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Meat Thermometer Puzzle
Old 05-26-2014, 09:07 AM   #1
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Meat Thermometer Puzzle

I have a ChefAlarm meat thermometer and a traditional instant read thermometer.

When I calibrate them in a glass of water, both are accurate and agree on temperatures. They read 32 degrees in an ice bath, and 212 in boiling water.

But in use, I get wildly different readings:

TempDiscrepancy.jpg

In that picture, the traditional probe is correct. The two probes are touching.

Why do you think that is?

My theory is that it's caused by the exact position of the probe, but a 30 degree discrepancy?
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Old 05-26-2014, 09:22 AM   #2
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Interesting. I'd say it absolutely has to do with the exact placement of the probe. That cut of meat looks thin (tough to say exactly from that angle), and if it is, your margin of error is quite small. A thicker cut will give you a larger sweet spot.

This also reminds me of the old adage: "A man with one watch always knows what time it is. A man with two watches is never sure..."
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Old 05-26-2014, 09:31 AM   #3
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I agree that the picture makes it look like the meat is only about half an inch thick, maybe ⅝", so there is no way I would use anything but my Thermapen to check it.
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Old 05-26-2014, 10:08 AM   #4
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The two probes are touching. Could one of them be more sensitive to the conductivity of the other probe? Do they get the same readings when probing separately, say take a measurement with one then a few seconds later with the other?
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Old 05-26-2014, 10:17 AM   #5
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Due to a typo error, the thread title reads "Meath Thermometer...". At first glance, I thought I saw "Meth", and wondered what T-Al was up to.

Anyway, the thinness of the pork chop (I think that's what it was) caused the error, as these thermometers have relatively large probes with a significant thermal mass. Also one can never be sure if the probe tip has penetrated to the other side with a thin piece of meat.

I usually probe from the edge to be sure that an inch of probe is centered in the meat, even when working with an 1" thick beefsteak. Of course this is easy with a Thermapen, but I often sear steaks on a pan, then put in a broiler to finish and need to use a thermometer with a remote probe that can go in the oven.
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Old 05-26-2014, 10:27 AM   #6
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Could it be that the response time of the two devices is different?

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Old 05-26-2014, 11:14 AM   #7
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Quote:
Originally Posted by TromboneAl View Post
...
My theory is that it's caused by the exact position of the probe, but a 30 degree discrepancy?
Yes, easily. I agree with others on this.

In addition, the mechanical probe probably needs to be submerged more in whatever you are measuring. The electronic ones are typically more localized at the tip.

So there are really two 'sweet spots' to deal with - the 'sweet spot' in the meat, which is tiny in a thin cut like that, and the 'sweet spot' of the sensor.

Although I doubt it is a major problem, a two point calibration doesn't tell you all that much about the accuracy in the 150F range. You could have a linearity issue in the middle. A rough check is to put both probes in a hot water bath of ~ 150F, and see if they agree. Yes, they could both be off by the same amount, this isn't a 'traceable' sort of test, but it gives a reasonable confidence factor, especially if you have a third one to check against. Sorry if that was TMI, but metrology/calibration was a major segment of my working life, and it's hard to shake old habits!

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Old 05-26-2014, 11:55 AM   #8
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Could it be that the response time of the two devices is different?

-gauss
That would be my guess as well
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Old 05-26-2014, 11:58 AM   #9
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That would be my guess as well
I assumed that T-Al did wait until both readings settled - but you know what happens when you assume!


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Old 05-26-2014, 12:04 PM   #10
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... and the time for him to take out the camera or the smartphone. I do not think the thermal lag can totally explain the 30F difference; the response of even the slow probes is measured in seconds, not minutes. The exact location of the sensing element inside the thin piece of meat is important.

And then, who knows how big the sensing element of the mechanical dial thermometer is? Is it a rod or coil that runs the length of the probe?

By the way, the Thermapen uses a thermocouple (junction of dissimilar metals), while most of the less expensive probes use a thermistor (a resistor with a negative temperature coefficient). A thermocouple can be made extremely small, hence has a very small thermal mass, but takes more sensitive electronics and also requires an accurate cold junction compensation. The thermocouple probe also allows the Thermapen to measure up to 572F.

