Thermometer (In)Accuracy... (Briefly Revisited)

[Peter B.]

Yesterday, I received three new stove thermometers ordered directly from the manufacturer. The single flue probe I've been using is about twenty years old, and not only did I think it might be time to replace it, but with several monitoring points on my stove, I thought it would be of interest to compare simultaneous readings at the different locations.

But first, I put all the thermometers - old and new - in the kitchen oven at 250*, along with an oven thermometer as well. Here (approximately) is what I got from each:

Oven - 225*
(New) stovepipe surface - 300*
(New) cat probe - 500*
(Old) flue probe - 600*
(New) flue probe - 700* (+)

In actual use, the thermometers also provided disparate results and different readings when swapped from one stove location to another for direct comparison. Even the two flue probes - old and new - didn't agree in temp ranges over 500*.

Needless to say, these 'findings' inspired a *whole* lot of confidence... and I have NO idea which thermometer I can actually trust... if any.

Yes, I was aware that the thermometers would not be perfectly accurate, and that only relative readings from a single thermometer in a single location were really meaningful for stove operation, but I sure didn't expect such a wild range of readings.

I would have spent my money better either buying two <identical> thermometers or a single IR to make periodic readings to compare with the old flue probe.

Lesson learned. I've reverted to simply using the old flue probe... because at least it provides 'familiar' readings.

Don't know offhand what to do with the others...

Peter B.

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[begreen]

Wow, that's certainly confidence inspiring. I bought a cheap Drolet surface thermometer from our local True Value for a friend last year. According to my IR thermometer, it checked out within 20 degrees at 550.

Have you confirmed the accuracy of the oven thermometer? Have you contacted the manufacturer of these thermometers?

 

[edthedawg]

I wonder more about the disparity between Tmetal and Tgas. Were the measurement contact points of the probe thermometers hanging in air? Or resting on a sheet? on a rack?

 

[SteveT]

I wonder more about the disparity between Tmetal and Tgas. Were the measurement contact points of the probe thermometers hanging in air? Or resting on a sheet? on a rack?

This makes sense to me. Perhaps the very high reading was not incorrect, it was that the probe had in fact heated to that level by absorbing radiant heat of the calrod.

The best way to calibrate (although it is for only a single temperature) is to suspend the probe into boiling water. It must be suspended so that it doesn't pick up heat from the container. Obviously you're looking for 212F (minimal variation for pressure altitude).

 

[Corey]

225 / 230 / 250, I'd expect something like that from relatively inaccurate 'stove' thermometers. The 500,600, 700 readings from the probes seem way out of line. Is this is some type of thermocouple with a display unit? If so, are you sure the TC probe and the display are matched. You might see numbers like that if you have a type K probe with a type J readout (or pick two other letters...B, E, J, K, N, R, S, T), etc Many times, the display will have switches or a way to program it for a specific TC type. One other option would be to put a volt meter across the probe leads and read what the mV output actually is. If course if they're some type of mechanical thermometer, this is all out the window!

 

[Peter B.]

Wow, that's certainly confidence inspiring. I bought a cheap Drolet surface thermometer from our local True Value for a friend last year. According to my IR thermometer, it checked out with 20 degrees at 550.

Have you confirmed the accuracy of the oven thermometer? Have you contacted the manufacturer of these thermometers?

I don't have a method of verifying the accuracy of ANY of the thermometers at present, I'm afraid.

No, I haven't contacted the manufacturer... yet. I'm not out a huge pile of cash, and I wonder if it's really worth doing so... but I confess, I'm not real happy with the products supplied, when I had previously trusted their name (deliberately unmentioned here).

--

I wonder more about the disparity between Tmetal and Tgas. Were the measurement contact points of the probe thermometers hanging in air? Or resting on a sheet? on a rack?

The oven test was done with all thermometers resting on a metal sheet (with probe tips touching the sheet)... and you're right, that may have provided some bogus results, but as an example of actual use disparity, in the same location, the new flue probe and the new cat probe read about 300* apart. The stovepipe surface thermometer reads (roughly) half of the flue probe thermometer at the same height (to be expected ???).

