I stated in the vac kiln thread that RH sensors are a lot of trouble. I need to add to that statement.
In a vac kiln, the first problem you have is trying to seal the type of sensor that is intended to be inserted in duct work. Any little bit of air leaking past the sensor screws up the reading. Then, condensation from the chamber wall can screw up the reading. Then cooling from evaporation from pressure chamber changes can screw up the reading. See a trend here?
In a DH kiln, you can stick a probe from your control room into the kiln and get a good reading. And that's it.
The RH sensors that Nyle uses are connected to the controller only by wire so the problems related to sticking them through the wall or side are not applicable. However a sensor that is suitable for a kiln is relatively expensive. They have to tolerate higher tmeperatures and be accurate at high RH (90% range). And stay in calibration for some time. Most aren't able to do this.
The sensors that will work are around $350.
You guys spend too much on sensors ;D
Jason
But we're the 'plug and play' type and must pay for the convenience. :D
I just got back from changing an RH transmitter on a DH kiln. This customer has several 'Drylines' and a 'Nyle'. I had converted them from wet bulb to RH sensor about 6 years ago. I think this was the first sensor we had to replace and it was broken physically. At any rate, I told the customer that the replacement and the spare that he bought were somewhere around $300 each. His reply? "I don't know why anybody messes around with wet bulbs."
Anyone here use the radio shack sensors? They are about 75-90 for the wireless unit and the transmittor. A buddy of mine says they work real good for the money..
In one of these threads, we were talking about 'digital hygrometers' and I mentioned that I was working with a company who had a problem. The company is Delmhorst and I mention them now because they now agree with me that a serious problem exist.
Delmhorst supplies a lot of moisture meters to the lumber industry. A digital hygrometer/thermometer from Delmhorst would naturally be trusted by the people with their moisture meters. Delmhorst's literature has said that their meter was accurate to 158'F. Today, they told me in writing that the operating range specification would be rewritten to 128'F and that they would correct the literature and spec sheets as soon as possible.
If you are using a Delmhorst HT-3000, be aware that it could be very inaccurate at temperatures above 128. The one I evaluated was good at 121 but completely bonkers at 130.
You know what? I think that thread was at that other forum.
Den,
What type of sensor do they use in their hand held unit? It may not be the sensor that is failing. It could be their circuitry on the front end that is failing. Usually the cheap sensors like the Rat Shack sensor uses a capacitave sensor that is in a oscillator circuit. As the humidity changes the capacitance changes thus changing the frequency of the oscillator circuit. The frequency is read by the microprocessor and is correlated to a RH value usually by use of a lookup table in memory. These oscillator circuits are extremely suseptible to drift over temperature if not designed properly.
Jason
Jason,
I've used three different brands of RH transmitters. All have had capacitive sensors but they are 'packaged' differently.
You are right. The sensor isn't usually the problem. The problem is usually the insufficient temperature compensation in the oscillator. One other problem I've seen is the capacitance of the potted leads in duct mounted sensors. With one brand, I've repaired 'dead' transmitters by repotting.
How did you make out with that Honeywell?
Den