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I purchased an analogue instant-read meat thermometer (link to the precise product).

I fried some diced chicken in a little lime juice and olive oil in a frying pan on a medium-high heat. I periodically measured the temperature by sticking the end into the centre of a piece of chicken.

At no point (even after 10+ mins) could I get the thermometer to read above 40 C (whereas it should reach at least 68 C), even increasing the heat had little effect. I ended up overcooking the chicken in an attempt to increase the temperature measurement.

I stuck the thermometer in boiling water to check it was working properly and it indeed gave a reasonable measurement.

Question: Can anyone explain what I may have been doing wrong?

Is there a knack to performing the measurements? Are the thermometers intended for measuring large pieces of meat only? Or did I likely fail to heat the chicken correctly? In which case, what should I do to get the temperature up? Like I said, even on a high heat still wouldn't push the temperature above 40 C.

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Here's a technical explanation as to what happened. An analog thermometer works on the principle that different metals expand more than others when heated. There is a bi-metallic strip in the pen part of the thermometer in the shape of a spiral coil, which is attached to a piece of metal which turns the pointer. The outside of the strip expands more when heated and contracts more when cooled than the inside, so when heated the coil will twist the pointer in the direction of the spiral.

Cutaway of an analog thermometer

As the pen part of the thermometer heats up the coil changes shape and adjusts the temperature. As you can see from the picture the sensitive area of the thermometer is the length of the coil, that entire area needs to be heated up to the same temperature in order to read correctly. In your case what you were trying to measure was too small and had to compete against the ambient temperature of the air.

Digital thermometers work on a different principle entirely, using an electrical device called a thermocouple on the end of a probe. enter image description here

The bead at the end is the actual sensor, and as it is much smaller it is therefore more precise than a bi-metal coil. However, I still think one of these wouldn't work because again the size of the meat isn't enough to heat the probe's sheath enough for the sensor to read correctly.

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With small pieces of food, you can tell that the meat is done by checking the color (it should be opaque for chicken), the ease of inserting a fort or knife (it should give easily), and the color of the juices coming out (they should be clear).

This article from the University of Georgia notes that using these methods with small pieces is a common thing; the above methods will help specifically for chicken. As well, it explains that the main concern with small pieces is the surface bacteria / contamination, rather than the inside of the pieces.

Back in times when having a thermometer was far less common, this is how housewives and other cooks would test their foods. If you read or watch anything related to cooking in the past, the material would either describe how to test for doneness using similar methods, or else exclude the fact altogether, assuming the person cooking the food would already understand these gauges of doneness.

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I don't believe that thermometer is designed to measure small pieces of meat. From the equivalent Amazon page:

"For best results, slide the entire shaded area of the probe into cooked meat, avoiding large bones."

If you want to measure something of that size, I would look for a thermometer that takes it's reading at the very tip, rather than what appears to be the average over a large portion.

Highly recommend something by Thermoworks (Thermapen, MK4 or ThermoPop). They're accurate, very fast and read right at the very tip. (No affiliation, just love the product).

Aside from that, I typically wouldn't use temperature to check whether a stir fried (small dice) piece of meat is cooked. I'd probably just cut one piece open and look for it to be cooked through. If it's small enough, by the time the outside is at the right colour, the inside should be cooked through. If you're not confident, and really want to check the temperature, you'll need a different thermometer.

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