Electromechanical temperature switches sense and monitor temperature, providing a reliable means of temperature control.

Figure 1. There are two types of electromechanical temperature switches: remote bulb and capillary switch and local mount.
Temperature is one of the most important and widely monitored parameters in process control. During the past few years, the trend has been toward using microprocessors, networking, software and other "smart" technologies that indicate, transmit, log and control temperature. Infrared measurement systems, PC-based loop control and thermal imaging are a sampling of techniques gaining in popularity for specialized applications. Yet, electromechanical switches remain a widely used temperature device in diverse applications. These components continue to enjoy wide implementation because they:

  • Resist vibration effects.

  • Are only minimally affected by ambient temperature changes.

  • Are able to provide excellent response time.

  • Incorporate a calibrated setting scale.

  • Do not require an external power source.

For process applications that can be served by devices with adjustable ranges from -65 to 600oF (-54 to 316oC), temperature switches provide reliable, effective temperature sensing and control.

There are two types of electromechanical temperature switches: remote bulb and capillary, and local mount (figure 1). Often used to measure and regulate temperature in ovens, dryers, air compressors, pumps and heaters, electromechanical temperature switches also are installed in heat tracing applications to counteract heat losses from process equipment and piping (figure 2).

Figure 2. Process equipment and piping is wrapped with heating tape to compensate for heat losses. As the temperature fluctuates, the temperature switch activates the heating tape.

Bulb and Capillary Switches

Used for remote installations up to 25' from the media, bulb and capillary switches have three basic components: bellows, capillary and bulb. These components are manufactured separately and assembled to create a hermetically sealed system that produces a linear movement of the bellows as bulb temperature changes. To produce this movement, the filling liquid must have a relatively constant coefficient of thermal expansion over the sensor's temperature range.

In a process, the bulb is mounted in a thermowell located in the media to be measured, then secured by a split nut or union connector (figure 3). The split nut fits over the capillary and simply holds the bulb in place. A union connector seals the bulb into the thermowell with a tapered Teflon seal that engages the fitting's taper. When the jam nut is tightened, the seal compresses against the capillary and creates a seal that will withstand 300 psi. The upper part of the bulb also fits into the union connector, so a union connector sometimes is used to retain the bulb in a pipe or tank and seal it from external leakage to 300 psi.

To ensure accurate readings with bulb and capillary switches, ambient compensation must be employed. In this switch design, changes in the ambient temperature affect the amount fluid expansion, resulting in "false" temperature sensing. For example, an extreme ambient change from -65 to 165oF (-54 to 74oC) will reduce the accuracy of most switches by 20% or more. If the temperature changes by only 70oF, accuracy can be reduced by as much as 15%.

Fortunately, the effects of ambient temperature swings can be overcome by using stacked, concave bimetal washers. Then, ambient temperature changes -- which cause the liquid in the bulb, capillary and bellows to expand or contract -- result in an equal, opposite reaction between the washers. This compensating action ensures high repeatable accuracy. For example, over most of the adjustable range, accuracy can be held to within +/-1% of full scale. And, accuracy at constant ambient is +/-0.5% full scale.

Figure 3. The sensor for the bulb and capillary switch is remotely mounted in a thermowell and secured by a split nut or union connector. The split nut fits over the capillary and holds the bulb in place, and the union connector is used to seal the bulb into the thermowell.

Local Mount Switches

Consisting of bellows, housing, body and fill tube, local mount switches are installed in the pipe or vessel. Like bulb and capillary models, local mount switches are manufactured separately and assembled to provide a hermetically sealed system. Brass and stainless steel are common construction materials. In this sensor type, the fluid surrounds the bellows (called an outside fill). Temperature increases cause the fluid to expand, and the bellows moves in the opposite direction. The bellows action due to temperature changes in local mount switches is the opposite of the bellows action in bulb and capillary switches.

In principle, operation of a local mount switch is identical to the bulb and capillary type. The only difference is that local mount switches are unaffected by ambient temperature changes. Because all of the fill fluid is exposed to the media temperature the bulb is sensing, there is no ambient temperature influence on the filling fluid. For this reason, no compensation is required.

Standard local mount switches are manufactured with a threaded connection or, when used in a local mount thermowell, the wrench flats and threads are removed and a groove provided (figure 4). A set screw in the thermowell engages the groove and secures the temperature switch in the well. This provision also allows the temperature switch to be rotated 360o in the well to ease electrical connections. The threads must be removed to ensure that the whole temperature-sensitive portion of the sensor is available to sense temperature.

Thermowells are used for several reasons:

  • To allow the temperature switch to be removed without losing media.

  • To provide extra corrosion resistance.

  • To protect the temperature sensor from media pressure that exceeds 300 psi.

  • To protect the temperature sensor from high velocity process flows or media that can wear and abrade the sensor.

For most switches, the bulb's wall thickness is approximately 0.030": Thicker walls would substantially reduce heat transfer through the metal. But, this relatively thin wall dimension imposes pressure limits: The amount of external pressure the sensors can withstand without deformation is 300 psi.

Response time is influenced by several factors. Defined as the time required for the sensor to react to a temperature change in the media in which the bulb is located, response time is a function of media viscosity, media velocity as it passes around the bulb, the bulb's wall thickness, and the temperature difference between the bulb and the media being measured.

Figure 4. Local mount thermowells allow the temperature switch to be removed without losing media.

Selecting the Right Temperature Switch

Choosing the proper temperature switch for a process requires careful analysis of the entire control system. The key to selection is understanding the application and the environment. Often, the simplest control is the best one to use, both for reasons of economy and for best results.

When evaluating your process and operating environment, ask the following questions:

Sensor

  • Should be sensor be remotely or locally mounted?

  • Where will the bulb be positioned?

  • Is a thermowell required?

  • Should a union connection or split nut be used?

Switches

  • Define the sensing requirements. Will the switches be used for control or limit duty only? If so, a single switch may be used.

  • Do you need additional limits or alarm functions? If so, a dual switch should be used.

  • What is the system's electrical rating?

Enclosure

  • Will the enclosure be NEMA 4 or 4X rated?
  • Do you need an explosionproof (NEMA 4, 7 and 9) enclosure?

Once you've defined the basic hardware conditions, consider the temperature sensing requirements. Select a switch that best meets the process's mid-range temperature readings to ensure optimum, repeatable accuracy in the extreme reaches of the process's temperature range. Also, be sure to select a switch range that allows the desired setpoint to fall in the mid-60% of the adjustable range.

Finally, consider the ambient conditions such as surrounding atmosphere and temperature. Determine whether the unit needs overtemperature protection, and if so, for how long. Also, decide if temperature indication is needed. Each of these additional features may add to the overall cost of the system but will ensure accurate, repeatable performance.