What is a thermometer?
A thermometer is a device that measures changes in temperature. (Temperature is the average molecular kinetic energy in a substance.) If a thermometer is calibrated to a known temperature, then it can be used to accurately determine absolute temperature.
All thermometers work by measuring a change in a material that reacts to temperature changes.
A mechanical or dial thermometer uses two strips of different metal bonded together. The different metals have dissimilar expansion rates as temperature increases. Because of this, the bimetallic strip curls when the temperature changes. When constructed in the shape of a coil, the curling metal can be used to alter the angle of a needle. A dial thermometer is constructed by placing the needle in front of a static background and marking the background with known temperatures.
Bulb thermometers are based on the expansion of liquids. A liquid is encased in a thin transparent tube with a small reservoir at one end. As the liquid in the reservoir expands or contracts due to temperature change, the level of the liquid in the tube also changes. The thermometer is read by matching the level of the liquid with markings that label known temperatures. Mercury is a common liquid used in bulb thermometers, but should not be used in the kitchen because mercury is a poison and unsafe if the thermometer is broken.
Most bulb thermometers for kitchen use are filled with an alcohol (stained red for easy reading).
Electronic or digital thermometers operate based on an electronic component that reacts to different temperatures. Most digital kitchen thermometers use a thermistor (thermoresistor) - a device whose resistance changes due to temperature. When a current is passed through the thermistor, a voltage drop occurs. This voltage can be measured and a small microcomputer calculates the temperature based on the voltage. The temperature is then displayed for the user to read. Another technique, is to take advantage of the principle that joining two different metals causes a voltage potential between them that is dependent on the temperature of the junction (Seebeck effect). By reading the voltage between two metal wires joined at one end, the temperature can be calculated. Unfortunately, connecting a voltmeter to the thermocouple introduces more thermocouple junctions which alter the readings. By using a technique called cold-junction compensation (outside of the scope of Cooking For Engineers), the voltage can be properly read and temperature accurately calculated. Because of their high accuracy, thermocouple thermometers are popular in engineering and scientific practices, but their high average sales price (well over $100) generally precludes them from home kitchen use.
Many other types of thermometers using other methods of measuring temperature (like infrared thermometers that measure surface temperatures) are available, but not commonly used in a household kitchen.
Instant-read thermometers is a marketing name given to any type of thermometer (digital or dial) designed to be plunged into the food being cooked when it is nearly finished cooking to determine internal temperature. The thermometer does not work instantaneously and usually should not be allowed to persist in cooking temperatures (the thermometer needs to removed).
Probe thermometers are digital thermometers with probes connected to long wires. The probes hold a thermistor and is placed within the food while a connected base station reports the current temperature. Almost all probe thermometers have an alarm facility to alert the cook when a preprogrammed temperature has been reached.
Fry or candy thermometers are designed to measure the temperature of a cooking liquid (usually oil or sugar). The thermometers are partially submerged into the liquid and the temperature monitored during the cooking process.
When using a thermometer, speed is an important factor. Ideally, we'd like to know the temperature of the oil or the roast at this instant. If a thermometer takes a long time to report the temperature, the food could become overcooked or lose a lot of heat while you're waiting for a reading.
I pulled out five different thermometers (3 instant-read, one probe, and one fry/candy thermometer):
- ThermoWorks Thermapen - A digital instant-read thermometer that uses a thermocouple; $85
- ThermoWorks RT-301 Low Cost Pocket thermometer - A digital instant-read thermometer that uses a thermistor; $14
- Polder Cooking Thermometer - A digital probe thermometer with timer, clock, and alarm - $20
- Taylor Classic Style Meat Dial Thermometer - A basic dial thermometer - $10
- Taylor Candy and Deep-Fry Thermometer - A fry/candy bulb thermometer - $13
I tested the speed and accuracy of the thermometers by submerging them in ice water at 32°F (0°C), ambient temperature water at 70°F (21.1°C), and boiling water at 212°F (100°C). I tested the time it took for the thermometer to produce an accurate reading from (1) ambient to freezing temperature, (2) freezing to boiling temperature, and (3) ambient to boiling temperature. I also recorded the time it took for the thermometer to reach an approximate reading by recording times at 40°F (4.4°C) and 200°F (93.3°C). Two of the thermometers were incapable of indicating freezing temperatures, so I could not record the ambient to freezing time. However, I did place those thermometers in the ice water for several minutes before plunging them into the boiling water for the freezing to boiling times. All of the thermometers reached the reference temperatures as expected.