--

This makes sense to me. Perhaps the very high reading was not incorrect, it was that the probe had in fact heated to that level by absorbing radiant heat of the calrod.

The best way to calibrate (although it is for only a single temperature) is to suspend the probe into boiling water. It must be suspended so that it doesn't pick up heat from the container. Obviously you're looking for 212F (minimal variation for pressure altitude).

Yes, I could try to verify that specific single temperature point, but if there is such wide variation in readings at other temperatures, it's not of that much use.

Peter B.

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[Jags]

Get enough of them and you can use the little round ones for poker chips. Eh, just trying to help. :shut:

I get my confidence by using the IR thermo pointed at my little round rutland. They read within ~20 deg F of each other at almost any temp that I can expect. Do you have any burning friends that may have an IR to borrow?

 

[Peter B.]

Get enough of them and you can use the little round ones for poker chips. Eh, just trying to help. :shut:

I get my confidence by using the IR thermo pointed at my little round rutland. They read within ~20 deg F of each other at almost any temp that I can expect. Do you have any burning friends that may have an IR to borrow?

I guess it's (long) past time I bought one of my own...

I know there are dozens of threads here on recommendations (and sources) for IR's, but if no one minds too much, I'll ask again. (From another thread of my own, I learned that the less expensive ones won't read much beyond 1000*, but I'd like to be able to do that at least.)

Thanks Much.

Peter B.

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[Jags]

Mine is one that "won't" hit your 1000 mark. It will read pretty accurate to about 850. Its a hobo freight cheapy.

 

[begreen]

I have the ATD-701. It was about $40 and works pretty well. Tops out at around 1022.

(broken link removed to http://www.tooldesk.com/products/productDetail.aspx+id+2084)

 

[EddyKilowatt]

Do your probe units have a visible bimetal coil located right behind the dial?

I've been kind of wearing out my welcome around here pointing out that those 'probe' thermometers sold in the stove trade are in fact surface temperature thermometers, 'calibrated' to estimate pipe internal temperature based on surface temperature.

I arrived at that conclusion by doing exactly the same oven test you did (though with a sample size of one), and getting very similar results... along with noticing that the 'probe' was steel, not at all a good conductor of heat compared with copper and therefore pretty much useless for conducting the internal pipe temperature out to the bimetal coil located behind the dial.

I would agree with what you already said, they are adequate for comparative measurements in a single location on a single stove, but any hope of actual accuracy is pretty much zero.

Go for an industrial unit from Omega, Taylor, or Tel-Tru, made of stainless and with the bimetal element down inside the probe where it belongs.

Eddy

 

[madison]

Suggest using a pizza stone, or some large solid metal object to rest/attach the probes, allow the oven and stone to reach the desired and equilibrate. As well as possibly trying a higher temp to compare the devices.

 

[Peter B.]

For what it's worth, I thought I'd provide this further observation...

I 'tested' another set of thermometers in the kitchen oven at various temps and discovered that probes don't (remotely) reflect accurate temperatures when fully enclosed within a heat source. On the other hand, stovetop/stovepipe (magnetic) thermometers should read reasonably closely to the oven temperature.

Again, probably <none> of the inexpensive thermometers we use with our stoves are likely to be dead accurate, but can only provide relative reference temperatures... unfortunately.

I'd prefer better overall accuracy, but I guess it's not available on the cheap.

Peter B.

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[Cluttermagnet]

I have a Condar probe thermometer (broken link removed to http://www.condar.com/probe_meters_woodstoves.html) I'm very happy with it. Have not made any measurements to confirm accuracy, yet I have a strong gut feeling it is pretty darned accurate. They claim +/- 5 percent of full scale. Full scale is 1700 deg F, so that's +/- 85 degrees.

They recommend burning with flue gas temperatures between about 400 to 900 deg F for good efficiency/ low creosote. (Just guidelines- consult the stove manufacturer's instructions) They say that flue gas temperature should run about 50 percent higher than the stove surface temperature.

If it were me, I'd not write off any of those new thermometers just yet.

I'm measuring flue gas temperature only. It behaves rationally- what I see indicated correlates well with what I see going on in the firebox, and also the presence or absence of visible chimney smoke, etc.

 

[Peter B.]

Just to clarify...

