To restore the accuracy of a bimetallic thermometer, the core procedure is simple: you need to recalibrate it at a known “reference temperature” by adjusting the calibration nut (usually a hex nut) directly below the dial.

Ice-Point Method: immerse the temperature sensing area in a mixture of crushed ice and water, and after the pointer is stable, rotate the nut with a wrench or a tool on the protective sleeve until the dial reading is accurately displayed at 32 °F (0°C).

Of course, you can also use the Boiling-Point Method, which is to adjust the reading to 212 °F (100°C) of boiling water. But there’s a big hole here-you have to correct for pressure changes based on the local altitude.

If the thermometer in your hand does not have a calibration nut or a central adjustment screw, it is irreparable. In this case, don’t worry, just change to a new one.

Find The Key Calibration Mechanism

Before doing it, confirm the adjustment mechanism. Most professional-grade bimetal probe thermometers can find this calibration nut where the dial head and metal rod are connected.

• Hexagon nut: This is usually a small hexagon nut that connects the thermometer surface and the probe.
• Tools: The current design is very user-friendly. Many thermometers have a built-in wrench bayonet on the protective sleeve, which is specially used to jam the nut. If you lose the protective sleeve find an open-end wrench or adjustable wrench to fix it.

The principle of operation is simple: turning this nut is actually physically rotating the dial or internal coil to realign it with the pointer, thereby “zeroing out” the error “.

Freezing Point Method

The freezing point method is the most reliable way to restore accuracy. The reason is simple: Whether you’re at sea level or in the highlands of Denver, the physical properties of the ice-water mixture are constant 32 °F (0°C).

Follow the steps below, basically no error:

• Make ice slurry: find a large cup and fill it with crushed ice (note that it is crushed ice, not large ice cubes). Add clean tap water until the glass is full, but make sure the ice is not floating; it should take on the form of a thick mud.
• Immersion into the temperature sensing area: insert the metal rod of the thermometer into the mixture. Make sure the temperature sensing area (usually 2 inches at the bottom of the rod) is completely submerged. This is essential: never let the probe touch the bottom or the wall of the cup, otherwise the heat conduction will directly bias the reading.
• Wait for stabilization: Hold the thermometer steady and wait for the pointer to stop completely. This usually takes 30 seconds to a minute.
• Adjusting nut: Keep the probe immersed in ice water and hold the calibration nut firmly with a wrench or protective sleeve tool. Rotate it until the pointer is aligned at 32 °F (0°C).

Boiling Point Method

If you really don’t have ice on hand, you can use the boiling point method. But this method is very sensitive to environmental factors, must pay attention.

• To prepare the boiling water: Boil a pot of clean water until it is completely boiling.
• Immersion probe: Insert the temperature sensing area into boiling water. Similarly, do not touch the wall or bottom of the pot.
• Altitude correction (the most error-prone step): At sea level, water boils at 212 °F (100°C). Important: If you are at high altitude, the boiling point will decrease (about 1 °F for every 500 feet of elevation). You must first calculate the correct boiling point for your location, and then adjust it.
• Calibration: After the hands are stable, turn the calibration nut until the dial shows the boiling temperature you calculated.

When Should We Give Up On Repair?

Not all bimetallic thermometers can be repaired, and there is no need to repair them. Carefully examine the underside of the dial:

Sealing device: If the metal rod is directly connected to the dial case, the hexagonal nut cannot be found, or the entire unit is wrapped in plastic and there is no adjustment screw, it is a permanently sealed design.

Replacement recommendations: If these sealed units are not read correctly, it usually means that the internal bimetal coil has been aging, or has been dropped and caused damage. In this case, the accuracy cannot be saved.

Author:David Miller

Hi, I’m a Certified Quality Control Specialist with over 15 years of experience in industrial instrumentation and food safety compliance. I specialize in the maintenance and calibration of precision measurement tools, dedicating my career to ensuring that professionals understand the proper protocols for restoring accuracy to essential equipment like bimetallic thermometers.