About Maximum Registering Thermometers

Maximum registering thermometers (including autoclave maximum registering thermometers) are thermometers designed to indicate the highest temperature attained during a process. They are typically used in processes in which it is difficult to measure the temperature during the process. Autoclaves and sterilizers are typical applications.

The construction of the thermometer incorporates three distinct differences from normal “indicating” thermometers:

1. The maximum registering thermometer has a constriction, or restriction, in the capillary, normally about 1/2″ or so above the juncture of the bulb to the capillary.

The intent of this restriction is to permit the mercury to pass through under pressure (during heating of the thermometer, or conversely, to return the mercury to the bulb after use by using centrifugal force). The constriction should NOT permit the mercury to retreat by force of gravity.

2. The area above the mercury column is a partial vacuum.

On regular indicating thermometers, the area above the mercury is filled with pressurized nitrogen, which is why the mercury column retreats when cooling, even in a horizontal position.

3. Calibration: the major manufacturers design the thermometer to indicate the temperature to which it has been exposed not while in the process, but after having been removed from the process and permitted to cool.

In production, the way this is accomplished is to subject the thermometer (with no scale markings) to the desired temperature(s) in a precision bath, allow it to come to equilibrium, then carefully remove it from the bath and gently place it in an upright position to cool. After cooling is complete, a mark is made on the stem of the thermometer where the meniscus of the mercury resides. This mark is used for placement of the scale.

In use, the thermometer is reset for the next use by ‘shaking’ it, similar to the motion one uses to ‘shake down’ a fever thermometer, thereby generating centrifugal force that impels the mercury downward, through the constriction, and into the bulb. Note that not all the mercury returns to the bulb, however the top of the mercury column should reside below the scale of the thermometer. The thermometer is now ready for use.

Place the thermometer in your process and permit the process to run its course. When finished, gently remove the thermometer and slowly and carefully place the thermometer in a vertical position, taking care not to jar it. When cooled to room temperature, read the thermometer. It’s reading should indicate the highest temperature attained in the process.

When you read the thermometer is not important. The mercury will remain at this level until it is reset, or until it encounters severe vibration or jarring.

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