Splitting RTD Input

rguimond

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I have two 4-wire RTD inputs leading to a digital indicating thermometer. I want to convert the RTD signal to a 4-20mA signal using a signal converter (http://www.automationdirect.com/adc...nt/Signal_Conditioners/Universal_Input/884114)

Can I simply bare the leads from the RTD and insert them into the correct points on the signal conditioner (connect in parallel), or do I need a signal conditioner that provides RTD and 4-20mA outputs?
 
No. Have you ever tried to read a single resistor with two different meters at the same time? Spoiler alert: It doesn't work very well. :D That is what you would be doing if you connected them in parallel.

You will have to read it with one device and then condition the outputs as needed.
 
I'm not sure exactly what you have or what you're trying to do.

It sounds like you have 2 RTD's and one digital indicating thermometer. What type of input does the digital indicating thermometer take? RTD? T/C? 4-20mA?

Are you trying to connect an RTD to the digital thermometer that takes a 4-20mA input?

What are you trying to do with the 2nd RTD signal?

The signal conditioner will convert ONE RTD to a 4-20mA temperature signal.

One can not parallel RTDs, as Mickey has pointed out.
 
danw said:
I'm not sure exactly what you have or what you're trying to do.

It sounds like you have 2 RTD's and one digital indicating thermometer. What type of input does the digital indicating thermometer take? RTD? T/C? 4-20mA?

Are you trying to connect an RTD to the digital thermometer that takes a 4-20mA input?

What are you trying to do with the 2nd RTD signal?

The signal conditioner will convert ONE RTD to a 4-20mA temperature signal.

One can not parallel RTDs, as Mickey has pointed out.
Click to expand...

I'm designing an automated cut-in / cut-out system for a Pasteurizer. My regulatory body, CFIA, is demanding that I use the signal feeding the indicating thermometer to feed the PLC that's going to record the temperature at which the cut-in and cut-out occurs. We're using and Anderson DART 741, which accepts a duel-element 4-wire RTD. I don't think it's available with a 4-20mA pass-through.

Hopefully, I'll be able to convince CFIA to allow me to connect the PLC analog input in series with the 4-20mA signal that supplies the STLR...
 
rguimond said:
I'm designing an automated cut-in / cut-out system for a Pasteurizer. My regulatory body, CFIA, is demanding that I use the signal feeding the indicating thermometer to feed the PLC that's going to record the temperature at which the cut-in and cut-out occurs. We're using and Anderson DART 741, and I don't think it's available with a 4-20mA pass-through.

Hopefully, I'll be able to convince CFIA to allow me to connect the PLC analog input in series with the 4-20mA signal that supplies the STLR...
Click to expand...

If you want the 4-20mA signal to the PLC, you must change the temperature digital indicator to 4-20mA input and booth (PLC + Indicator) must be in the same current loop. Why buy such an expensive converter?
 
nonuke said:
If you want the 4-20mA signal to the PLC, you must change the temperature digital indicator to 4-20mA input and booth (PLC + Indicator) must be in the same current loop. Why buy such an expensive converter?
Click to expand...

If the DART would accept 4-20mA, that's exactly what I would do. However, the DART will only accept RTD input. It actuslly accepts two RTD inputs.

As far as I know, Anderson's DART http://www.andinst.com/PDFs/3348.pdf thermometers are the only acceptable temperature display device for use with a PMO (Pasteurizied Milk Ordinance) system. I stand to be corrected.
 
Last edited:
rguimond said:
If the DART would accept 4-20mA, that's exactly what I would do. However, the DART will only accept RTD input. It actuslly accepts two RTD inputs.

As far as I know, Anderson's DART http://www.andinst.com/PDFs/3348.pdf thermometers are the only acceptable temperature display device for use with a PMO (Pasteurizied Milk Ordinance) system. I stand to be corrected.
Click to expand...

You can add to the 4-20mA current loop of the RTD, the analog PLC Input and a new converter that will convert the 4-20mA to RTD. On the RTD side of this converter, you can connect you indicator. Two converters LOL.
 
Best Solution:
IF you don't want to loose the accuracy of your probes, you should just add another RTD with a 4-20mA converter already in it, and connect this new probe to the PLC.

I looked at the PDF file, and you can buy the indicator unit with 4-20mA re-transmission to connect to the PLC (option 3).

3 w/ 4-20mA Retransmission
(only with code FD2 or FD3)
 
Last edited:
maybe,

http://www05.abb.com/global/scot/scot203.nsf/veritydisplay/a967ddc6b9735560c1257a56004a4693/$file/DS_C1950-EN_L.pdf
 
nonuke said:
Best Solution:
IF you don't want to loose the accuracy of your probes, you should just add another RTD with a 4-20mA converter already in it, and connect this new probe to the PLC.

I looked at the PDF file, and you can buy the indicator unit with 4-20mA re-transmission to connect to the PLC (option 3).

3 w/ 4-20mA Retransmission
(only with code FD2 or FD3)
Click to expand...


Yep, saw that, but it cancels out the PMO classification
 
The Dart shows a re transmission option... Thats how we have done it in the past with pasteurizers, both probes into compliance recorder (using ABB thingy in NZ..) and retransmit out to PLC with 4-20...
 
Lemming said:
The Dart shows a re transmission option... Thats how we have done it in the past with pasteurizers, both probes into compliance recorder (using ABB thingy in NZ..) and retransmit out to PLC with 4-20...
Click to expand...

I can certainly add the re-transmit board - PN 56001P0032. I'll have to confirm with CFIA that they're OK with it no longer being PMO certified.
 
Or use a relay out on the dart and digital input on the PLC... Should clarify, we have used the retransmit for loop control in the PLC, but the compliance shut off was a digital input from the ABB recorder..
 
No you are not allowed to change anything on this apparatus as it is used to comply with the PMO.
when using the retransmitter the dart is still PMO, the PLC is not but that is not needed.
 

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