Can RTD's that output 0-10v each be put in series

Can RTD's that output 0-10v each be put in series such that you can put them in a loop and use a voltage transformer to bring it back to the 0-10V range, which would give me an average temp reading of the area... Yes, No, Maybe???Any solutions asap PLZZZ.

Not practical. RTD's don't put out 10 volts. They are resistors. The modules would output 10 volts. Supplies for both modules would need to be isolated (at least one of them) so the voltages could add. Then you would use a voltage divider with resistors - transformers are only for AC.

You could put both RTD's in series prior to the module, then compensate in software for the difference.

The proper way to do it is with 2 Modules, and 2 inputs on the Analog card. Average the signals in the Ladder program.

http://www.rtdcompany.com/items/index.cfm?CAT_ID=30 and this might help you too!

mailman166 said:

Can RTD's that output 0-10v each be put in series such that you can put them in a loop and use a voltage transformer to bring it back to the 0-10V range, which would give me an average temp reading of the area... Yes, No, Maybe???Any solutions asap PLZZZ.

Click to expand...

I beleive he refers to a RTD transmitter having a signal of 0-10 Vdc.

In this case the answer is "NO you cannot".

The transmnitter is a source of "something". When you put such source in serie with other source, they all still want to supply there own "something".

Darwin tells it like it is, survival of the fittest.

On of them source will die and your serial loop will be no more...

For instance in this case, to have a measure signal or 0 to 10 volts, the transmitter acts as asource of current. For a signal of 10 volts, it will raise its current output until it measure 10 Volts. Since this current is linked serially with the other transmitters which are trying hard to do the same thing with there own values... Houston we have a problem!

Correct - I assumed he was talking transmitters.

The theory is simple. Current isn't really a factor. If you have two 9 volt batteries and clip them together, you have 18 volts. That's possible because they are isolated. The two batteries will add. They won't fight each other (unless one of them is half dead, but that's another matter).
If the negative of both batteries is grounded, then connecting them together will short out the first battery.

I assume the minus side of the transmitter's output is referenced to ground. If they are totally isolated, then they could add. I don't know exactly what brand of transmitter we're considering, so I can't say positively that it would work. They certainly aren't designed for series connection, and you would have to build a resistor network in a box to average the outputs back down to 0-10. This is crazy.
I'll agree with your reply that you can't do that.

Judging by randylud's post, averaging RTD's prior to the transmitter appears common. That's much safer and cheaper than trying to average transmitter outputs.
If you insist on a transmitter on each RTD, then give them each a dedicated input on the analog card, and do the math in the PLC.

randylud said:

http://www.rtdcompany.com/items/index.cfm?CAT_ID=30 and this might help you too!

Click to expand...

This one intrigued me, so I did a little checking.

First of all, you can't put the RTD's in series because the resistances would add and you'get a high temperature reading. You can't put them in parallel because the result would be a low total resistance and a low temperature reading.

You can't start putting 0-10VDC transmitter outputs in series. At best they would add and give you bogus results. At worst you'd have a short and cook some transmitters. If you put them in parallel I have no idea what you would get, but I doubt if it would be an average. Again, you would likely let some smoke out.

Now, about those averaging RTDs. I contacted the manufacturer with some dumb questions, and they gave excellent response. Neat concept. It is actually a single wire wound RTD with the wire "stretched" out over several inches. It effectively averages the temperature over the length by averaging the increase in resistance. Very clever! However, it is not the same as averaging several different RTDs at several locations.

Good one, Tom. I was bit confused too. No schematics at the website, so how did they average them? They show a few RTD's and a handybox. Hmmm. Maybe they're making custom units that add up to 1000?

Now I know. One RTD - one transmitter - one analog input. Average in software. That's just how it's done - no deviations.

keithkyll said:

One RTD - one transmitter - one analog input. Average in software. That's just how it's done - no deviations.

Click to expand...

I don't think the averaging is done in software. The averaging is physical, like Tom says, over a linear distance, as the RTD wire element is either heated or loses heat to its environment. Minor resistance changes along its length affect its total resistance, like resistors in series.

It's like an "averaging pitot tube" that has tiny holes drilled in it so that it "averages" the flow across the pipe for a single DP reading.

Dan

No argument there. Tom is talking about a single RTD with a stretched element to create a single, "averaging" RTD. Randylud posted a link to it. I checked and was confused as to what they were trying to tell me. Tom called and got the info. It's new to me. I've never heard of an averaging RTD until now. I don't think I would trust it.

I was refering to the original poster's question using multiple RTD's. I believe multiple RTD's is the proper and accepted way to do it. With multiple RTD's, you would average them in software.

It's all about isolation

In the Electrical Lab when I was doing first year
electrical engineering there was a large panel. Several
outlet terminals had both + and - 110 Volts DC. Behind
the panel out of sight was a bank of large glass cased wet
cell batteries.
Well, some bright spark decided he wanted 220 Volts DC and
proceeded to wire two pairs of the 110 VDC terminals in series.
He certainly got his "bright spark".