I recently became interested in the subject of consumer and commercial food safety and have been studying it in my spare time. What puzzles me is why food (and I assume other substances as well) held at a constant temperature in a refrigeration unit do not have a homogenous temperature throughout. The most dramatic example of this fact that I’ve encountered so far is a YouTube video I found that demonstrated how to properly calibrate a foodservice thermometer. (I wish I could find the video again so I could better illustrate what I’m talking about, but somehow it’s been lost in the aether. Despite repeated attempts, I just can’t find it.) The video was produced by NIOSH, so there’s little question as to its authoritativeness. The narrator of the video took a plastic cup and filled it with crushed ice to within about half an inch of its rim. Then he filled the cup almost to the very rim with cold tap water and stirred it vigorously for several seconds. He then attached a probe to a dual-function infrared thermometer similar to this one, which is a model commonly used in the foodservice industry. He first took a reading near the bottom of the cup, which read 32° F, as you would reasonably expect. Then he took several more readings throughout the cup—some near the middle, and some closer to the top. Despite the fact that the cup had been freshly stirred, he obtained readings all over the place—a couple of the higher readings were even in the low fifties! This even happened when he took subsequent readings back at the bottom of the cup. He explained that this is a perfectly normal natural phenomenon, and illustrated why it is important to know what you are doing when you calibrate a thermometer. But why is this? What causes the wild variations in temperature in a medium that intuition suggests should be homogenous?
I must have done something wrong, or the titled was deliberately truncated. The last word in the title is ‘sampled,’ followed by a question mark. Possible to fix?
You reached the maximum number of characters. I edited out “significantly” to make the rest fit.
Many thanks, and may the birds fly upside-down over your car.
It is quite impossible for anything held at a constant temperature long enough to not be at a uniform temperature throughout. However…
“Long enough” has no simple meaning. It means however long it takes for all temperature variations to even out through the heat flow they generate, and that depends on how much heat there is to flow, and how fast it flows, and nature of the eventual sink (or source) for heat. For example, a small glass of water standing on the counter in a shaded room should be in equilibrium within seconds to minutes. At the other extreme, the Earth’s core has barely cooled since the formation of the Solar System, and will probably be only a little bit cooler when the Sun turns into a white dwarf in 10 billion years or so.
“Uniform” is a bit in the eyes of the beholder. In fact there will always be fluctuations in the measured temperature, momentary excursions away from the mean, and the size of these fluctuations varies with the size of the system and its coupling to any other fluctuating heat reservoirs in the vicinity. Intrinsic fluctuations (due to the molecular nature of matter) are too small to be measured within ordinary instruments, usually, but coupling to external heat reservoirs is another story – if there were, for example, currents of warmer and cooler air in your fridge coming from a fan, recently inserted warm items, door opened a few moments ago, et cetera, that would considerably increase the fluctuations expected in something that may otherwise have been at equilibrium.
Finally, and perhaps most relevant to the example you cite, temperature is not identically equal to what is shown on a thermometer. It’s possible to not use the thermometer right, or for it not to be working right. In this case, it occurs to me that measuring the IR spectrum of light from your cup of water and translating it to a temperature may be more than the simple software in the beast can handle accurately. One assumes it’s programmed to assume the object is a black body, but your plastic cup may not approximate that, or more precisely may approximate it to different degrees at different places on its surface, particularly if there are external sources and sinks of heat, e.g. air currents. Imagine, for example, that a momentary puff of warmer air momentarily warms a patch of plastic at which you happen to be pointing your IR thermometer gun. There may be a very local very shallow and short-lived excursion in temperature, but of course that may be all it takes for your reading to be inaccurate.
It’s tough to tell without the video, but my guess (working in the food service industry) is that the video was trying to demonstrate why you shouldn’t use an infrared thermometer except in specific cases. One specific case is the first one you saw…the product is freshly stirred and you want to get an approximate temp (is the cold food well below 41? Is the hot food well above 135? or if it’s heating or cooling is it heading in that direction?). Also WRT them being commonly used, you’ll notice nearly every chef you see in a restaurant carries a probe thermometer in his or her sleeve pocket.
Did he then pull out a probe thermometer and insert it into the ice water? Probably after showing you the dimple on the probe and explaining that it needs to be inserted to that point. Then it either read 32 exactly or the video was about calibrating and he turned the nut on the back and set it to 32.
Anyways, an infrared thermometer only measures surface temp. All he was doing was getting the temp of the outside of the cup. You already know that it’s well above freezing since water condensates on it, not freezes. Also, those infrared thermometers, as nice as they are, are notoriously bad and temping things when they’re not being aimed directly at the product. You can either have the problem you saw, or reflection issues when people try to do it through glass cases or other things.
(Joey P, certified food safety manager)
You might be just the perfect person to ask. I’ve read the entire ServSafe text, and I’m part way through a second reading. But I’m looking for something a little ‘meatier’—something more technical, and geared towards somebody who has a fairly solid grasp of the basic biological sciences. Can you recommend anything? What do you think my next logical step should be?
I’ve always wondered about this.
Back in college Thermo lab, we’d use, for example, a cup of water and ice as a 0C reference point. The prof. would just dump the ice into the water and immediately start doing thermocouple calibration. Didn’t seem right to me.
I’m not following what’s being asked and assumed here. It appears that the OP watched a video of a demonstration of how different types of thermometers will give different temperature readings of a known constant temperature fluid when used or calibrated improperly… but is thinking the video demonstrates that those thermometers were actually giving accurate readings and that the temperature of food held at a constant temperature actually fluctuates for some unknown reason.
But I might be confusing something here… how do you “attach a probe” to an infrared thermometer… how do you get temperature readings from the bottom, middle, top of a cup of liquid with a sensor designed to measure surface radiation/heat… what’s the evidence that a bowl full of jello that’s set in the fridge overnight would read 4C in one spot but 14C in another?
I would say that food items held for multiple hours or days at constant temperature DO have constant temperature throughout. You would notice a difference when putting food of 1 temperature into a place with a different constant temperature: a thawing turkey in the fridge will, while it’s thawing, have varying temperatures depending how deep inside you measure, and that same turkey being placed back in the fridge after it’s been cooked will also have varying temperatures throughout while it cools back down. But after sitting in the fridge for a week and reaching equilibrium, it will indeed be the same temp throughout …although improper use of a thermometer might make it appear not to.