Interpolate THIS!

[soupy1957]

Yesterday was cool enough that the wife REQUESTED a fire. "Fine" by ME!! (lol). Anyway, I burned for a good portion of the day, and welcomed the chance to compare the Rutland thermometer's readings to the Laser's readings.

I built a hot but small fire, and kept it that way (a couple splits at a time, down to embers; rinse/repeat). I experimented more with the damper positions, and was running the fire with the damper ALMOST all the way closed, but not quite (enough to still see flames, but not enough to smother the fire).

It was consistently true that the Laser would read about 50º less, (reading the temp by pointing the laser at the burning wood, thru a glass door), than the Rutland. It (the Laser) was also showing me temps of about 50º less than the stove pipe thermometer.

I tried the Laser on the top and sides of the stove, and the readings were significantly lower than the readings I was getting thru the glass (somewhere in the neighborhood of 350º or so.

Interpolate this data for me, if you will................

-Soupy1957

 

[fishingpol]

Soupy,

We use laser thermometers at work for reading air temperatures from vents on heating and cooling sytems. I know that these units have to be about 8 inches from the surface or vent that is being read to be the most accurate. The beam exiting the therm. is to my understanding like a cone getting wider the further away from the unit. The further away the object is from you, it will pick up temperatures from items surrounding it. Many techs that work on a/c systems always use a thermometer that contacts copper refrigerant lines. I never did ask why. I have heating pipes with a thermometer built into a 90 degree elbow on the system. Our laser thermometers always read a few lower different than the therm.

I can only speculate that piping of various material reduce actual temperature readings. I would also speculate that heat rising off the stove or pipe would bring in slightly cooler air from below or around it causing a difference in temperature.

It can also be that one of the units may be off a few degrees. A probe thermometer will only move as fast as the mechanics inside it which is most likely a spring. Digital thermometers give you more of a "real time" reading, as they are able to read and translate temp. readings quickly, as in split seconds.

50 degrees does not seem like a lot unless you are running on the high end of your stoves capacity and are concerend about overheating. I always err to the side of caution. All of these are just observations I have made at work.

Jon

 

[Pagey]

Shooting a double wall stove pipe with an IR gun is "meaningless" compared to the data offered by the probe thermometer which is actually reading the gases inside the pipe as opposed to the outermost layer/surface of the 2 walls. I would fully expect it to be lower. I would also expect that the inside of the firebox is much, much hotter than any surface reading from the plate steel. If I recall, you need about 1,100F inside to ignite the lovely secondary combustion. 1,100F on the top plate, however, would be detrimental to the stove's health.

 

[CountryBoy19]

First of all, I don't think you can "read the temp of the fire by pointing the thermo through the glass". You're actually getting the temp of the glass. IR does not pass "through" glass, it reflects off of it. The "laser" thermo that you have is most likely an IR thermo and therefore you're just getting a glass temp when you point it at the glass.


2nd point (this is my theory as to why you're getting lower temps with the laser), is that the magnetic thermo that you have is actually slightly insulating the stove-pipe and therefore that thermo is reading hotter underneath where the probe is. The laser is taking a reading on exposed pipe, where the air is continuously cooling the pipe down. Does that make sense?

 

[soupy1957]

The more I thought about it this morning, the more I agree about the stove pipe. Since it's a double-wall, the Laser temp reading SHOULD be cooler.

The "distance" from the source, with the Laser is definitely a consideration. I tried it from about 6 ft away, and then about a foot and a half away (the results as stated in the OP were the result of the closer proximity).

Are there any issues with taking the Laser read of the logs, thru the glass? (In terms of accuracy of the read, or deflection, or whatever, because of the glass).

1100ºF seems very high to me, and not the typical responses I see in here, for burn temps that are "cruising speed."

-Soupy1957

 

[Pagey]

The more I thought about it this morning, the more I agree about the stove pipe. Since it's a double-wall, the Laser temp reading SHOULD be cooler.

The "distance" from the source, with the Laser is definitely a consideration. I tried it from about 6 ft away, and then about a foot and a half away (the results as stated in the OP were the result of the closer proximity).

Are there any issues with taking the Laser read of the logs, thru the glass? (In terms of accuracy of the read, or deflection, or whatever, because of the glass).

1100ºF seems very high to me, and not the typical responses I see in here, for burn temps that are "cruising speed."

-Soupy1957

I'm not saying that your stove top should "cruise" at 1,100F. I am saying that in order for secondary combustion to kick off and maintain itself in a non-cat stove, you need to reach about 1,100F or so inside the firebox. But the firebox is full of fire brick, which insulates your stove. The plate steel won't be anywhere near the temps needed for secondary combustion.

See this: (broken link removed to http://www.chimneysweeponline.com/horeburn.htm)

Quoting from that link: "Technique #2: The trapped heat reburn (raise the temperature inside the stove)
A secondary combustion chamber is incorporated into the design of the stove, at the top of the firebox. Space-age insulation is installed on top of the chamber, to "trap" rising heat from the primary fire in the chamber below. The secondary air inputs in this design are fed by passageways that are exposed to the heat from the primary fire, thus preheating the secondary air to help ensure that secondary ignition temperature is achieved and maintained. In operation, the rising exhaust from the primary fire encounters the pre-heated air in the superheated secondary combustion chamber and ignites, at a temperature of about 1100 degrees. Trapped heat stoves are more difficult to design, but easier and less expensive to operate and maintain, as there is no need to engage a catalytic converter, or clean it and replace it as needed. Trapped heat stoves are known for higher heat output and a livelier, more attractive fire view. All of the woodburners we sell use this technique."

 

[oldspark]

Oh no, you should have deleted the comment about cat stoves, a "cat fight" might ensue! Not sure why but they do not like cat stoves on that site.

 

[North of 60]

Yesterday was cool enough that the wife REQUESTED a fire. "Fine" by ME!! (lol). Anyway, I burned for a good portion of the day, and welcomed the chance to compare the Rutland thermometer's readings to the Laser's readings.

I built a hot but small fire, and kept it that way (a couple splits at a time, down to embers; rinse/repeat). I experimented more with the damper positions, and was running the fire with the damper ALMOST all the way closed, but not quite (enough to still see flames, but not enough to smother the fire).


-Soupy1957

Are you seeing the flames from your secondaries or the wood? What is the stove top temp in comparison with the thermo and the infrared?

 

[grommal]

The more I thought about it this morning, the more I agree about the stove pipe. Since it's a double-wall, the Laser temp reading SHOULD be cooler.

The "distance" from the source, with the Laser is definitely a consideration. I tried it from about 6 ft away, and then about a foot and a half away (the results as stated in the OP were the result of the closer proximity).

Are there any issues with taking the Laser read of the logs, thru the glass? (In terms of accuracy of the read, or deflection, or whatever, because of the glass).

1100ºF seems very high to me, and not the typical responses I see in here, for burn temps that are "cruising speed."

-Soupy1957

Your reading will be more indicative of the glass temperature than the wood temperature. Infrafed energy does pass through stove glass to some extent, but the glass is not invisible to infrared radiation like it is to visible light frequencies.

The other issue with infrared surface temperature measurement is the issue of "emissivity". Different surfaces (textures, colors, materials) have different emissivity in IR wavelengths. What this means is that different materials that are the exact same temperature will read differently to the IR device. Likewise, two different materials that give the same reading on the IR device could be very different in actual temperature. Every IR thermometer is calibrated assuming a given emissivity. Errors can be many tens of degrees for surfaces that are several hundred degrees F, and the errors get larger with higher temperature.