Temperature sensing for a heap leach

Hi All, A pilot plant is being constructed in Mongolia, which has heap leach (insulated). The intent is to monitor the heap temperature (~ 16 points)
and record the temperature (manual operation). The Client does not wish to install 16 Temperature gauges (but rather 16 temperature sensors and one hand held monitor).
I am thinking of installing 16 RTDs and wiring them back to a control room using RTD extension cables. Using a hand held RTD monitor the operator can measure and record all 16 temperatures. Although this sounds quite simple, I have the following concerns:
1. The outside temperature can fall to -30 deg C during winter. Are RTD extension cables capable of withstanding these lower temperatures?
2. Will the hand held monitor work properly during winter (compensating for the ambient outside temperature, whilst the measured temperature will be between 30 & 55 deg C)?
3. Is there a better way of monitoring these temperatures? As usual, being a pilot plant, the Client is frugal with the dollars and cents.
All pointers and suggestions are appreciated.
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On Sun, 26 Feb 2006 13:48:34 GMT, the renowned "Raj Sreenevasan"

Yes, of course. Specify wire that has insulation that won't crack at low temperatures, but -30C is really not particularly low.

With the information you've give, I can only say 'maybe'. Specify an adequate wire gauge for the length of runs from an accuracy perspective, especially if it's a 3-wire system. Four-wire sensors are much less sensitive to mismatch or changes in wire resistance, but are rare in industry, IME.

It sounds like a reasonable approach to me. They can always put 16 indicators or a recorder or a data aquisition system on the same sensors later, provided you are sensible enough to stick with industry-standard sensors.

Best regards, Spehro Pefhany
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Spehro Pefhany wrote:

...

...
Properly configured, a three-wire sensor can give accurate measurements over long runs even with fine wires. In the four-wire Kelvin arrangement, excitation current flows in two wires, and two sense wires sample the sensor drop without influence from any drops in the excitation leads. With a three-wire arrangement, there are still two excitation leads, but one of them has no parallel sense lead. If both excitation leads have the same resistance, however large, accurate readings can be made. Most RTD readers have appropriate circuitry to do that.
I won't go into the circuit, but the basic idea is simple. Current is driven through the excitation leads. Both the sum of the drops across the sensor and one excitation lead, and the drop across the other excitation lead are measured through the sense wire. Since the drops across both excitation leads are the same, the sensor voltage can be isolated from those measurements.
Jerry
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On Sun, 26 Feb 2006 11:09:31 -0500, the renowned Jerry Avins

The circuitry can ONLY do first order compensation in a 3-wire system-- mismatches in the lead resistance caused by small differences in length, resistance per foot, or tempco (which is affected by annealing) will all show up as errors in the reading. He's got an unknown run length, relatively large variations in leadwire temperature and the readings he wants to get are all in a narrow range. Caution is called for.
Best regards, Spehro Pefhany
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Spehro Pefhany wrote:

Of course four-lead RTDs are inherently more accurate, but often we need to settle for three leads, especially when RTDs are built into equipment we buy. (I have in mind a 250 HP motor.) When I connect to three-wire devices, I try to be sure that all the leads are from the same roll of wire and make up my own cable. I check long runs and use the best-matched pair of three for excitation. Can one do better than that?
Jerry
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