Kitchen Thermometers

Since a major aspect of cooking is the heating of ingredients, temperature is an important topic. In cooking, we are primarily concerned in the temperature of our cooking environment (such as deep frying oil) and the temperature of our food (like a roast). With some foods, if the temperature of an ingredient has passed a certain point, it's texture and flavor changes such that we call it overdone. With other dishes, like pot roast, a minimum temperature must be reached to produce the desired effect. With lean steaks and roasts, there is a very small window which enables us to serve the dish as the perfect temperature. A thermometer is an invaluable tool in the kitchen.

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.

Some thermometer types
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).

Since a major aspect of cooking is the heating of ingredients, temperature is an important topic. In cooking, we are primarily concerned in the temperature of our cooking environment (such as deep frying oil) and the temperature of our food (like a roast). With some foods, if the temperature of an ingredient has passed a certain point, it's texture and flavor changes such that we call it overdone. With other dishes, like pot roast, a minimum temperature must be reached to produce the desired effect. With lean steaks and roasts, there is a very small window which enables us to serve the dish as the perfect temperature. A thermometer is an invaluable tool in the kitchen.

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.

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Some thermometer types
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.