Calculate humidity

chrismarek · Nov 21, 2013

Is there a way I can calculate humidity with out a humidity sensor?

I currently program to a machine that has a performance map that changes based on temperature and humidity and need a way to estimate humidity decently accurate. Currently inlet temperature and barometric pressure are measured. Is there a way I can gather other data to estimate humidity, don't have anything to find dew point either. Our product is shipped all over so I need to have a fit/ equation for the plc to adjust to the conditions.

Tom Jenkins · Nov 21, 2013

There is no way to accomplish this without a humidity sensor or a wet bulb temperature. At a given barometric pressure and ambient temperature the humidity may theoretically range from 0% Relative Humidity (%RH) to 100% RH. In real conditions the range is more likely to be from 20% RH to 80% RH, but that is still too broad a range for an estimate.

Try Omega Engineering - I've used their humidity sensor with good results.

chrismarek · Nov 21, 2013

Thank you for the info Tom. The omega sensors work with PLC? 4-20mA or 0-10V or anything like that?

BobB · Nov 22, 2013

2 x RTD - wet bulb/dry bulb. Humidity sensors need calibrating regularly - hate the damn things sometimes!

Tom Jenkins · Nov 22, 2013

BobB said:
.... hate the damn things sometimes!

I have had reliability issues as well. This unit seems to hold calibration well and hasn't given me too much trouble. Of course, in my application I'm not looking for dead nuts accuracy.

http://www.omega.com/pptst/hx71.html

dploof23 · Nov 22, 2013

Tom Jenkins said:
I have had reliability issues as well. This unit seems to hold calibration well and hasn't given me too much trouble. Of course, in my application I'm not looking for dead nuts accuracy.

http://www.omega.com/pptst/hx71.html

I use that same unit in an application here. So far it has been in service just shy of two years with no issues yet.

Can you hear me knocking on some wood?

Dave

TConnolly · Nov 22, 2013

BobB said:
2 x RTD - wet bulb/dry bulb.

BobB,

Have you ever done this? I've toyed with the idea of using an RTD to put a wet bulb at a cooling tower inlet louver with a valve to periodically wet the wicking. I'd be interested in knowing how others have done this.

danw · Nov 22, 2013

TConnolly said:
RTD wet bulb at a cooling tower inlet louver with a valve to periodically wet the wicking.

I've seen that done in a bakery in the high humidity room where they let the dough rise. They had a dry bulb, a PT100 RTD, and a wet bulb wrapped in a wick. They used a toilet float valve to keep water in a container with the wick immersed part way in the water.

They were using a stand alone Honeywell PID controller that had the dry bulb/wet bulb conversion to RH to blow external (relatively dry) air or moist air that was moistened by being blown over and around a series of heated, wetted canvas frames.

shooter · Nov 23, 2013

you will need one extra input , or from a humidity sensor (with the old hairs or a electronic one.
or another tempfeeler with a wetted sock on them as suggested above as long as the sock is wet it is reliable.

rootboy · Nov 24, 2013

We've used Rotronics with excellent results. Pricey, but reliable.

Calculate humidity

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