I am trying to write some code to read a WaterLog H-377 Temperature Probe with a CR800, but am having trouble getting it correct.
Here is my code:
'Make a halfbridge measurement of the mV
BrHalf (DH_377,1,mV2500,1,Vx1,1,2500,False,0,250,1,0)
'Resistence of reference resistor (using 2500mV excitation)
Rt=20000*((2.5/DH_377)-1)
'Now convert the resistence to the temperature in Deg Kelvin
Wat_Temp=1/(0.0010291636+(0.0002391251*(LN(Rt)))(0.0000001566*(LN(Rt))^3))
'Convert to Degrees C
Wat_Temp=Wat_Temp-273.15
Where DH_377 is the voltage reading (in volts) of the H-377
The H-377 calls for 5V excitation but the limit on a CR800 is 2.5V or 2500mV. I am not sure if that is causing a problem with the coefficients within the Steinhart and Hart equation or what, but I am not getting the correct temperature. Any help would be greatly appreciated.
A couple of things:
First-
Your S&H equation needs another "+" sign between
1/(0.0010291636+(0.0002391251*(LN(Rt))) and (0.0000001566*(LN(Rt))^3)).
When you insert the "+" the equation results in about 25 degrees C for a Rt of 10000 ohms.
Second-
I realize "Rt=20000*((2.5/DH_377)-1)" is pretty much the equation they give in the H-377 manual to determine resistivity of the sensor but instead try:
Rt=20000*(DH_377/(1-DH_377))
The Half Bridge instruction returns a ratio of voltage received to voltage sent.
Hope this helps, good luck.