Hello All,
Does anyone know how to incorporate an Omega 44008 Thermistor into the CR 1000 datalogger? I'm new to the CR1000 as well as SCWin, so I'm also not sure how to incorporate this new sensor into the sensor library. Any help would be great. The Steinhart-Hart Constants are as follows:
@R25C = 30000:
A= 9.376E-4
B= 2.208E-4
C= 1.276E-7
If any other information is needed to determine this interface, please let me know. Thanks in advance!
This is not as straight forward as it seems.
I don't think you can simply add sensors to the sensor list in SCWin, but you can use the "Generic Measurements" section instead.
To measure the thermistor correctly, you have to first connect a precision resistor between the the measured port (SE) and the ground. Try SE2 as it is closer to the ground than SE1.
A 1k resistor should be okay to use.
The other leg of the thermistor should be connected to one of the VX channels.
The measurement to use is a "half bridge"
The idea is to compare the voltage drop from the precision resistor to that of the thermistor and calculate the thermistor resistance from the results.
You will then have to calculate the temperature using a polynomial with your supplied coefficients. "Calculations and Control", "Calculations" "User entered".
Cheers
Grant
I believe Grant's wiring would work with a single-ended measurement with excitation. But for a half-bridge measurement the fixed resistor should be placed between the excitation channel and the single-ended measurement port and the other end of the thermistor run to analog ground.
See page 66 of the current CR1000 manual:
http://www.campbellsci.com/documents/manuals/cr1000.pdf
Page 66 also shows how to calculate the resistance of the sensor from the measurement. Which can then be plugged into your S&H equation.
It does not matter too much whether the thermistor is at the top or bottom of the bridge as long as you use the right version of the equation to solve for the unknown resistor (the thermistor). The only difference the position of the resistor can make is in the required settling time. This is discussed in length in some of our logger manuals in relation to measuring high impedance sensors, the take home rule generally being you should try to keep the source impedance at the logger input low and constant if you want a fast settling time. If you are unsure just use a longer settling time.
If you are going to use the 44008 which has a resistance of 30k at 25 C, I believe, I would use a 30k precision resistor. As a rule of thumb I normally recommend to use a resistor that equals the resistance at the mid-temperature of the range you are interested. Then measure the output of the sensor on a range equal to the excitation voltage, e.g. 2500 mV.
BTW make sure you code in the Steinhart-Hart equation for those parameters - this is not a polynomial.