temperature/ humdity control

[techpia]

I want to PID control the temperature/humidity simulateneously usng USB based DAQ

I have NI USB 6211. so I wonder

1) is it possible to get the input of temperature and humidity using 6211; if possible how to get the convert the voltage to C or %RH. 

2) what RTD/thermocouple and humidity sensor is applicatble, and for thermocouple, how to set the cold junction

3) I want to get the DC output voltage from same usb DAQ and will connect solid state relay for control of heater and humidifier, is it ok?

4) if NI USB 6211 cannot handle this job, any recommendation?

[cirrusio]

techpia,

This should be fairly straightforward depending on the sensors you are using.  Temperature is easily monitored using a voltage divider on a circuit containing a thermistor (I am not sure if there is a requirement to use an RTD or thermocouple, but a thermistor should cover a wide range of temperatures; not sure if you can use a thermocouple or RTD).  

What kind of accuracy do you require on your humidity?  If high, then you will require a dew point hygrometer; if lower, than a capacitance based one such as those sold by Vaisala should be sufficient.  In either case, you should be able to find one with an analog output and with a scale defined by the manufacturer.

Depending on any additional AI, you should be able to monitor the voltage using the NI 6211 as long as the DC output is within the AI range.

Anyway, hope that helps.

Cheers, Matt

[Henrik_Volkers]

Most humidity sensors already involve a temperature sensor. Due to the high temperature influence of Hrel and the tempco of the H-sensor itself 😉

To control temperature AND humidity I found best to calculate absolute water in air and use that for the humidity control.  

[cirrusio]

That is typically what we do here also, Henrik.  However, calculating the water vapor mixing ratio can be quite tricky depending on what temperature your system operates on given the some of the inherent difficulties for solving the Clausius-Clapeyron equation explicitly and the  wide variety of water vapor saturation pressures parameterizations available.  You just have to be explicit with your calculations, whatever you do .

Cheers, Matt