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RA01 PT100 measurment issue with CR1000X


firmin Dec 6, 2018 08:33 AM

Hello,

We try to use a PT100 through a 4WPB100 bridge terminal to calibrate our RA01 Hukseflux radiation sensor.

Both PT100 and 4WPB100 resistances are ok.

For the wiring:

PT100 + (red)> L7 ( 4WPB100)

PT100 + (white)> H8

PT100 - (blue)> AG ( 4WPB100)

PT100 - (green)> L8

We use the folowing instruction:

BrHalf4W (RA01_TC,1,mV200,mV200,8,Vx1,1,4000,True ,True ,0,250,1,0)
PRT (RA01_TC,1,RA01_TC,1.0,0)

With a result of: RA01_TC = -241.8689 °C

We don't understand what is going wrong ?

There is not yet documentation for CR1000X datalogger on this topic and for example we have no idea witch excitation voltage is suitable for example.

Best Regards,

Firmin


aks Dec 6, 2018 02:51 PM

The manufacturer of the sensor should provide  the excitation current or maximum power that could be dissipated in the sensor without significant heating. Once we have this current you can calculate the excitation voltage using the total bridge resistance. 4 volts is too high of an excitation voltage for 200 ohm total resistance in the bridge. To be in safer limit I will use 1 volt.

It looks like your channel allocation for the measurement is wrong.

Please try the followinfg instruction (note the Diff Ch 7 instead of original 8).

BrHalf4W(RA01,1,mV1000,mV1000,7,VX1,1,1000,True,True,3000,15000,1.0,0)


aps Dec 7, 2018 03:10 AM

Most PT100s are calibrated at 1 mA excitation current.   The 4WPB100 actually includes a 10k resistor in series with the wire that goes off to the excitation (Vx) which acts as a current limiter.  This means that you can use the maximum excitation voltage possible in this case (4000 mV) and still be less than half the industry standard excitation.   Self heating is further reduced with Campbell loggers because the excitation is only switched on during the measurement which, unless you are doing measurements in a never ending loop, results in the sensor only being powered for small fraction of time, meaning the heat input is typically be a few percent of being continuously powered sensor.

The other issue to consider is that you need to try to set the excitation so the voltage measuremed across the PT100 is near full-scale of one of the logger input voltage ranges.   This is because, with our older loggers, the measurement resolution in terms of temperature may not be as high as required for half bridge measurements of these types of sensors.  With older loggers such as the CR1000 the excitation is often set lower than the maximum possible (typically 2100 mV) so the  25mV input range can be used yet the sensor still be used to measure the full range of ambient temperatures without the logger over-ranging.

With the CR1000X resolution is not a limiting factor (due to its 24 bit A/D convertor) .  This means you can set the excitation to the highest value and still be able to measure PT100 sensors to >600 C on the 200 mV range.

So you should be OK with your orginal excitation and input range but you need to change the channel as advised above and connect the Vx wire to Vx1 (which I a sure you have done).

Also use PRTCalc which is the more uptodate replacement for the PRT instruction.


JDavis Dec 7, 2018 09:25 AM

I recommend using different variables for the result of BRHalf4W and PRT. Then, you can see the unchanged result of BRHalf4W. That number should be close to 1.


firmin Dec 8, 2018 05:15 AM

Hello,

All is working now by changing the diff channel to 7.

I set the excitation voltage to 4000mV.

Thanks for your help.

Firmin

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