Wet bulb thermometer using a thermocouple

[Kcwoodbutcher]

Is it possible to make a wet bulb thermometer using a thermocouple rather than a glass thermometer? What considerations should be noted if this is possible?

[BBTom]

The thermocouple MUST have a stainless tube, the plastic ones will eventually absorb water.  The TC and wet sock must be where there is plenty of airflow. but not directly in front of a fan.  The wet sock needs to be wet with distilled water or mineral/calcium buildup will require sock replacement every couple loads.

[Den Socling]

If possible, use an RTD instead. They are a whole lot less trouble.

[TimRB]

Even easier would be to use one of the semiconductor temperature sensors.  They only cost a couple bucks, are accurate and linear, and can be used with nearly any voltage, so battery operation is easy.

http://www.national.com/mpf/LM/LM35.html#Overview

Tim

[Den Socling]

That LM35 looks interesting but I wonder how you would get one in a wick. Maybe mount it on a SS rod and put the wick over the rod?

[TimRB]

That LM35 looks interesting but I wonder how you would get one in a wick. Maybe mount it on a SS rod and put the wick over the rod?

They are three-terminal devices that come in various packages.  If you pick one of the transistor-style packages it's easy to solder some wires to the leads, stick the thing inside a short piece of plastic or glass tubing, and seal it with silicone.  I've kept them immersed in water for days on end this way.

That would work for a wet bulb/dry bulb application, but it would behoove you use glass or metal, and  try to make the package small, so as to reduce the thermal time constant.  You don't want to be sitting there all day waiting for your wet bulb temp to stabilize.  A little ingenuity goes a long way here.

Tim

[BBTom]

You can get one of the chinese temperature controllers for under $40 on ebay.  the TET612 if I remember right,  it will read most any thermocouple or an RTD.  use one controller for the dry bulb, controlling the temperature, and another one for the wet bulb controlling the vent blower, and you have a kiln control system. 

If you want both of them in the same package, you can go to
http://www.dwyer-inst.com/Products/Product.cfm?Group_ID=537&Product_ID=910&sPageName=Ordering
and get the same unit used by Nyle for the Woodmizer model kiln.  This unit uses a humidity sensor instead of wet bulb. 

There are lots of solutions out there, but like the wood doctor pointed out in kiln drying operator class.  "Do you want to trust $10,000 worth of lumber to a $29 radio shack thermometer?  You only have to buy a good controller once to do hundreds of thousands of $ worth of lumber."

[Den Socling]

Heck as long as we are spending woodbutcher's money, lets use a couple loop controllers with RTD and humidity transmitter inputs and proportional outputs.  :D Wait! Lets use a PLC and add all of the bells and whistles.  8)

Seriously, to answer the original question, I don't use thermocouples because the leads of the extension ables are solid and are not "regular" wire and the joint where the extension goes on the thermocouple are a problem. Otherwise, you can get thermocouples sealed in a SS rod and ready for a wick and reservoir.