Digital Cat Probe

[Ashful]

Just so I understand completely, the front of the cat is the inlet? That seems very odd to me, as -- aside from some radiation -- I'd expect temps to be fairly cool on the inlet side of the cat.

It is surprising the two different probes read very differently, but it sounded like you have a few readings that agree with the thermocouple, and they all disagree with the old mechanical Condar. Are you sure that one is not just wrong?

 

[mellow]

The thermocouples never go past 1000 degrees, while the 6" Condar bi-metallic probe will read up into the 1700's while the cat is active. That to me says the probe is correct, but how is it reading those temps and the thermocouples do not?

 

[Ashful]

Your cat is likely peaking at 1700F if your wood is good, but I also would not be surprised that the temperature on the inlet side of the cat is closer to 1000F. Could the Condar probe be somehow falsely compensated, if it is designed specifically for measuring on the input side of the cat?

I'm trying to think of some way that you could test your probes, but nothing accessible in the household comes to mind, that runs a known temperature above 1000F.

 

[mellow]

I do not think it is anything special, Condar mentions temperatures behind the cat for the bi-metallic probes as well.

(broken link removed to http://www.condar.com/cat_meters_woodstoves.html)

One of the differences is they use copper for the probes, not sure if they have copper in the thermocouples or not.

 

[Ashful]

The thermocouples are usually manufactured from stainless tubing, for good reason. You need very poor thermal conductivity down the length of the probe, to get a reading at the tip only. It's a matter of aspect ratios, with much surface and minimal transmission depth thru the tip, versus much transmission length and minimal cross section down its length.

 

[raybonz]

The thermocouples never go past 1000 degrees, while the 6" Condar bi-metallic probe will read up into the 1700's while the cat is active. That to me says the probe is correct, but how is it reading those temps and the thermocouples do not?

Are you sure you're set up for Fahrenheit and not Celsius? 1000 degrees C = 1832 degrees F.. Also if your temperature is set up for Fahrenheit have you tried adjusting your offset to reflect what you think your temp is at that time?

To check your setup place the thermocouple in boiling water it should read 212 degrees F or 100 degrees C.

Ray

 

[mellow]

So I did some tests over the weekend to test accuracy of the thermocouple vs the bi-metallic, thinking that one HAD to be wrong, well as the pics will show, both are pretty close to each other.

Tested the Thermocouple in the oven set at 550. It reads 554, close enough and verified.


I however could not do this test with the probe due to it was reading over 2000 degrees with it put on the same place in the rack in the oven, I discounted this result as the probe tip should be the only thing receiving that kind of heat and it throws off the bi-metallic if the whole probe is put in the oven.

Test the thermocouple over high heat on the range:


Test the bi-metallic probe over high heat on the range:


As you can see both read about the same temperature, so either both are off or both are correct in high heat, I don't have a way to verify the results are correct, but find it interesting that both read the same temperatures. I did get both of them cherry red before reading the final results, I must say my hands were pretty hot holding that bi-metallic, it takes A LOT longer to get up to temp.


So this brings me full circle to my issue, if BOTH are reading temperatures correctly then why am I getting two very different results from reading my catalytic combustor at the same exact spots?

 

[Ashful]

hah! Very frustrating, indeed. Is it possible the shaft of one is contacting more stove body metal than the other, where it penetrates the outer casing of your stove, thus cooling the probe body?

 

[mellow]

This picture is not correct, but this past weekend I put the whole damn 6" thermocouple IN one of the cat cells, it was just small enough to fit, as pictured I tried doing it in the middle but the damper when open would push it back out. With it inserted into the cell it reads about the same temps as usual, hovers around 800-980 F during peak burns, However if an active flame hits the cat the results jump up into the 1000 range.

 

[Ashful]

I'm really surprised the wire/probe junction holds up to those sort of temperatures. Typically, that part of the probe body is rated much lower.

 

[mellow]

At this point I don't care if it holds up or not, it is a last ditch effort to see what works. I too am at a loss as to why it won't read correctly when inserted directly into a cell on the cat.

I will shoot a message to BK and Woodstock and see if they can direct me to someone that can explain why this is not working.

 

[Ashful]

Might also be good to forward a link to this thread over to the folks at Omega engineering. If someone knows a thing or three about thermal measurement, it's them.

 

[mellow]

I have given up on this project for my Appalachian 52 Bay Insert, none of the manufacturers know how to make this work from the front and default back to the Condar bi-metallic probe as the solution, Buck included. I am not about to drill a hole through the thick top plate of this insert just to find out if the thermocouple will work from behind the cat, that is the only way to access it.

All the manufacturers seem to be happy with just offering bi-metallic probes, no one seemed to like the idea, which I find odd.

I ruined the 6" thermocouple by cutting it thinking maybe making it shorter would be better.

Maybe someone down the road will read this thread and have the solution.

 

[Ashful]

Sorry to hear that!

Me? I'd probably drill the top. It's an insert... who will see it?

 

[mellow]

Problem being it is an insert, I would have to drill through the top plate, and then there is 2" of space for the air jacket that goes over the top of the firebox, then drill into the top of the firebox.

IF it was only drilling out the top plate then I would consider it.

 

[mellow]

Just a heads up, I am hoping to bring this issue up at the Stove Challenge, how can I get a digital cat probe to read correctly from the front like the Condar.

I have beat my head against a wall trying to get this to work with limited success. Experts please re-read this thread and let me know what you think, I can not get any of the Manufactures to talk to me about this.

 

[Ashful]

I remember this thread. Good luck! As to the actual probe to use, I worked with Omega (and burned up several probes), in the search for the right one. Unfortunately, it will now take me some digging to find what probe we settled on, but I have it recorded somewhere, if you need the info. If you can tolerate a larger probe, the endurance troubles are less an issue, but I needed a 1/16" diameter probe to fit the hole I had drilled thru the center of my probe hole plug bolt.

 

[mellow]

I can drill out the hole on the front, that is not a problem. As for endurance that hasn't been an issue so far, the only thing that has broken the thermocouples so far is me cutting them. They are holding up very well with all the abuse I have been putting them through.

 

[Ashful]

Cutting them? You have to weld a new bead, if you do that. Any EE worth his salt knows how to do this with a car battery.

 

[Todd]

With all the tinkering I've done with cat probes I'd say go by your thermocouple readings. I have noticed the Condar probes read high because the radiant stove heat effects the spring coil.

I also think you would get the most accurate reading if you drilled down through the top and got the probe 1/2-1" downstream of the cat. Measuring closer than that is not recommended by Applied Ceramic or Sud Chemie.

 

[mellow]

I have noticed the Condar probes read high because the radiant stove heat effects the spring coil.

I thought that might be an issue as well so I pulled the probe back some and made an air stop around it to block heat and still got the same results.

As for drilling out the top, that is not possible with these type of stoves due to the air jacket running on the top.

 

[mellow]

There are many a stove out there with this type of design, Buck being one of the largest manufacturers. I tried talking to Buck's engineers but they acted like they could care less about having a digital read out with high/low temperature alarms on their stoves and told me to stick with the Condar basic probe, it really is a shame most of these guys are still stuck in the 80's.

 

[Todd]

You got me thinking of doing this with my Keystone. I have an access hole in the back of the stove for the probe but it doesn't line up very well with the cat so I'd have to put a slight bend in the probe. Do you think it would hurt the probe to put a slight bend in it?

I hope you get something figured out for your stove and I wish manufactures would come up with a better cat probe.

 

[mellow]

Shouldn't hurt the probe to bend it, heck I bent the crap out of mine and it still works. Been using it on my smoker over the summer.

 

[Ashful]

Most probe manufacturers specify a minimum bend radius. I have a 1/4" or 3/8" radius bend at 90 degrees in each of mine, so the stove can be pushed back almost against the wall, with the probe sticking out the rear.