Pt100 configuration issues (USB-6008)

[Fatsat]

Hello, 

Im using the USB-6008 DAQ device (I know it isn't precise and all) and I use the Pt100 (QAP2010, Click for datasheet).

I've connected it like this.

Now in LabVIEW, these are the only options (New DAQmx task -> acquire signals -> Temperature -> RTD).

If i choose for Custom settings, I get these options (I don't know which variables those are).

I'm using a little greenhouse where i need to measure temperature and humidity and control the environment (by using a ventilator to cool the greenhouse and heat cartridges to heat it).

My goal is to read the temperature using a Chart on the front panel.

Can someone help me how to configure/choose the right options? If any more information needed i'll provide them asap!

[RavensFan]

Check the datasheet that comes with the RTD.  Those values define the temperature vs. voltage curve of the RTD.

[Fatsat]

There are no such values in the datasheet

[RavensFan]

They should have put them in there.  They are actually buried into the charts of resistance vs. temperature.

I'm not sure your wiring diagram is correct.

If you want to sue the USB-6008, then I'd recommend finding a good RTD signal conditioner to convert the measurement from the RTD (which is usually a part of a Wheatstone bridge circuit) to a voltage the the USB can read.  Like this http://www.omega.com/pptst/DRF-RTD.html

[johnsold · Accepted solution]

A quick search for RTD Class B shows the accuracy and the coefficient. As Ravens Fan pointed out the resistance versus temperature is plotted in the data sheet.  For greenhouse temperature controi a simple calculation of the slope from that graph is more than good enough.

Note that the USB-6008 has limited current drive on the analog outputs and that it controls only voltage. So you will need at least an external resistor and possibly and external power source to excite the RTD.  Those options you linked may not be applicable to the USB-6008, which is a very simple device. You may need to just measure the voltage and calulate first the resistance and then the temperature.

Lynn

[CarlosOtiniano]

http://www.ni.com/white-paper/7130/es/ an PT100 has an internal resistance which varies with temperature, the link tells you how to accomplish one with Wheatstone bridge configuration, the task is similar to a gauge 

[dsgh]

Dear Carlos, 

Do you know if this white paper is still available? I am trying to do something similar, but I am unable to load the file in your link.

Many thanks,

Dan

[arteitle]

The USB-6008 doesn't support reading RTDs directly, as you obviously know given how you wired it up with an external power supply and resistor. Per your datasheet, a Pt100 has a resistance of about 110 Ω at 25 °C, so your analog input will see about 2.96 V. The RTD resistance increases by about 0.4 Ω per 1 °C rise, so a temperature change of 1 °C will result in a voltage change of about 4.4 mV. You'll be using the +/- 4 V input range on the USB-6008, and its 12-bit resolution (when using a differential input) gives you a voltage resolution of 8 V / 4096 = about 2.0 mV, or a temperature resolution of about 0.45 °C. Or in other words, you'll be able to measure in half-degree increments if you use a differential input. Your diagram shows a single-ended input (since one end is connected to GND) which halves the resolution. I'd suggest you work out the equation for calculating temperature from voltage based on your circuit, and use that in your program to convert the analog input measurement back to temperature.