Thermapens rule!

PS. See the following illustration from Thermoworks.



For more info, see: http://www.thermoworks.com/products/...en_sensor.html
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Old 05-26-2014, 12:16 PM   #11
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Another point regarding any measuring instrument, unless it has been periodically calibrated against a legitimate standard it can be off.
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Old 05-26-2014, 12:30 PM   #12
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T-Al claims that his thermometers agree at water freezing and boiling temperatures, so I do not think they can differ by 30F at somewhere in between.

In another thread, I reported seeing a 10F difference between two electronic meat thermometers. I checked them by immersion in a glass of boiling water, and recorded the readings through the range as the water was cooling down. The "standard" was an engineering thermocouple probe hooked up to a DVM. I may repeat this experiment with the Thermapen included.
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Old 05-26-2014, 01:19 PM   #13
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Originally Posted by braumeister View Post
I agree that the picture makes it look like the meat is only about half an inch thick, maybe ⅝", so there is no way I would use anything but my Thermapen to check it.
+1

The precision of the probe is everything. The temp change from edge to center can be significant in meat.
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Old 05-26-2014, 01:28 PM   #14
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The temp change from edge to center can be significant in meat.
But of course!

Like 30F or more easily.

That's how one can sear the outside, and the center stays pink.
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Old 05-26-2014, 01:47 PM   #15
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T-Al claims that his thermometers agree at water freezing and boiling temperatures, so I do not think they can differ by 30F at somewhere in between. ...
No, I would not expect that much delta in the mid-range, I was just throwing it in for completeness. It might account for a few degrees though.

Nice diagram of the thermapen. I think the mechanical type like he shows are even more extreme. Those have a bi-metal (or maybe just something sensitive to thermal expansion?) strip that is twisted (like an auger) so it turns the dial as it expands/contracts. That strip is likely an inch long, I seem to recall taking a broken one apart one time.

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Old 05-26-2014, 01:58 PM   #16
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My first thought was that the ChefAlarm probably has the sensor at the tip, while the conventional one measures behind the dial, with the rod being a thermal conductor. I would then suspect that the ChefAlarm was a more accurate representation of the meat temperature. T-Al, why did you think that the traditional one was more accurate?

Possibly one way to test this would be to place only the tips into boiling water (with the rest of the rod cooler), vs. placing the entire rod in the water. My guess is that with the entire rod, both would agree, and with only the tips, the ChefAlarm would be higher.
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Old 05-26-2014, 02:01 PM   #17
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Yes, when the thermal sensing element is bigger than the object being measured, well, one ends up measuring partly the air temperature as the probe is not entirely immersed in the object.

Talking about the outside of the meat being much hotter than the interior, when I stuck the meat with the Thermapen, I observed that its reading first jumped up as the tip traversed the outer layer, then settled down to a lower temperature as I put the tip to the center of the cut. And this was a 1"-thick cut, not a roast. That Thermapen's response is that fast.

And as the probe was removed, the reading dropped back to that of the ambient air within a few seconds. Good stuff!
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Old 05-26-2014, 03:24 PM   #18
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If we keep up our nagging, eventually most FIREes will own a Thermapen. But in the meantime, I can understand the call of frugality.

So I would like to point out that if you go to their website, you can sign up for their email list. I typically get an offer for discounted products at least once, often twice a year.
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Old 05-26-2014, 03:29 PM   #19
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Thermoworks does have less expensive models with very reasonable prices. I am thinking about getting one, just for the heck of it.

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Old 05-26-2014, 03:36 PM   #20
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Originally Posted by braumeister View Post
If we keep up our nagging, eventually most FIREes will own a Thermapen. But in the meantime, I can understand the call of frugality.

So I would like to point out that if you go to their website, you can sign up for their email list. I typically get an offer for discounted products at least once, often twice a year.
LOL! I confess I have two. TWO! Earlier this year I picked up a "refurbished/demo" unit of their new backlit Thermapen for my outdoor grilling. A modest discount, but what the heck!
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