Glossary
In our ongoing effort of providing knowledgeable service and assistance, Selco Products has developed the following Glossary of Terms. This valuable guide covers basic terminology and common industry terms used for many of our products, including thermostats, thermal cut-offs, thermal protectors, control knobs, and digital panel meters.
From prototypes to volume production requirements, we have structured our service and engineering support operations to provide a virtually seamless means of meeting your changing needs. We service OEM and end-user companies with equal ease, delivering price/performance benefits and value-added features that simplify both the product selection and manufacturing processes.
Regardless of your application − HVAC, medical, electronics, professional audio, industrial instrumentation, food service, appliance, process controls or telecommunications − we welcome the opportunity to assist you finding the quality product that is right for your application.
Thermal Products:
| Ambient Temperature: |
The surrounding temperature or environmental temperature. |
| Bimetal: |
Two different metals that bond together to form the disc that actuate the thermostat when reaching a specified temperature. |
| Close On Rise: |
Normally open contacts. When the temperature rises to the specified temperature the contacts close. |
| Contact Resistance: |
The resistance measured in ohms from across the contacts |
| Dielectric Strength: |
The voltage that an insulating material can withstand before breakdown occurs. |
| Differential: |
The difference between opening and closing temperatures. Also known as hysteresis. |
| Electrical Contact Rating: |
The maximum amps and volts that the device has been tested to withstand. |
| Epoxy Seal: |
A seal to protect against dust and moisture. |
| FLA: |
Full Load Amps, the amount of current on a motor under its normal operation. |
| LRA: |
Locked Rotor Amps, the amount of current a motor will pull in a locked condition. |
| Manual Reset: |
The thermostat will remain open until reset manually. Also known as a high limit. |
| Open On Rise: |
Normally closed contacts. When the temperature rises to the specified temperature the contacts open. |
| Phenolic: |
An insulating thermo-set plastic used for the body of specific thermostats. |
| Snap Action: |
Fast sudden change in temperature that will cause the bimetal to snap |
| SPST: |
single pole/single throw. One set of terminals to activate or deactivate a circuit. |
| Surface Mount: |
Designed to sense the surface as opposed to the air temperature. |
| Tolerance: |
An additional range above or below the nominal set point. |
NTC Thermistors:
| NTC Thermistor: |
Thermally sensitive resistor that exhibits very predictable resistive values over a specific temperature range. The electrical resistance output of a NTC thermistor is very high at low temperature and decreases as the temperature rises. View Selco NTC Thermistors. |
| Base Resistance: |
Zero power resistance at 25 degrees C. A 10K thermistor has a base resistance of 10,000 Ohms at 25 degrees C. |
| NTC Resistance-Temperature Curve: |
The resistance of an NTC thermistor will track an established curve that is higher at low temperatures and will descend and flatten as the temperature increases. The R-T curve is determined by composition of the base metals and binders that make up the thermistor. Typically, a thermistor manufacturer will offer a number of NTC curves as derived by the material composition of the thermistor materials that they manufacture. Selco currently offers eleven different curve materials. Selco can also formulate other thermistor material compositions to match customer curve requirements. |
| NTC Resistance-Temperature Table: |
An R-T table provides a convenient method to determine a thermistors resistance output at specific temperatures over a given temperature range. Typically, an R-T table is available for each thermistor curve and can be accessed on the web site or in the catalog. |
| Tolerance: |
A term describing how accurately the NTC thermistor tracks the R-T curve over a temperature range or at a single temperature point. Tolerance can be expressed in a percentage such as +/- 1% or in degrees C such as +/-.1 degrees C. |
| Thermal Time Constant: |
The amount of time, expressed in seconds, for a thermistor to indicate 63.2% of a newly impressed temperature. The time constant of a thermistor is directly affected by the mass of the thermistor and the thermal coupling to the environment. Measurements are typically taken in still air or in a stirred oil bath. |
| Dissipation Constant: |
The power required to raise the temperature of a thermistor 1 degrees C above the surrounding environment. Power is expressed in watts. The dissipation constant of a thermistor with a 0.095" OD, coated with epoxy, is typically 13 mW/degrees C in stirred oil and 2mW/degrees C in still air. |
| Beta Value: |
A one way to characterize its resistance temperature relationship. Beta is calculated as follows: βT2/T1= Ln(RT2/RT1)/(1T2-1T/1) (see Selco Technical Notes calculation) Temperature is in degrees Kelvin. RT1 is the resistance at T1 and RT2 is the resistance at temperature T2. Selco's Beta values are calculated using 25 degrees C/85 degrees C. |
| Stability: |
The stability of a thermistor is achieved when the thermistor is operated at temperatures within the specified operating range. The stability of Selco's thermistors is the result of years of experience in thermistor manufacturing coupled with stringent process controls to ensure excellent field stability. Selco's thermistors typically exhibit less than 0.02 degrees C thermometric drift per year when operated at temperatures less than 50 degrees C. |
| Maximum Temperature Rating/Recommended Operating Ranges: |