The tests were run several times and the average times are shown in the following table. The times in [brackets] represent the time it took to reach the ballpark value of either 40°F (4.4°C) or 200°F (93.3°C).
|Model||Ambient to Freezing||Freezing to Boiling||Ambient to Boiling|
|ThermoWorks Thermapen||3 sec [1 sec]||3 sec [1 sec]||3 sec [1 sec]|
|ThermoWorks RT-301||31 sec [15 sec]||24 sec [13 sec]||18 sec [9 sec]|
|Polder Probe Thermometer||16 sec [9 sec]||32 sec [11 sec]||36 sec [10 sec]|
|Taylor Dial Thermometer||N/A||80 sec [24 sec]||70 sec [22 sec]|
|Taylor Candy Thermometer||N/A||45 sec [17 sec]||33 sec [14 sec]|
The Polder probe thermometer took around four seconds to produce any change in reading. For example, after taking the probe from ice water and plunging it into the boiling water, the base module continued to register 0°C for four seconds before changing to around 60°C. I think the slow update time caused the thermometer to lose a few seconds when it might have been internally registering boiling or freezing when the display was not.
Also, I donned heat resistant gloves while testing the Taylor dial thermometer in the boiling water because it required me to hold the thermometer at the edge to the hot pan. The radiant heat and steam would have been dangerous without protective gloves.
As you can see from the results, there is no comparison between the Thermapen thermocouple thermometer and any of the others. The speed is amazing - about one second for a ballpark value and two more for dead accurate. Three seconds is such a short amount of time that the time it takes to position the probe in the middle of the roast is almost the same time it will take for the Thermapen to take a reading. At $85, it is quite expensive, but compared to other thermocouple thermometers, this device is a bargain. It also had the highest maximum temperature (572°F or 300°C) which makes it capable of measuring the temperature of deep frying oils as well.
The Taylor dial thermometer was so slow that I was afraid that my gloves would start getting too hot for me to hold the thermometer. Its thick probe also makes it useless except for large roasts, except you'll need to open the oven door for a full minute before you could guess if your roast was between medium-rare or medium.
If you only have space for one thermometer, save up and treat yourself to the ThermoWorks Thermapen. It's fast. Very fast. It also comes calibrated with documentation to prove it. You can even calibrate it yourself (which is a feature missing in almost all digital thermometers)! The probe tip is very thin (like a thick needle), so measuring the temperature of a 1/4 in. steak is no problem at all. The temperature range is greater than most thermometers and it's also really easy to read. Recently, I've seen other companies selling unbranded thermometers of similar shape and design to the Thermapen, but at a discounted price. I've heard these thermometers may use thermistors and are much slower, so it's best not to take a chance and buy direct from ThermoWorks.
A fry/candy thermometer is also nice to have around if you do any confectionery or deep-frying in a pot.
The Polder probe thermometer is one of the lower cost probe thermometers I've seen and works like a charm. It is a bit slow so I always set it a few degrees below my target temperature. If you make roasts or anything that cooks for a long time (even bringing a large pot of water to a boil), this is your "fire-and-forget" thermometer.
The others? Save your money.
[Link to Amazon.com to buy this thermometer]
ThermoWorks RT-301 Low Cost Pocket thermometer
Polder Cooking Thermometer
Taylor Classic Style Meat Dial Thermometer
Taylor Candy and Deep-Fry Thermometer