PT100 Multiplexers

Q

Thread Starter

Quasar

I am looking for a PT100 Mux with atleast 30 inputs. Does anyone know of any such product. Is the following solution workable? a) Digital Outputs to drive relays. b) One Analog Input channel c) PT100s connected to the analog input channel through relays. d) Switching of the PT 100 is performed by the DO signal. Is this a workable solution?
 
H

Hakan Ozevin

Yes, it works. We applied it to a customer and it works for years without any problem. Since temperature change in nearly all processes is relatively slow, you can multiplex Pt100's, either for alarm or control purposes. Consider that you cannot decrease the multiplexing interval as you wish, otherwise contacts will wear out too early. Thus you have to choose relays with high quality contacts that will not change their resistances in time and select the highest possible multiplexing interval. If possible, use a four-wire connection, feed the Pt100's with an external current source and multiplex only the voltage read from the Pt100's. But 2-wire and 3-wire connections will also work with less stability. The most important point that you must consider in the software is the delay (say 10-30 msecs) when the contacts are closing and opening. You can also check www.ziehl.de that has a standard product for this purpose. By the way, I don't think that this is an acceptable solution for thermocouple case. ++++++++++++ Hakan Ozevin
 
R
That'll work -- it's basically how we do it. You have to excite the PT100's too though, since they're resistance devices. Plus if they're going to be any distance from the measuring device you'll want to perform a differential measurement -- one relay contact for the positive end and another for the negative one. Rick Daniel Intelligent Instrumentation
 
A

Alastair Fordyce

Have a look at the 2100-M-R analog multiplexer modules from Intech Instruments www.intech.co.nz We have used these units and their predecessors very successfully for many years. Best regards, Alastair Fordyce REA, NZCE,CQA Systems Engineer Bremca Industries Limited www.bremca.co.nz PO Box 7169 Christchurch NEW ZEALAND tel +64 (3) 332 6370 fax +64 (3) 332 6377 e-mail [email protected]
 
R
> By the way, I don't think that this is an acceptable solution for > thermocouple case. Actually this technique works just fine for thermocouples if you take some care -- we've been doing it for 17 years. We normally use solid state multiplexers rather than relays, but the principle is the same. You just need to make sure that your connection points are all at the same temperature to avoid cold junction errors. If the relays you use have significant thermal offset properties, that can induce errors if the two relay contacts are at different temperatures. Otherwise, no problem! Rick Daniel Intelligent Instrumentation
 
H

Hakan Ozevin

I want to know if the voltage drop on the triac or SCR cause any problems. Or are you using a special SSR? Best regards, Hakan Ozevin
 
Generally, what is the maximum distance that we can extend an RTD cable? If extending an RTD cable generates a constant offset due to the additional resistance, isn't it possible to introduce a constant compensation for it in the software? As I see, the only limitation might be the voltage induced for resistance measurement.
 
H

Hakan Ozevin

Yes, you can compensate the voltage drop on the cable by software. However, in any case I would not exceed 200 m and always use shielded cable, even it is 1 m. Hakan Ozevin
 
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