I've owned a similar flue probe for twenty years and have relied on it to provide reasonably accurate readings... though I've never had a way of calibrating it.

In actual use (not kitchen oven tests), one of the new probes reads about 100* less than the old, while two other new probes (one of them essentially identical to the old) read 200*-300* higher under the same circumstances... usually at temps above 500*... a rather important range for monitoring purposes.

At this point, I have no idea which - if any - of the thermometers to trust as 'accurate'.

So, the old probe with the 'familiar' temp references remains in service.

--

Why so many thermometers in the first place?

I had hoped to find two that closely agreed, so that I could monitor two different points - stovetop internal and flue internal (at 12" above stovetop) to make some sort of meaningful comparison of temperatures at those points.

I can now do that with two of the new probes, but their actual accuracy is still very much in question.

Plus or minus 5% error through the range is NOT what I'm seeing.

Again, they still provide <relative> references, but accuracy is what I was seeking.

Peter B.

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[oconnor]

You could always get one like mine - not really a thermometer, more of a trend indicator - at least it is since I baked the paint of it during an overtemp.

I must say though, it seemed accurate - 900*F seemed about right given the glow.

 

[Cluttermagnet]

Let me mention a couple more things Condar says on its package for the 3-39 probe thermometer. Point number 1- the indicated flue temperature will be "slightly high" if you fail to observe the minimum 18" above the top of the stove. It may read about 6 percent low if placed in a horizontal flue pipe. Point number 2- due to hot corrosive flue gasses, if "used continuously, there is the possibility in some installations that the stem will corrode and cause innacurate readings when in service for more than 4000 hours." (There are about 8760 hours in a year) What does the stem of your old one look like? Clean, or beat up?

Yeah, I hear you about the significant differences in the temperatures indicated by the new ones. Apparently, one or more of them are crap. But don't rule out that your old one may have drifted significantly off of accurate, if it ever was. ;-)

 

[Todd]

To clean the crud off your temp probe, soak it in a vinegar and water solution overnight. Don't scrape them, they have a metal coating over a copper probe and you will srew up the readings. I know I've done it.

 

[EddyKilowatt]

I'd prefer better overall accuracy, but I guess it's not available on the cheap.

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Alas, Tel-Tru is no longer selling industrial glass-and-stainless dial thermometers direct from their website. That's where I got mine a few years ago.

I do see that Omega Engineering stocks, and will sell direct,

this little guy for $40 (Q-series):

Available with 4" stem in a 150-750F range, which is the range I use and have found adequate when I have good draft. These units are also glass-and-stainless, claimed accurate to 1%, intended for industrial use, and also actually measure the temperature right inside your flue rather than estimating it from outside.

Omega also list 200-1000F models for their Q, G, and H series (1", 1-3/4", 2-5/16"), but not stock. But with summer coming on, having to wait a few months (assuming they'll take your order) might not be the end of the world.

So there are options out there, for a few bucks more. I don't claim that anybody needs to measure their stack temp to one percent accuracy, but I appreciate that these units are telling me what is actually going on inside the flue, and they are lifetime stainless construction. (Ahem, as long as you remember to remove them before running the cleaning brush thru the pipe... don't ask me how I know.)

Eddy

 

[begreen]

I'd prefer better overall accuracy, but I guess it's not available on the cheap.

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Alas, Tel-Tru is no longer selling industrial glass-and-stainless dial thermometers direct from their website. That's where I got mine a few years ago.


Available with 4" stem in a 150-750F range, which is the range I use and have found adequate when I have good draft. These units are also glass-and-stainless, claimed accurate to 1%, intended for industrial use, and also actually measure the temperature right inside your flue rather than estimating it from outside.

Omega also list 200-1000F models for their Q, G, and H series

Eddy

Good info. If my Sandman ever dies, I will be getting a TelTru. They look like fine instruments.

(Ahem, as long as you remember to remove them before running the cleaning brush thru the pipe… don’t ask me how I know.)

Ouch!!

 

[dante2]

I got an IR therm at Autozone for 19.95. It's their brand (OEM) and it's good to 689F at least I think so I haven't got the stove that hot yet. It's within 20F of the rutland that I bought at ACE.