If Goldilocks had broken into the home of three bears who had inexplicably left slabs of meat cooking in an oven, to know which one was done just right, she would have needed a digital thermometer. If the internal temperature were too high, then it would be overdone and unpleasant to eat. If the internal temperature were too low, then it wouldn't be cooked enough to destroy harmful germs like Salmonella and E.coli.
Why digital thermometers
Internal temperature is the only reliable way to tell if meat is done properly. Meat colour, juice colour, and feel can all be a deceptive way to judge if food is over or under done. A master chef with familiar equipment and regular cuts of meat can use experience to produce a Goldilocks result, but using a digital thermometer is a safer route for the rest of us.
Food poisoning, which people often incorrectly attribute to “stomach flu” is unpleasant and can even be fatal, particularly for young children, the elderly and those with compromised immune systems. Digital thermometers are the best way to measure temperature. Dial thermometers or other gadgets are unreliable.
Now, I'm generally familiar with how mercury expands or contracts in an old school thermometer, but I wondered what the story was with digital thermometers.
A Hot Couple
The fastest and most precise digital thermometers use a thermocouple. When different metals are connected, heat causes this system to generate a tiny voltage that can be calibrated to measure temperature. Thermocouples have two little wires of different metals welded at the tip. The most common form is Type K, involving nickel and chromium. The sensor is small, so you can measure thinner cuts of meat without leaving big holes. This also lets you check a number of locations in the meat. These “instant read” thermometers often look like a wand (incendio?!) and some are foldable. They are used near the end of the estimated cooking time to check the temperature rather than left in.
Thermistors are short for thermal resistors. They measure how resistance to electrical flow changes with temperature. The device sends an electrical current through a wire in the probe which has a resistor in the tip. The resistor is a ceramic semiconductor bonded at the tip with temperature sensitive epoxy. The meter measures the voltage across the resistor.
The relationship between voltage and temperature is not a straight line, so you need a computer to calculate it. Thermistors are cheaper than thermocouples but they may be slower, slightly less accurate and thicker, yet not as rugged. They are best for leaving in a large roast. They usually have a metal probe that houses the thermistor, connected wirelessly or through a cable to a readout which is outside the grill or oven, in the comfort of your kitchen or patio.
Resistance is not futile
RTD stands for resistance temperature detector. It is also based on the change of electrical resistance with temperature. They are only made from metals like platinum, nickel and copper. They have a slower response time and so are of the leave-in type. They seem to be more common for industrial uses, in part because they have a wider temperature range than thermistors.
The commercial products often do not describe what mechanism is being used. Probably other features like usability, features, and price will concern you more. But now, just for fun, you can also grill the salesperson about their thermometer knowledge.
If you have any hot tips on using digital thermometers or really gross stories about getting sick from eating undercooked meat, share them in the comments below.