Dry Bulb & Wet bulb hygrometer or RH/EMC Probe for control

[HencoV]

Hi Guys,

I have always had some reservations about using RH/EMC probes instead of Wet Bulbs for control. This, in my opinion is probably because the controls I came across that uses RH/EMC control was very slow to react, and "over vented" from an early stage in drying. I suspect it was a lack of proper understanding of the programmer on the effect of RH on venting and energy usage. So..to put my own internal conflict at rest, I decided to run some tests

I'm busy running a comparison in one of our kilns using both Wet Bulb, as well as RH probe for control purposes to try and establish the best way forward.  I'll will post my findings here in a month or so.

In the mean time:What are your views, pros & cons of each of these technologies. From a control and accuracy point of view, which would you rate best? Share your experience please

[MichaelM]

First, let's assume they are both accurate or at least give the same RH value and the difference is in the rate of response.

If the sensor changes, then the valve response needs to change.  This involves changing the PID settings in the controller.  I would recommend asking the manufacturer about this and do not change the values yourself.  There are three numbers for each control loop.  One regulates how much the actuator opens for a certain error. The second integrates the error over time and opens or closes the actuator in response to that value certain.  The last uses how fast the temperature is changing to effect a change in the actuator position.  The kiln control will be a mess if these settings are not correct (overshoot or slow to come back to set point).  "Error is how far the kiln is away from set point.