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AM16/32 Multiplexer with thermocouples and other sensors ?.


jackyjoy123 Jan 4, 2021 12:21 AM

Hello,

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

jackyjoy


JDavis Jan 4, 2021 05:28 PM

Thermocouple wire between the multiplexer and the datalogger will create an offset voltage to compensate for an offset that happens where the thermocouple sensors connect to the multiplexer. If temperature is the same on the input and COM terminals on the multiplexer are equal, offsets are equal and they cancel each other out.

Other sensors would not have the thermoelectric effect where they connect to the multiplexer. So they end up with an offset added to there measurement(and not cancelled) due to the thermocouple wire between the multiplexer and datalogger. The offset would appear in the measurements.

To not need thermocouple wire between the multiplexer and datalogger, you would need the temperature of the multiplexer to use as a reference. Common practice is to insert a model 107 temperature probe under the foam cover for the multiplexer.

Recommendation would be to just add a second multiplexer.

PS. Be aware that the 4-20mA sensors will create an offset on the analog ground. The offset would be apparent in all single ended measurements. Doing all your other measurements as differential is recommended.

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