Lifter;
The meat thermometers, both instant read and keep in the meat, work by the same mechanism. That is true of both the digital thermometers and their analog counterparts. First, let me qualify my expertise in this field. I hold a Bachelor of Science degree in Electrical Engineering Technology from Lake Superior State University, with my senior sequence comprised of Control Systems.
Now, the digital thermometers work through an electronic componant known as a thyristor. The amount of current that passes through the componant is proportional to the temperature of its p-n junction. That is, the hotter it gets, the less resistance it has. The current is supplied by a battery, feeds through the thyristor, and powers a device (a transducer) that transforms the current into the digital numbers we see on the gauge. It is good for literally tens of thousands of cycles before breaking down. It is usually made of a silicon wafer doped with both positive ions on one side, and negative on the other, hence the p-n nomenclature.
The analog thermometers work on the priciple of material expansion/contraction characteristics relative to temperature. All materials expand and contract based on the ambient temperature of their surroundings. Metals tend to expand and contract rapidly due to there natural conductive properties. In the thermometer, two metals are joined together with welds to form what is know as a bi-metalic strip. The two metals have differing know expansion rates. As the strip is heated, one metal explands more rapidly than does the other, forcing the strip to bend at a predictable rate. The needle mechanism is spring loaded, with the end of the bi-metalic strip touching one side. The bending of the strip moves the end of the needle over the marks on the thermometer face. The thermometer is carefully calibrated at the place of manufacture to assure a reasonable degree of accuracy.
That same bi-metalic strip is what controlls many home furnace temerature controls, most toasters, toaster ovens, etc, and have a useful life of I don't know how many years. My furnace control is the bi-metalic strip type and has worked without fail in excess of thirty years now. I expect my toaster to last another ten years at least. The technology is very reiliable, if not quite as accurate as its digital cousins.
As for juice leakage, when the thermometer probe is inserted into the raw meat, there is virtually no pressure to cause fluid spillage. As the meat temperature rises, the outer surface heats first. The individual tissue cells swell, effectively sealing against the probe. There is very little fluid loss. A good way to check this is to examine the meat right around the probe. If the meat is clean, that is free of coagulated and cooked blood, then there has been little or no fluid loss. To prove this, pan fry a pice of meat. When the meat starts getting hot, poke it and immediately turn it so the pierced side is against the pan surface. You will notice that cooked blood darkens the pan surface and the meat around the pucture.
I tried the instant read thermometer along side my in the meat thermometer. I made the mistake of grilling with smoke without covering the glass of the stay-in thermometer and it became unreadable due to smoke stain. The roast was very juicy looking and I thought it should be done. When I poked it with the instant read thermometer, juices literally gushed out of the roast and fell into the fire. I lost much of the liquid that could have been used as a gravy base after carving.
Try it for yourself. Before I share my information, I make sure that I speak from experience and often, from the results of experimentation. I'm just that kind of guy. I have to know why things work as they do.
But I'm also the kind of guy, who when presented with a differing opinion, eagerly looks at the opposing point of view. That's often how I learn new things, and make my own knowledge base greater.
Now I can't really say that enough meat is lost, when poking with an instant read thermometer, to justify a great discussion on the topic. But then again, I'd just rather not take the chance. I don't like dry meat. I suspect that there isn't a great difference in end product though.
Oh, and know that I often respond to discussions this way. I am a scientific kind of guy. I have been so for as long as I can remember. But I am humble as well. I have much to learn in the world of cooking, and life in general.
Seeeeeeya; Goodweed of the North
