OPUS

Specialty Item Thermometers

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Thermometers measure the temperature of air, gas, water, and other liquids and solids. Thermometers are placed at those points where operating personnel would normally be the most interested in verifying the input/output temperatures of individual equipment and overall system functions.

Installation of Separable Sockets (Thermometer Wells)
The installation of separable sockets (thermometer wells) within a piping system is a recommended practice. A separable socket is an accessory installed to avoid system shutdowns for thermometer removal. The socket becomes an extension of the thermometer bulb assembly and maintains piping system integrity when instrument removal is necessary for calibration or replacement. In many cases, spare sockets or test wells are installed in the piping system for temporary or possible future use during system start up and operation. Generally, thermometer wells used with industrial thermometers, are furnished in standard 3/4” NPT, and are available in steel, brass, and types 304 and 316 stainless steel.

Two categories of thermometers which are used for temperature measurement in mechanical systems are the industrial glass stem type and the dial type. The glass stem thermometer is the common liquid in glass thermometer one generally visualizes when thinking of a temperature indicator without specifying a specific type. A bulb containing liquid is connected to a small glass capillary tube extending the length of the glass stem. The stem or its backing is marked with a temperature scale. The liquid within this glass capillary tube rises or drops depending on the temperature of the substance that is in contact with the bulb.

Industrial glass stem thermometers are universally accepted for installation on all types of building construction including but not limited to process piping installations, tanks, and air ducts. Industrial glass stem thermometers feature a metal protective case with a clear acrylic window. A mercury filled tube inside the housing is backed by a clearly marked scale. The bulb chamber is tapered to fit a matched taper of a separable sock et providing metal to metal contact for maximum sensitivity.

The liquid in industrial glass stem thermometers is generally mercury which expands when heated and contracts when cooled. Since the freezing point of mercury is  40°F, thermometers intended to read temperatures in this range or lower contain alcohol rather than mercury. Industrial glass stem thermometers are manufactured for fixed position installations, or with an adjustable angle feature for better instrument face viewing.

Dial type thermometers contain elements or tubes which cause a pointer in a dial face to move. Dial type thermometers are available in a wide variety of stem lengths, dial sizes, and temperature ranges. Dial type thermometers are generally easier to read than glass stem thermometers, and as a result, they are generally more expensive. Two types of dial thermometers are available for mechanical systems. They are bimetallic, and gas or liquid filled.

Like the industrial glass stem type thermometers, the bimetallic dial type is used for direct mounting within the system which is to be monitored. Bimetallic dial type thermometers use a bimetallic temperature sensing element in a stem that bends or twists with any temperature change and then transmits this change to a pointer through a mechanical linkage.

Gas filled or liquid filled, dial type thermometers may be used for either direct mounting within the system which is to be monitored, or for remote mounting using a capillary tube generally covered with a braided metal protector. Gas  or liquid filled, dial type thermometers contain a bourdon tube (see above), and a temperature sensing system consisting of a bulb and capillary tube which are charged with either a liquid or a gas. Temperature changes at the bulb cause the contained liquid or gas to expand or contract which causes changes in the pressure exerted within the bourdon tube. Movement caused by this change causes a pointer to move over a graduated scale similar to a pressure gauge except that the thermometer dial is graduated in degrees.

The greatest advantage of gas  or liquid filled, dial type thermometers is that they can be used to read temperatures from a remote location. They should be recalibrated occasionally due to the presence of mechanical linkages within the thermometer.



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