Showing posts with label temperature switch. Show all posts
Showing posts with label temperature switch. Show all posts

Temperature Switches

temperature switch
Temperature switch
(United Electric Controls)
A temperature switch detects the temperature of some substance. Temperature switches often use bimetallic strips as the temperature-sensing element, the motion of which actuates one or more switch contacts. An alternative design uses a metal bulb filled with a fluid that expands with temperature, causing the switch mechanism to actuate based on the pressure this fluid exerts against a diaphragm or bellows. This latter temperature switch design is really a pressure switch, whose pressure is a direct function of process temperature by virtue of the physics of the entrapped fluid inside the sensing bulb.

The “normal” status of a switch is the resting condition of no stimulation. A temperature switch will be in its “normal” status when it senses minimum temperature (i.e. cold, in some cases a condition colder than ambient). For a temperature switch, “normal” status is any sensed temperature below the trip threshold of the switch.

switch contacts

Like all other process switches, temperature switches exhibit a certain amount of deadband in their switching action. A temperature switch that trips at 300 F rising, for example, will not reset at 300 F falling. That switch would more likely reset at some lower temperature such as 295 F. With mechanical switch designs, some amount of deadband is inevitable due to friction inside the mechanism. However, process switch deadband is actually a useful characteristic as it helps avoid repeated “nuisance” alarms from happening.

To understand this concept, it is helpful to imagine a scenario where the process variable is at or very near the trip point. For our hypothetical temperature switch with a trip point of 300 F (rising), imagine a situation where the process temperature is precisely 300.0 F. Any further rise in temperature will of course trip the switch (sounding an alarm). With no deadband, however, the switch will immediately re-set when the temperature falls back down to 300.0 F. This means the switch may possibly “cycle” back and forth between its trip and reset states with just a minute change in process temperature (300.0 F to 300.1 F and back again). If the temperature switch is activating an alarm every time it trips, it will create a series of alarm events prompting operators to repeatedly acknowledge the alarm. This is a nuisance to operations personnel, as it distracts them from addressing what they already realize is a process problem. It is better for the switch to trip at 300.0 F rising and remain in that tripped state until the temperature falls down to some degree substantially below the trip point. This way, the operators only receive one alarm event rather than multiple alarm events for each process temperature excursion.

Some mechanical temperature switches come equipped with a separate adjustment for deadband (also called differential). Setting this deadband adjustment in a mechanical temperature switch requires the technician to repeatedly subject the sensing element to a rising and falling temperature, to check that the switch trips at the proper setting and resets at the proper setting. This is analogous to cycling the process variable back and forth when adjusting the “zero” and “span” settings of an analog transmitter: checking to see that the transmitter repeatedly outputs a 0% signal at the lower range value (LRV) and a 100% signal at the upper range value (URV). 

For discrete temperature-sensing applications demanding high accuracy and repeatability, electronic temperature switch circuits using thermocouples, RTDs, or thermistors may be used instead of a mechanical (bi-metallic or filled bulb) sensing element. The operation and configuration of discrete electronic temperature switches is very similar to that of continuous electronic temperature transmitters.

An example of an electronic temperature switch module is the United Electric One Series shown below:

UE Series One Electronic Temperature Switch
UE Series One Electronic Temperature Switch

With electronic temperature switches, the adjustment of deadband (differential) is both precise and flexible. Unlike mechanical switches where deadband is primarily a function of friction, and therefore liable to change over time as the device wears, electronic switching circuits may be precisely set for any trip and reset points along its measurement range, remaining very stable over time.

For more information about temperature switches, contact Miller Energy by visiting https://millerenergy.com or by calling 908-755-6700.


Text adapted from "Lessons In Industrial Instrumentation" by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License.

Bulb and Capillary Temperature Switches

general purpose industrial temperature pressure vacuum switch
General purpose temperature switch with bottom connection
for capillary and bulb specific for each application.
Image courtesy United Electric Controls
Not all processes or operations require the use of state of the art technology to get the desired results. Part of good process design is matching up the most appropriate methods and technology to the operation.

One method of changing the state of a switch in response to a process temperature change is a bulb and capillary temperature switch. The switch operation produces a state change in the mechanical switch when the temperature of a process control operation crosses a certain threshold. Bulb and capillary switches have the advantage of operating without electricity, simplifying their application.

The physical operating principle behind the capillary thermostat relies on the use of a fluid. The fluid inside the thermostat expands or contracts in response to the temperature at the sensing bulb. The change in fluid volume produces a force upon a diaphragm or other mechanical transfer device. The diaphragm is connected to, and changes the status of, an adjoining circuit using a snap action switch. For example, a main use of the operating principle in action is when a commercial food company relies on the capillary switch to control temperature related to processing and distribution. Each individual use of a bulb and capillary thermostat is specifically designed based on manufacturer and industry specifications, all of which apply the same physical principle of fluid based physics.

Because of their simplicity and comparatively modest cost, commercial versions of bulb and capillary switches find application throughout residential and commercial settings. Some common applications include warming ovens, deep fat fryers, and water heaters. The HVAC industry uses capillary and bulb switches because the rate of temperature change found in their applications fits the adjoining range offered by the bulb and capillary type switches. Operation of the temperature switches is subject to a few limitations. The switching point is often fixed, so the application must be without a requirement for an adjustable setpoint. The temperature range over which the switches are suitable is comparatively limited, with a matching of the bulb and capillary fluid system to the application temperature range a necessary task in product selection. Within its proper sphere of use, though, bulb and capillary temperature switches offer simple, reliable operation, with little requirement for maintenance.

Bulb and capillary switches are typically used to evaluate average temperature and are especially useful for applications where the temperature is to be maintained at a well-known, consistent value. The bulb portion can be configured to accommodate mounting within the media to be controlled. The devices can be applied effectively to liquid and gaseous media when the proper bulb is used.

Industrial versions of bulb and capillary switches are fitted with appropriate housings for the installation environment. Hazardous location installation can be accommodated, as well as high current ratings and auxiliary functions. There are almost countless variants of bulb and capillary temperature switches available. Don’t overlook these simple mechanical devices as candidates for application in any temperature control process. Share your application requirements and challenges with product specialists for useful recommendations.


Myths About Process Switches

general purpose switch for temperature pressure differential pressure
Example of a general purpose switch for industrial use
Courtesy United Electric Controls
We may have developed an obsession with data and information. Understatement, right? Whether a process variable is high or low no longer is satisfactory. We want to know how high, how low, how long, how fast is it changing, and more. In many cases, this is useful information that can be applied toward goals of safety and efficiency. Let us not, however, overlook the possibility that some facets of an operation are best served by that old soldier, the process switch.

Process switches are readily available for temperature, pressure, and differential pressure in ranges to suit almost any application. They are rugged time and field proven devices designed to do one thing extremely well. Process switches will reliably and instantly change the state of their mechanical switch when the process value reaches their setpoint. Once properly installed and set, process switches require little, if any, maintenance and can provide extended periods of reliable service.

United Electric Controls, globally recognized leader in the manufacture of process switches for temperature, pressure, and differential pressure, authored an informative piece that debunks some myths about process switches. The piece is included below and makes interesting reading.

Establishing the best instrumentation and control layout for a process benefits from careful consideration of many factors. Share your requirements and challenges with process measurement and control specialists, combining you own process knowledge and experience with their current product application expertise to develop effective solutions.