Specifying Bimetal Thermometers
Bimetal thermometers are used in many in-dustries, including oil, gas and petrochemical refining; food and beverage processing; paper and pulp; pharmaceutical; and a host of OEM applications including boilers, compressors, pumps and brewing equipment. The primary function of a bimetal thermometer is to provide local temperature indication on process surface areas that are easily accessible for temperature readings.
The term bimetal refers to the two dissimilar metals bonded together to form a helical coil. The bimetal thermometer's operating principle is rather simple: The two bonded metals that make up the bimetallic coil expand at different rates when exposed to a change in temperature. This dissimilar expansion causes the helix to wind up or unwind, depending on the direction of temperature change. Each bimetallic coil is carefully calibrated to a specific temperature range. The coil's thermal expansion creates the necessary torque to drive the shaft attached to the pointer. The pointer indicates a temperature change on the dial face (figure 1).
Prior to PurchaseWhen specifying bimetal thermometers, four general specifications must be met initially:
- Model type.
- Stem length.
- Temperature range.
- Mounting thread.
Although most bimetal thermometer requirements do not deviate from industry standards, there are applications that require nonstandard specifications. Being familiar with the following terms will help you achieve maximum functionality and satisfaction out of your equipment when specifying a conventional or unconventional bi-metal thermometer.
Back Connection. The back connection or angle form is the simplest and most economical form, good for most general purposes (figure 2).
Adjustable Angle. The adjustable angle form can act as either a bottom or a back connection, depending on how it is adjusted. This form is useful when the specifier is not certain how the thermometer is to be oriented in the process (figure 4).
Range. Temperature ranges for bimetal thermometers start as low as -100oF (-70oC) and can have maximum scale reading of 1,000oF (550oC). In choosing the correct temperature scale for a bimetal thermometer, the operating temperature of the process must be identified by the end user. Although accuracy is 1% across the full temperature scale, the operating range should ideally be read mid-scale, be-tween 10 and two o'clock on the dial face.
Stem. Depend-ing on the temperature range, standard and nonstandard stem lengths for bimetal thermometers start at 2" and are available up to 80". Standard stem diameter is 1", and 3/8" is sometimes specified. Stems typically are constructed of 300 series stainless steel. The length of a stem is measured from the top of the threaded connection to the end tip of stem. If a thermowell is already in place, it is important to match the stem length and diameter. Keep in mind, a thermowell is recommended in any pressurized application.
Mounting Thread. The most common thread is 1/2" NPT, as this fits into most thermowells. Other standard and metric threads such as 1/4" NPT and 1/2" BSPP also are available upon request.
Dial Size. Industry standard case diameters for hermetically sealed bimetal thermometers are 3, 4 and 5". Smaller dials are available from 1 to 2.37".
Factors to ConsiderIn addition to familiarizing yourself with functionality terms, consider the following criteria and make sure the equipment you choose can withstand these requirements.
For correct temperature measurement, the tip of the thermometer stem must be immersed at least 2" in a liquid and 4" in a gas process. For certain temperature ranges such as 0 to 100oF, 25 to 125oF or 0 to 50oC, the bimetal thermometer must extend further into the stream: The minimum stem length is 3.5" for a liquid and 5" for a gas.
The industry standard for bimetal thermometer accuracy is +/-1% of the full scale (Grade A per ASME B40.3). Temperature readings normally are stabilized within 40 sec.
It is important to note that the temperature of the thermometer head should not exceed 200oF (93oC) or 10oF if filled with silicone-dampening fluid. The bimetallic coil at the tip of the thermometer stem should not be exposed to temperatures exceeding 50% over-range or 800oF (427oC).
Vibration is the biggest enemy of the bimetal thermometer. Silicone filling is recommended for heavy vibration as this will extend the life of the instrument. Silicone dampening prevents wear and fatigue of the instrument's internal parts and reduces pointer flutter.
For safety reasons, a thermowell is recommended in any pressurized application. Use of a thermowell allows the thermometer to be removed from the process without spilling the contents or shutting down the process.
Although its overall appearance and use has not changed much in more than 100 years, new design innovations such as hermetic sealing and silicone filling have given bimetal thermometers renewed utility and applicability. For applications where electronic output is needed and the local indication of a bimetal thermometer also is re-quired, a dual mode thermometer performs well. It combines a thermocouple or RTD output signal with bimetal dial indication to provide two outputs from one thermowell. Thermocouple output also can be converted to 4 to 20 mA by means of a small transmitter in the attached connection enclosure.
The bimetal thermometer is a reliable temperature-sensing device that continues to have an important role in process industries. Its simplicity, durability, and lack of power requirements will preserve its place as a mainstay for industrial temperature indication.
For more information on Reotemp's bimetal thermometers:
Circle 402 on the Reader Action Card
Call (800) 648-7737.