Problems with Condar digtal temp probe

[liberty]

Just installed the theromocouple in my Fireplace X 36" Elite catalytic fireplace and when I plug it into the display unit I get a constantly changing temp when the unit is cold. If I remove the thermocouple from touching the metal it reads cool as it is supposed to. Once the thermocouple touches metal again the readings go erratic. I tried a seperate thermocouple with the same results. Any ideas?

 

[ShoerFast]

Static electric, or RFI from florescent lighting.

Or as simple as stray eddy-currants, have you tried running a couple grounds to the unit and the fireplace?

 

[liberty]

The house is new construction with modern wiring and properly grounded. Even if I turn off the power to the blower via the wall switch it still does it.

 

[ShoerFast]

Try totally unplugging it?

New dose not mean right.

This might sound troubling to you, but really not the end of the world.

The temp-probe may have in internal grounding problem, as simple as the board maker has a bad hi-impedance resistor that was wired to take care of it.

If that is the case, they should be happy to exchange it for you.

 

[liberty]

I removed the ac power plug and used the optional 9 volt battery and it works fine. I checked the output of the ac adapter and it is 8.9VDC. Also, where does the thermocouple have to go to get a good reading. Mine sits around a half inch in front of the catalyst.. Is this right?

 

[ShoerFast]

You isolated the fluctuations, just find the stray-currant source but i have no idea about placement.

 

[chowdozer]

Just installed the theromocouple in my Fireplace X 36" Elite catalytic fireplace and when I plug it into the display unit I get a constantly changing temp when the unit is cold. If I remove the thermocouple from touching the metal it reads cool as it is supposed to. Once the thermocouple touches metal again the readings go erratic. I tried a seperate thermocouple with the same results. Any ideas?

A TC works on the Peltier Junction principle. In a nutshell, two dissimiliar metals in contact with each other create a voltage that varies with heat. It's in the millivolt range, so very small. Different metals create different voltages so different types are not interchangable. You can see this on your car, m/c, saw etc where two dissimiliar metals are joined (corrosion). Cig lighter fridges use a Peltier too, but backwards. You should isolate the junction from other metals. In the past 15 years, the only time I have seen a TC read erratically was when I keyed up one of those "Family" type two way radios.

It could be a ground loop, but I kinda doubt it.

I use Chromel Alumel or type K which has the yellow insulating jacket with a red stripe. The red is (-). It is robust, accurate to about 1 deg C without calibration.

If you have a type K, you can check it's accuracy with an ice water bath and a boiling water bath (0C&100C). Just straightline the curve and extrapolate. Type K's are pretty liner.

 

[liberty]

Sorry for the stupidity on my part. What should I do? I disconnected the power completetly from the fireplace and it still does it with the ac adapter and not with the 9 volt battery. Is it possible to have a bad ac adapter?Thanks

 

[chowdozer]

Sorry for the stupidity on my part. What should I do? I disconnected the power completetly from the fireplace and it still does it with the ac adapter and not with the 9 volt battery. Is it possible to have a bad ac adapter?Thanks

What type of TC is it? Can you post a pic of it? Is it a wire with a bead on the end or does it have a mounting tab?

First thing to do would be isolate it from everything. Hook up the AC adapter, set it on the kitchen table and see if it works. Verify with varied temps.

 

[ShoerFast]

What type of TC is it? Can you post a pic of it? Is it a wire with a bead on the end or does it have a mounting tab?

First thing to do would be isolate it from everything. Hook up the AC adapter, set it on the kitchen table and see if it works. Verify with varied temps.

Yup!

Try a different A/C adapter, my bet that it's sending stray currant surges, double the bet that there is a AC currant carrying with the 9V DC ?

If you a Fluke or other good meter would show you a frequency of the fluctuations?

 

[chowdozer]

Yup!

Try a different A/C adapter, my bet that it's sending stray currant surges, double the bet that there is a AC currant carrying with the 9V DC ?

If you a Fluke or other good meter would show you a frequency of the fluctuations?

If you put a DVM on ac and measure a dc plug-in wall tranformer, you should get zero. If you get any ac voltage, it's toast.

 

[liberty]

I get 2 VAC.

 

[chowdozer]

I get 2 VAC.

You found the problem.