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AM16/32 Multiplexer with thermocouples and other sensors


Uri Oct 19, 2017 09:08 AM

I'm seting up a station with the folowing sensors:

9 Thermocouples

4 CS500 T-RH sensores

4 Vaisala CO2 sensores (4-20mA output)

1 CS106 barometer

Since the CR1000 doesn't have enough analog channels for all the sensors I will use an AM16/32 multiplexer (I already have all the sensors, CR1000 and AM16/32).

Now I'm confused about the correct way to wire all the sensors and also the wiring between the CR1000 and the AM16/32. According to the AM16/32 manual (appendix c) a proper use of thermocouple will be to use a thermocouple wire to connect between the AM16/32 ODD H/ODD L and CR1000 H1/L1 (when datalogger reference junction is used), but in your tutorial video the presenter is using a simple wire.

Which is correct?

It is also wirtten in the manual "It is not recommended to make measurements of any other sensor type through the AM16/32B if thermocouples are measured with respect to the datalogger reference (the signal wires between the datalogger and AM16/32B are made of thermocouple wire). Two problems would arise due to the properties of thermocouple wire"

Can I use both thermocouple and other sensors with the AM16/32?

Thanks

Uri


JDavis Oct 19, 2017 09:16 AM

You can use regular wire between the AM16/32 and the CR1000, if you have a temperature measurement of the AM16/32 to use as the reference for thermocouples.


Uri Oct 19, 2017 10:00 AM

Thanks JDavis.

I don't have a temperature measurement on my AM16/32.

In this tutorial the AM16/32 is wired with regular wire and in the following programing tutorial the reference temperature is the CR1000 and not a temperature measurement of the AM16/32.

What do you recommend me to do?

thanks

Uri


JDavis Oct 19, 2017 01:52 PM

Any difference in temperature between the AM16/32 and the CR1000 would offset all of your thermocouple temperatures.


Uri Oct 19, 2017 03:02 PM

So I have only 2 options?

  1. Use regular wires for connection between the AM16/32 and the CR1000 and add a temperature measurement of the AM16/32 (connected to the CR1000). That way I can use both the thermocouples and other sensors on the AM16/32.
  2. Use Thermocouple wire to connect between the the CR1000 and AM16/32 but use only thermocouples on the AM16/32.

JDavis Oct 19, 2017 03:18 PM

Yes


Uri Oct 20, 2017 04:11 AM

Thanks JDavis for all the answers.


Cristian Nov 27, 2017 12:40 PM

Hello JDavis,

Do you know why a temperature reference is not needed using thermocouple wires betwenn the multiplexer and the controller and why we need it using regular wires?

If I keep the multiplexer and the controller(cr1000) in a enclouser, How much different could the temperature be between them? when is an AM25T worth of buying? 

Thanks,

Cristian

   


JDavis Nov 27, 2017 01:30 PM

The requirement of where the reference temperature should be measured is due to the thermoelectric effect. It it the nature of how thermocouples work. Dissimilar metals generate a differential voltage when in contact with each other. The terminals on the multiplexer and datalogger are all copper. 

When you wire into the AM16/32, and back out with thermocouple wire, the voltages going in and back out cancel each other. You need a reference temperature where it stops being thermocouple wire.

There are two reasons for using a AM25T; fast channel switching time, and a good reference temperature measurement. The channel switching time is only important if you need to do your measurements fast. The AM25T has an aluminum heat spreader with a reference temperature sensor bonded to it. It ensures you don't have gradient errors across your terminals.

Compare the accuracy of the AM25T reference temperature to the panel temperature on the CR1000. The difference might not be significant for your application.


Cristian Nov 27, 2017 02:55 PM

ok. Thanks JDavis!

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