Glossary of Technical Terms

P

Potentiometer (POT)
A component that changes resistance as a function of rotational or linear position. The pot varies the flow of energy through the system. The potentiometer may be mounted on the controller panel circuit board or remotely mounted on a control panel If mounted separately, the pot is provided with a pointer knob and dial calibrated in degrees F or C.

Power Consumption
Maximum wattage (W) used for a device within its operating range during steady state signal conditions.
Proportional Band/Gain — Proportional band is the portion of the controllers span around set point where the output is proportional to the error signal. Width of the band is expressed in degrees, millivolts or in percentage of span. Width of the proportional band depends on the controller amplifier gain; as the gain increases, the proportional band becomes narrower, and as the gain decreases, the band becomes wider. While both terms are commonly used, one is in inverse function of the other.

Proportional Control
The controller proportions the rate of energy flow into the workload, thereby alleviating large temperature overshoot, It prevents continuous oscillation through the set point and ensures accurate, stable temperature measurement. The correlative action is related to deviation from the set point.

R

Refresh Rate
The rate at which a thermocouple or thermistor signal is sampled and analyzed.

Repeatability
The closeness of agreement among the number of consecutive measurements of the output for the same value of the input under the same operating conditions, approaching from the same direction, for full range traverse. It is usually measured as non-repeatability and expressed as repeatability in percentage of span. It does not include hysteresis.

Response Time
Total time in which a system temperature goes through one overshoot and undershoot cycle.

Resistance Temperature Detector
Resistance temperature detectors, or RTDs, are devices which measure temperature by means of a change in resistance vs. temperature. