Thermometer to put on glass?

mnowaczyk · Dec 31, 2009

I have an early 80s insert with single-pane glass and want a thermometer to mount on the glass.

(I'm not sure if newer models have double-pane glass. I realized my glass is expelling a good amount of heat to the room. I realized this when I held our oven thermometer on the dirty glass for a few minutes resulting in a crystal clear circle where the thermometer was held. I assume the glass must be cooling from the room temperature, resulting in buildup.)

I have very little firebox metal on the top front corner of my stove, and was not getting as hot of readings as I was getting on the glass. So I think that's probably not the most accurate place for a thermometer on my insert.

Are there thermometers designed to sit on the glass? Maybe with a metal block for the back of the glass? One that would not scratch the ceramic glass? Or is this something others do themselves? Maybe put another magnet on the inside of the stove since the magnet is probably not that strong on the magnetic thermometers?

Or is there a better probe type of thermometer that should be used on an insert instead?

derecskey · Dec 31, 2009

They make these for aquariums. Don't see why someone couldn't make one for fireplaces. Just have to figure out a way to mount it, as you said, and gear it for higher temp ranges.

mnowaczyk · Dec 31, 2009

derecskey said:
They make these for aquariums. Don't see why someone couldn't make one for fireplaces. Just have to figure out a way to mount it, as you said, and gear it for higher temp ranges.

Never thought of using my insert as an aquarium! I'll need to check my seals first.

begreen · Dec 31, 2009

A handheld infrared thermometer would work well in this situation. Look up IR Thermometers. Mine is an ATD-701, but there are several mfgs and places to by for about $40. Or WES999 has made some homebrew digital probe units.

mnowaczyk · Dec 31, 2009

BeGreen said:
A handheld infrared thermometer would work well in this situation. Look up IR Thermometers. Mine is an ATD-701, but there are several mfgs and places to by for about $40. Or WES999 has made some homebrew digital probe units.

I've read that some folks think IRs read all over the map. Maybe they are being picky. Is this IR precise within 50-100 degress in your opinion? I'm hoping to keep above 400, but below 700. So 100 degrees could make a difference for me.

Thanks,
Mike

begreen · Dec 31, 2009

Mine is pretty accurate. It's designed for automotive use where a couple degrees can be the difference between boiling or not. I can't speak for other ones.

Reading accuracy also depends on where one takes the reading. Reading off the glass or reflective surfaces for example may give inconsistent readings.

Battenkiller · Dec 31, 2009

DelBurner said:
BeGreen said:

A handheld infrared thermometer would work well in this situation. Look up IR Thermometers. Mine is an ATD-701, but there are several mfgs and places to by for about $40. Or WES999 has made some homebrew digital probe units.

Click to expand...

I've read that some folks think IRs read all over the map. Maybe they are being picky. Is this IR precise within 50-100 degress in your opinion? I'm hoping to keep above 400, but below 700. So 100 degrees could make a difference for me.

Most all of the units are very accurate, within +/-10ºF at stove/flue pipe normal temps. Like any tool, you have to learn its limits and how it works to get the most out of it.

Readings that are taken at a 90º angle to the surface to be measured will give the best results. Don't stand back too far and expect to get a small target area. Mine has a 20:1 distance to target size ratio. That means that at 20", the target area is 1" across. Some of the cheaper units only have a 8:1 or even a 6:1 ratio. Don't expect to target a specific spot on your glass from the sofa 8' away. The target area will be 12" across on a unit with an 8:1 ratio. It doesn't matter how small the pointing laser is, the sensor is behind the lens, not in the laser itself.

This can also cause pointing errors when you go up real close for a reading. Just like a telescopic rifle scope that is sighted in at a certain distance, don't expect to hit the target from a foot away when the rifle is sighted in at 100 yds. Best to ignore the laser (on mine you can turn it off) and just point the lens itself from up close.

Some folks say you can't get a reading off glass because it is transparent. The device measures the IR radiation being emitted from the surface. Various surfaces have different degrees of emissivity. Most units are calibrated for an emissivity of .95 (1.0 being a theoretically perfect "black body" emitter). Below is a link to a table of emissivities for various materials:

http://www.infrared-thermography.com/material-1.htm

As you can see from the table, glass has an emissivity of between .92 and .94 depending on the glass surface. Close enough for country. Firebrick only has an emissivity of .68, so you will need to extrapolate to find the actual surface temp of firebrick if that is of interest to you.

The long and short of it is, no matter how hot the temp is on the inside, the IR thermometer will only read the IR being emitted from the very surface. Highly polished metals like aluminum and silver have extremely low emissivities (near .02), but it is extremely unlikely you will encounter these materials in this form in your home. The table I linked to will help you as you go pointing the thing around the house trying to see how hot things are in the stove room - like your wood stash or your $3000 plasma TV. If they have a low emissivity surface, they may be hotter than you think.

My IR thermo is the Sixth Sense JT300. It measures from -76ºF up to 1400ºF. It has a 20:1 distance to target ratio. It has adjustable emissivity, so you can read your firebrick temps directly. It has several modes and accepts K-type probes (it comes with a low temp probe) so you can use it to take measurements inside your flue pipe (like WES999 does) by purchasing a $30 K-type high-temp thermocouple. It is on sale at Instrumart for $119 with free shipping and a nice case that will hold any gadgets you may acquire for it. That's a lot compared to what most folks are using, but if you are a curious sort like me and want "last word" accuracy, this is a reasonable price compared to most of the industrial grade units that do the same thing.

BK

SolaGracia · Dec 31, 2009

One question I have when reading these posts is---when you point the IR therm. at the glass, or you measuring the temp of the glass? Is this what the concern is about? I would like to know how to measure the internal temp of my Regency I3100. I want to know when to choke back the air control, and also avoid overfiring. Problem is, I asked Regency how one might do this. Their response? "There is no good way to measure temps on an insert." Thanks. "Oh", they say, "and by the way, make sure you don't overfire or your warranty is void."

Right now I have a magnetic thermometer mounted just above the door and below the blower outlets. I read temps here at a peak of about 400F, down to 300F during the course of a good burn. If only I knew the correlation between the temp at this location, and the internal burn temp, I would be able to rely on it for temp indication. If somehow I could measure flue temp, and compared it to the temp on the front of the unit, would this reliably set my temp ratio that I need to know?

Battenkiller · Dec 31, 2009

SolaGracia said:
One question I have when reading these posts is---when you point the IR therm. at the glass, or you measuring the temp of the glass? Is this what the concern is about? I would like to know how to measure the internal temp of my Regency I3100. I want to know when to choke back the air control, and also avoid overfiring. Problem is, I asked Regency how one might do this. Their response? "There is no good way to measure temps on an insert." Thanks. "Oh", they say, "and by the way, make sure you don't overfire or your warranty is void."

Right now I have a magnetic thermometer mounted just above the door and below the blower outlets. I read temps here at a peak of about 400F, down to 300F during the course of a good burn. If only I knew the correlation between the temp at this location, and the internal burn temp, I would be able to rely on it for temp indication. If somehow I could measure flue temp, and compared it to the temp on the front of the unit, would this reliably set my temp ratio that I need to know?

One of the reasons why I never used to use a thermometer is that I knew I would drive myself nuts checking it. Once I had three of them (the IR, one on the pipe and one on the stove), I drove myself bonkers.

There is only so much any thermometer is going to tell you. At best, they will give you a good indication of whether or not you are within the parameters recommended by the manufacturer. Evidence of a strong, clean burn (which was my old thermometer), experience, and an absence of damage will tell you even more.

Yes, the IR will only give you the temp of the glass. And only the outside surface of the glass, not the inside surface. Low emissivity glass (not used in stoves for obvious reasons) might read 400ºF on the surface while the actual temperature is 800º inside the glass itself.

Rest assured, if you are getting a proper burn, your internal temps will be above 1200ºF. My IR maxes out when I point it directly at the coals. They are significantly higher than 1400º. If they are glowing a bright yellow white, they are over 2000º. What good does this info do you you? Besides, different parts of the stove will hit different peaks at different times depending on how long they have been exposed to either the flue gases, the hot coal bed or both. These same parts will heat up faster or slower and hold the heat for shorter or longer times depending on their metallurgy, shape, size, location, and mass. Some of these parts can take the heat for years, other will need to be replaced as the stove is used over the years. That's a design issue, not a burn issue. These things are forever only in that they can be rebuilt as long as parts are readily available. Besides clean burning, efficiency and heat output, good designs are made to withstand lots of heat and to be rebuilt in an easy manner when the parts wear out.

Follow the suggestions of the smarter folks on this board (the pros and dealers, certainly not me) and keep your stove within these parameters and you shouldn't damage your stove. For my stove, my stove repair guys (30 years in the business) tell me to keep the flue temps between 400º and 650ºF on a magnetic thermometer 18" above the flue collar and just forget about the stove top temp entirely. But do I listen to them? :roll:

So when I see I'm getting a nice steady 400ºF three hours into a burn and then I point my IR at the door and get a reading of 875º.... who's picking up the bill for my therapist? But we all worry a bit about our stoves, and I'm no exception. If you really need to know how hot the internal parts are, get an IR, open the door and quickly shoot the part in question. Maybe this will give you the peace of mind you seek, and that will be a good thing.

I learned all I needed to know about my stove from the IR at this point. My magnetic thermos are calibrated to it. They are about 15ºF off from each other and less than 25º off from the IR throughout the burning range. I found out my stove develops hot spots during a burn and that they are somewhat predictable and that they haven't hurt my stove in the least. Now it's just another one of my expensive toys. When I can use it to figure out when the wife is in the mood... well, then I'll be getting somewhere.

mnowaczyk · Jan 5, 2010

great info BattenKiller! Thank you. It is interesting to have a concept of how hot those coals can get. I agree that the temperatures inside a stove should not be a significant concern because they should be all over the map. I assume that in general, you want to keep all the air inside the stove nice and hot to make sure any smoke gets completely burnt (cleaner for your chimney and the environment). My (unexperienced) opinion is that the best way to measure the air temperature of the stove would be to measure the stove itself. It seems that most people shoot for stove temps above 400 degrees, and it seems that people like to keep them below 700 degrees. It seems like keeping a 600 degree stove for hours on end would be a wet dream. Maybe "wet dream" is a bad term, when that would require very dry wood. It seems that most people are happy with a constant stove temperature of 400-475. So I guess what I will shoot for is a constant glass temperature above 450. The last couple times I propped my oven thermometer up against the glass with my coal shovel, I was getting 450-500 on burns that looked a little lazy to me. I guess my stove might be hotter than I expect... or the glass is simply hotter than metal... I learned that before I wrote this post.

So now I'm starting to wonder if the glass is a bad place to measure, especially with an IR. Here are a few reasons: As mentioned, the inside tep of the glass might be 800 and the outside 400. The 70 degree room air should cool glass pretty quickly, so I agree with this concept. Add the fact that my glass cleans itself up in a matter of minutes when I put the thermometer on it. It leaves a little circle where it gets my oven thermometer over 400 degrees, and I assume both sides of my glass are running over 400+ degrees. Meanwhile other areas of the glass are stick dirty... and maybe both sides of the glass are still under 400 degrees... therefore my glass is acting as the one cool side of my stove where smoke is condensing quickly and easily... the explanation for why it's so hard to keep my glass clean. (Ah ha !!!

) So the temperature readings I'm getting with a glass-mounted (HOT) thermometer will likely be very different from those read with an IR thermometer, where the glass is cooled by the room air. That's interesting, and maybe a good reason not to use an IR on the glass. I wonder how different IR temperatures would be on metal?

On a separate topic... I think I'd prefer to have a thermometer that's permanently mounted on the stove because it's more of a passive activity than an active one as I assume the IR would be. I agree that the IR will be a great toy, or tool in general to have around the house... especially at the $40 price mentioned (THANK YOU FOR THE INFO BEGREEN!). However, I agree that I'd probably drive myself nuts checking the temperature. I think that as a newbie, it would be good for me to understand the different temperatures. In the end, I think it will help me understand whether a lazy burn is acceptable, or not. With my tendancies for obsessive compulsive behavior, I think having a stove mounted thermometer would be quicker to read than an IR. I just think it would be quicker to take a quick look than it would be to go grab the IR, turn it on, move the fireplace screen, measure, put the fireplace screen back, and put the IR back. I also bet I'll take five measurements each time too.

So in the end:
- Nobody seems to have heard of double-pane stove glass. Right?
- Nobody knows of anyone who has permanently mounted a thermometer to their glass (at least while learning). Right?

THANKS FOR THE GREAT INFO EVERYONE!

mnowaczyk · Jan 8, 2010

I picked up a rutland magnetic thermometer at Home Depot today. What a P.O.S.! I can't get it to read anything over 300 degrees on the stove face because there's nothing to keep the metal element from cooling. I have metal andiron posts in the front of my stove, so I put the thermometer on one and closed the door for a few minutes to see the air temp in the front of the stove was over 800 degrees. I'm starting to wonder if that's the best thing to do with that $10 thermometer. (I know I overpaid.)

It's pretty interesting to see that my oven thermometer can get hotter reads because the back just has a few openings, whereas the Rutland is wide open to the room air... so I assume it would never be able to give you a real reading for the material to which it's attached. Funny that the oven thermometer does a better job at measuring stove face temp than one that's made to do that. LOL!

mnowaczyk · Jan 8, 2010

SolaGracia said:
One question I have when reading these posts is---when you point the IR therm. at the glass, or you measuring the temp of the glass? Is this what the concern is about? I would like to know how to measure the internal temp of my Regency I3100. I want to know when to choke back the air control, and also avoid overfiring. Problem is, I asked Regency how one might do this. Their response? "There is no good way to measure temps on an insert." Thanks. "Oh", they say, "and by the way, make sure you don't overfire or your warranty is void."

Right now I have a magnetic thermometer mounted just above the door and below the blower outlets. I read temps here at a peak of about 400F, down to 300F during the course of a good burn. If only I knew the correlation between the temp at this location, and the internal burn temp, I would be able to rely on it for temp indication. If somehow I could measure flue temp, and compared it to the temp on the front of the unit, would this reliably set my temp ratio that I need to know?

I'm seriously thinking about leaving the Rutland magnetic thermometer on the inside of my insert on one of the andirons. I can view it right through through the glass. Since it seems useless on the outside of my stove, what do I have to loose?

Oh BTW, I tried putting a strong magnet on the back side of the glass, and it did not hold up. The rutland magnet is pretty weak.

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