Most Selco thermistors may be intermittently cycled at temperatures from -50 degrees C to 150 degrees C. Stability is achieved when the thermistors are stored at less than 50 degrees C and operated continuously at temperatures less than 100 degrees C. For interchangeable thermistors, optimum stability is achieved when the thermistors are operated at temperatures within their specified temperature range. DT Series glass thermistors may be intermittently cycled at temperatures from -50 degrees C to 250 degrees C and operated at temperatures continuously below 220 degrees C. DT Series glass thermistors may be intermittently cycled at temperatures from -50 degrees C to 250 degrees C and operated at temperatures continuously below 220 degrees C. |
Control Knobs:
| Cap: |
An accessory that snaps onto the top of many Selco knobs; these are available in a multitude of colors, with and without indicator lines or dots. View Selco knob caps. |
| Collet Knob: |
A knob that is made up of a plastic outer body and internal brass hardware; the hardware grips the shaft 360 degrees to insure a secure fit; must be installed with a nut driver; requires separate cap on top |
| Conductive Knob: |
A knob that is made of a type of material that will allow it to transmit an electric charge |
| D Shaft: |
A round shaft with a flat on one side (also known as a flatted shaft) |
| Dial: |
A molded in feature or separate accessory on some knobs that greatly increases the overall outside diameter of the knob at the base and allows room for custom printing |
| Dual Concentric Knob: |
Two knobs that stack together to be used on a special dual concentric shaft; the top knob can be rotated independently of the bottom knob. View Selco dual concentric assemblies. |
| Inside height: |
The internal dimension from the bottom of the knob to the stopping point of the shaft, also known as "shaft depth" |
| One Shot Knob: |
A knob that is made out of a single type and color of plastic |
| Pointer: |
A molded in feature or separate accessory on some knobs that indicates the direction that the knob is facing, can either be at the base of the knob or molded down the side of the knob. View Selco pointers. |
| Potentiometer: |
A three-terminal resistor with an adjustable center connection, widely used for volume control in radio and television receivers |
| Pushbutton: |
A type of push-on knob that is designed for use on a pushbutton switch. View Selco push-on knobs. |
| Push-on Knob: |
A knob that is installed without the use of a tool; this type of knob stays on the shaft as the result of an interference fit. View Selco push-on knobs. |
| Set Screw Knob: |
A knob that is made up of a plastic or aluminum outer body and a side screw; the screw is tightened on one side which secures the knob to the shaft |
| Shaft: |
The portion of the potentiometer or switch that must be covered by the knob |
| Skirt/Nut Cover: |
A molded in feature or separate accessory on some knobs that slightly increases the overall outside diameter of the knob at the base; the purpose can be either cosmetic or to cover a nut on a front panel |
| Slider Knob: |
A type of push-on knob that is designed for use on a slide potentiometer. View Selco slider knobs. |
| Soft Touch Knob: |
A knob that has an outer body made of thermoplastic elastomer (with a soft, velvety feel), and an integral pointer or pointer/cap that is made of polypropylene. View Selco soft touch knobs. |
| Splined Shaft: |
A shaft that has a series of small ridges that run vertically up all sides, which fit inside a knob with similar ridges, enabling both to rotate together (also known as a knurled shaft) |
| Switch: |
A device used to break or open an electrical circuit or to divert current from one conductor to another |
| Three Shot Knob: |
A knob that is made out of three different types or colors of plastic (example: "Trio" knob). View Selco three shot knobs. |
| Torque: |
A turning or twisting force |
| Two Shot Knob: |
A knob that is made out of two different types or colors of plastic (example: soft touch knob). View Selco two shot knobs. |
Digital Panel Meters:
| Conversion Rate: |
The number of analog to digital conversions performed per second by a DPM. |
| Deadbeat Damping: |
In general deadbeat damping means that the instrument shall have no over-swing (0.2% or less). |
| Environmental Protection Requirements: |
NEMA 4 rated. Sometimes dust and water threatens the integrity of a DPM case, and protection is needed so the meter can continue functioning. NEMA has standards for the cast integrity of electrical meters. |
| Full scale: |
The high or upper input, the arithmetic sum of the two end-scale values. When zero is not on the scale, the full-scale value is the higher end-scale value. |
| Indicator Meter: |
A meter that indicates or displays information that it has received. |
| LSD: |
The "Least Significant Digit" usually the right most ac |
| Meter Relay: |
Is a meter that indicates or displays a value and then creates an output. Turns on alarm, on/off relays. |
| Off set: |
Is what you want the meter to display when you have the min or low input. Off set and full scale is adjustable is coming in from transmitter. You want to display in the engineering units. PSI, Temperature. You want to adjust to the off set. |
| Process Meter: |
Any type of meter that takes a process, temperature, flow, revolution, and pressure. A panel meter with scalable zero and span adjustment capabilities, which can be scaled for readout in engineering units for signals such as 4-20mA and 1-5V. |
| Range or Full Scale: |
The range of signal input, which can be measured before the instrument, goes into overload condition. |
| Span Adjustment: |
The ability to adjust the gain of a process or strain meter so that a specified display. |
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