Reading RTD w/ Micro800 System

jordanat2012

Member
Join Date
Aug 2018
Location
Illinois
Posts
22
Hi all,
I'm looking at starting a project that would require me to use an RTD to monitor temperature in an enclosed environment. I'd be using an Allen-Bradley Micro800 system (likely Micro820) to monitor the RTD. From what I've read on the topic, it looks like I'd need to use plug-in module 2080-RTD2.


Wondering if anyone has any experience with the Micro800 series and monitoring an RTD? Any type of special considerations I need to be aware of?


Thanks!
 
Thanks Firejo!


Have you used the 2080SC-IF4U before? Is it really as simple as reading an analog input?


I've pondered the Spectrum Controls units before, but never used one. How do you set them up for use in CCW? For the AB branded plug-in modules, they're shown as options on the setup menu. I assume there's extra steps for the SC modules to be loaded into CCW?
 
The 2085's are easier to use than the 2080's. The 2080SC-IF4U requires that you use the generic module configuration setup but the 2085SC-IF8U has a file to import into CCW making it appear in the module list. You can download manuals for both from the product page (linked earlier) and it covers how to install them. Once they are installed its just a matter of reading a tag.
 
So if I'm looking at the user manual for the Micro820 correctly (AB document 2080-UM005, specifically the I/O configurations outlined on page 3), it looks to me that there are 4 analog inputs onboard the 820. Theoretically, there should be no reason these analog inputs wouldn't be able to read an RTD?
 
Yes it should read it but the accuracy won't be as good (potentially not even close) and the math along with the calibration will need to be done in the code. Modules designed to take thermocouplers and RTD's have a much higher level of precision which is needed when you are looking at a signal in the 1 millivolt range and they (typically) do the math and calibration for you.
 
There are lots of theoretical and practical reasons why a voltage input does not measure temperature with an RTD

page 13 (pdf) of this document
https://literature.rockwellautomation.com/idc/groups/literature/documents/um/2080-um005_-en-e.pdf
says, "Analog In 0-10V"

An RTD is a passive resistor that changes resistance with temperature. An RTD does not generate a voltage (like a thermocouple does), so it is not compatible with a voltage input.

As background, all electrical measurements measure voltage. To measure a variable resistance, which is what an RTD is, a constant current is pumped through the resistance and voltage drop across the total resistance in the circuit is measured. Typically the constant current is on the order of 0.001A (1mA). The voltage drop through a 100 ohm Pt100 at the temperature of the freezing point of water is 0.1000V, with resolution needed in the uV range. And, if the accuracy of an RTD is required, then there has to be a circuit that also compensates for lead wire resistance.

A basic 0-10Vdc analog input does not generate a constant current nor does it compensate for lead wire resistance, so it is not suitable for measuring temperature using an RTD.

If you were to get a temperture transmitter (and a DC power supply) that can take an RTD input, then dropping the transmitter's mA current across a 500 ohm precision resistor would generate 2.0-10.0Vdc and use 80% of the analog input's span
 
Thanks for all of the help everyone! I've been tinkering with an RTD and PLC that I have laying around the house, but seem to be running into some odd behaviors. Hopefully someone here can help me out? I know this isn't directly a PLC question, but I thought I'd ask nonetheless.


I've got an Omega RTDM12 (data sheet attached) that I'm using. This is a 4-wire configuration that uses an M12 connector with lead (I purchased one of these from McMaster-Carr). I've wired the RTD following the schematic shown in the second attachment, which I believe I took from the National Instruments website.


When I wire everything and apply 10vdc, my multimeter shows the resistance spike to OL and shows the full voltage passing through the RTD. The RTD also heats up to the touch.


What might I be doing wrong here?

4-Wire RTD Wiring.JPG
 

Attachments

  • RTDM12 Spec Sheet.pdf
    217.3 KB · Views: 16
Yeah, I’ve looked at that one quite a bit. And when I’m ready to implement with PLC, that manual will definitely be useful.

But I’m struggling with it at an even more rudimentary level right now. I’m only looking at confirming the principles of the 4-wire RTD when using a multimeter. I know this forum is more for PLC questions, but I was hoping someone could help me with some basic principles.
 
A 4 wire RTD has two 'common' wires of one color on one side of the resistor element and two common wires of a different color on the other side of the resistor. The resistor element changes resistance with temperature (positive temperature coefficient - as the temperature increases, the resistance increases)

You can read the resistance of the resistor element with an ohm meter across any of the two different colored wires. The ohmmeter will always have an internal battery which sources a fixed, known current through the resistor, and the meter reads the voltage drop (Ohms Law I*R = E). A DVM's read out is converted from voltage drop to resistance, an analog ohmmeter's scale is printed in ohms taking into account the voltage drop-to-resistance conversion.

Conversion to temperature units will have an offset error due to the lead wire resistance which in the 2 wire ohmmeter measurement is added to the resistor resistance.

A 3 or 4 wire RTD analog input compensates for the lead wire resistance.

Most industrial grade analog inputs will do 3 wire RTD lead wire compensation, but many will not do true 4 wire lead wire compensation, using the assumption that all the lead wires the same gauge and length.
 

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