A bimetallic stemmed thermometer is a mechanical instrument used to measure the internal temperature of food and beverages. This device, recognized by its circular dial face and long metal probe, provides a direct reading. It is a reliable tool for monitoring the temperature of items during the cooking or holding process, typically measuring temperatures ranging from 0°F to 220°F (or -18°C to 104°C). Cooks and food service professionals use it to confirm that large cuts of meat or deep foods have reached the necessary thermal endpoint to eliminate pathogens.
How the Bimetallic Strip Works
The operation of this thermometer relies on the physical principle of thermal expansion, utilizing a component called the bimetallic strip. This strip consists of two distinct metals, such as steel and copper, that are permanently bonded together. Because each metal possesses a different coefficient of thermal expansion, they expand and contract at unequal rates when exposed to temperature changes.
The bonded strip is typically coiled into a helix or spiral shape and housed within the thermometer’s stem. As the temperature increases, the metal with the higher expansion rate elongates more significantly, causing the entire strip to bend or uncoil. This mechanical movement drives the pointer on the external dial face, translating the physical change in the coil into an observable temperature reading.
Primary Applications in Food Safety and Cooking
The bimetallic stemmed thermometer is widely used in food service for measuring the temperature of thick or deep food items accurately. Its long, robust stem is well-suited for inserting deep into large roasts, whole poultry, or stockpots of liquid. This allows users to verify that the center of a large item has reached a safe minimum internal temperature. The mechanical nature of the dial allows for continuous monitoring, though it should not be left in while the food is in the oven.
The temperature-sensing coil on the probe requires a specific depth, usually spanning two to two-and-a-half inches of the stem. This length requirement means the thermometer is not suitable for measuring the temperature of thin foods, such as hamburger patties or chicken breasts, where the sensing area cannot be fully submerged.
Compared to digital or thermocouple thermometers, the bimetallic model has a slower response time, often requiring 15 to 20 seconds, or sometimes up to a minute, to display the final, steady temperature. Despite this slower read, its simple, durable design and low cost make it a fixture in commercial and home kitchens for verifying temperature compliance.
Ensuring Accuracy: Calibration and Usage Techniques
To ensure reliable readings, users must follow specific techniques for usage and calibration. The temperature-sensitive area extends from the end of the probe up to a small indentation or mark on the stem, often called the dimple. When taking a temperature, the entire sensing area must be fully inserted into the thickest part of the food, avoiding bone, which can lead to an artificially high reading.
After insertion, the user must wait until the dial pointer stops moving, which generally takes between 15 seconds and one minute, before recording the measurement. Once the reading is complete, the thermometer probe should be cleaned and sanitized with hot, soapy water before being used again or stored. Regular cleaning prevents cross-contamination.
Calibration should be performed weekly, after the thermometer has been dropped, or after exposure to extreme temperature fluctuations. The preferred method is the ice-point method, which utilizes the known melting point of ice. To perform this, a container should be filled with crushed ice and clean tap water to create a dense slush.
The thermometer stem is then submerged into the slush so that the dimple is completely underwater, ensuring the probe does not touch the sides or bottom of the container. After waiting approximately 30 seconds for the pointer to stabilize, the dial should read 32°F (0°C). If the reading is incorrect, a small calibration nut located under the dial can be rotated with a wrench until the pointer is manually adjusted to the correct 32°F mark while the stem remains in the ice water.