Dear all
i need to use a RH and temperature captor from skye with a cr3000.
For the RH it's ok i use VoltSE instruction
but i cannot find the instruction method for recover the temperature
on the technical paper it's mentionned that's a 10kohm thermistor with an output of 0-1V on the range -40 +60°C. i have 2 wire signal and ground
i have try the instruction half bridge and add a resistor between VX1 and 8H but i only have the offset value
could you explain the method for recover the temperature?
while waiting to read you
regards
I believe this is just a bare thermistor probably connected between ground and the output pin. The 0-1V refers to the RH output.
If this is right (check with Skye to be sure) then you would need to form a half-bridge circuit using a precision resistor. 10kohms would be a good starting point. You then solve for the thermistor resistance following the instructions and examples for the half bridge measurement. Then if Skye or the internet can provide the exact parameters for the thermistor you can solve for temperature using the Steinhart-Hart equation.
Dear APS thak you for your answer
after verification,
the 0-1V refers to both the RH at the temperature
I try to do a half bride with a 10kohm resistor but i only have offset
do you think i need to add a multiplier and offset in the half bridge instruction?
I have find specification for the thermistor.
how can i solve the equation? i search but not sure!
regards
Please give me the manufacturers reference for the thermistor element or its resistance at three temperatures, say +60 and -20 deg C and I will check out to see if the have seen similar.
Dear APS
you can find specification at this address :
http://www.skyeinstruments.info/index_htm_files/rht+%20Relative%20Humidity%20and%20Air%20Temperature%20probes.pdf
many thanks
while waiting to read you
regards
If you connect a precision 10k resistor between a logger excitation (Vx1 in the example below) and the single ended input (SE1) and connect the thermisor output of the sensor between SE1 and an analogue ground, the instructions below, inserted in your program in the correct place should work. This is using the Steinhart-Hart coefficients used by the CR200 for our 109 probe which has the same thermistor, albeit in a different circuit configuration.
Sensor_resis should hold thermistor resistance and Sensor_temp the temperature in degrees C.
-----
Public Sensor_out, Sensor_resis, Sensor_temp
Const A = 1.129241E-3
Const B = 2.341077E-4
Const C = 8.775468E-8
....
BrHalf(Sensor_out,1,mV2500,1,Vx1,1, 2500,True,10000,_50Hz,1,0)
Sensor_resis=10000*(Sensor_out/(1-Sensor_out))
Sensor_temp=1/(A + B*LN(Sensor_resis) + C *(LN(Sensor_resis))^3) - 273.15
...
Dear APS thank you very much i can see the temperature changing.
this is a good point
but this is not the good parameters
The temperature releving are negative (approximatively -23°C) but i fact we need to have a temperature of 20°C
do you think it's normal?
do you give me a program for another temperature system (kelvin or farenheit?)
while waiting to read you
regards
As far as I can see the parameters are correct.
It is possible the sensor has some protective component in it that does not like the reversal of excitation so you could change the measurements instruction to this (with only a small change in accuracy):
BrHalf(Sensor_out,1,mV2500,1,Vx1,1,2500,False,20000,_50Hz,1,0)
This stops the excitation reversal and increases the settling time.
If that does not solve the problem please report that value of Sensor_resis as a known temperature so was can check the measurement of resistance is correct.
Note the above does assume the bridge resistor you are using has a resistance of 10000 ohms precisely.
Dear APs
Thank you for your reply
I'm sorry i have do a mistake.
i have insert a resistor of 1000 ohm... i'm sorry
the measurments are better with a 10000 ohm resistor...
Just one thing, in your instruction, i cannot put the excitation at 2500mV cr basic accept 1000mV or 5000mV
Do you think it's better 1000 or 5000mV?
regards
Sorry I did not spot it was a CR3000. Use 5000 mV excitation and 5000 mV input range.
BrHalf(Sensor_out,1,mV5000,1,Vx1,1,5000,False,20000,_50Hz,1,0)