal of the DAQ and the -ve of the 10v power supply to the negative of the channel. The +ve of the transmitter connected to the +ve of the transmitter.

Johnny

[K C]

What you need is a resistor in series with the 4-20mA loop. You measure the voltage across that resistor and you can calculate the current.

Use 250E for a voltage from 0-5V

Do you understand this ? If not I will make you a drawing. If I understand you correctly you have a passive transmitter

[johnnybond]

sorry i forgot I have a 100ohm resistor across the channel.

Is the problem that it is 100ohm??

[K C]

It depends what minimal resistor value your device accepts.

Can you draw the connecting or point it out in a manual of your transmitter. The connection you described is not clear to me.

Message Edited by K C on 12-06-2007 10:41 AM

[johnnybond]

Where can i find that?

Would it be in the manual or on a data sheet?

Johnny

[K C]

Yes, it must be in the datasheet of your transmitter. Can you tell me what transmitter you use ?

See also the edited part in the previous answer.

[johnnybond]

KC,

I have tried to attach files but the server keeps timing out.

I am using a druck 7517 transmitter.

Johnny

[K C]

OK, I think the resistor value is OK.

Are you sure you have the resistor, power supply and transmitter connected in series ?