RTD thermometer

[BrewingPadawan]

Who would be so kind as to tell me what the difference between these two units is and tell me anything about the difference in wiring of the two?

Your knowledge and input is appreciated.

Thanks everyone!

[rdcpro]

The one on the top is a 3 wire RTD. The one on the bottom, I believe, is a dual element RTD. That one is a little odd to me, because of the marker colors. Usually a dual element 6 wire RTD is really only two three wire RTDs. If I had to guess, the two elements are side-by-side, and the red wire is the "signal" wire and the green on either side is the common. They are connected together at the RTD itself.

If you google "difference between 3 wire and 4 wire RTD" you'll find lots of info about how lead length compensation works. If you have a good multimeter, you can figure out the wiring it's using. Measure the resistance of the two green wires on either side of the red one. They should read zero. Then you should see the 100 ohm platinum resistor from red to either green. You should see no connection at all between each of the two RTDs. That is, two green wires will be "shorted", the other two will be as well, but infinite resistance between the two groups.

For process measurement, you really only need a 3 wire RTD. 4 wire RTDs offer better compensation, but that's usually only done in the lab.

Regards,
Mike Sharp

[gitchegumee]

3-Wire vs. 4-Wire RTDs

Not a "dual element". Usually in a process setting like a brewery, 3-wire RTDs are considered "good enough". As stated above, 4-wire is for extreme precision and you don't need that. You can find lots of information about how to use 4-wire in a 3-wire circuit from your friends at Google. It's not difficult. Good luck.

[rdcpro]

Not a "dual element". Usually in a process setting like a brewery, 3-wire RTDs are considered "good enough". As stated above, 4-wire is for extreme precision and you don't need that. You can find lots of information about how to use 4-wire in a 3-wire circuit from your friends at Google. It's not difficult. Good luck.

At first glance I also thought it was a 4 wire RTD, but then I counted the wires...there are six. Four of one color, and two of another. If it's not a dual element, what is the wiring?

It can be verified with a multimeter, as I described above.

Regards,
Mike Sharp

[gitchegumee]

You are indeed correct.

I've missed the six wires--my mistake. Looked in the first picture like three wires with a jumper to field wiring. In this case, throw it away! A new 3-wire RTD isn't expensive. I've never toyed with a 6-wire, or "dual element" RTD. I wouldn't want to unless someone tells me that they are either less expensive or easier to wire. That isn't likely. We don't need extreme temperature control, and 6 wires have to be more difficult to connect, so stick with 3-wire. By far better than 2-wire, almost same price, and standard in almost all similar process controls. Or figure out how to use one circuit of the two? Dunno. Sorry for the mistake. Be happy for someone to tell me what this is and why/when it would be used.

[TGTimm]

Both of those are 3- wire RTDs--the lower one just has two RTDs in one probe/housing. You can easily use just one of them, but they might be at different depths within the probe, so find out if so, and use the one that's deepest.

[rdcpro]

If they came from the existing equipment in the background, the dual element RTD may be driving two different signals. E.g. a chart recorder and a process control signal. I've also seen dual RTDs for redundancy, but that's more likely in more critical process control like a pulp mill. But, if they came from existing equipment, the single and the dual may not be interchangable. Hard to know without more info from the original poster.

Regards,
Mike Sharp

[gitchegumee]

What?

Is that why someone would use a "dual element" RTD? A chart recorder or some other device? I've always used a single RTD and a controller to process the signal and drive other devices--like a recorder--from a 4-20 mA output. Or from a retransmit set of terminals on the controller. So I guess I've learned something today. Thanks!

[rdcpro]

Is that why someone would use a "dual element" RTD? A chart recorder or some other device? I've always used a single RTD and a controller to process the signal and drive other devices--like a recorder--from a 4-20 mA output. Or from a retransmit set of terminals on the controller. So I guess I've learned something today. Thanks!

I would probably do it like you suggest, use a single 4-20 mA signal, but I've seen them done this way before. My dad, who was a control systems engineer (and a metrologist) used to say, "if you really want to know what time it is, only look at one clock". But in this case, I suppose it has to do with how much accuracy you want. Converting an analog RTD signal to an analog 4-20 signal is going to introduce error, so you'd use 4-20 if you needed to take that signal some distance.

Given that breweries probably don't need extreme accuracy like you'd find in a standards lab, it's kind of surprising this type of RTD would be used. But again, I don't know the context of the OP's question. Are they RTDs he picked up somewhere, and wanted to use on his brewhouse? Or did they come off the brewhouse, and he needs to replace one?

Regards,
Mike Sharp

[BrewingPadawan]

Given that breweries probably don't need extreme accuracy like you'd find in a standards lab, it's kind of surprising this type of RTD would be used. But again, I don't know the context of the OP's question. Are they RTDs he picked up somewhere, and wanted to use on his brewhouse? Or did they come off the brewhouse, and he needs to replace one?

It was in a HLT with 2 electric heating elements. I think I'm just going to replace it with a 3 wire RTD and be done with it. After reading what everyone is saying, I don't think I need the potential added acuraccy and I have a couple extra 3 wire RTDs on hand anyway.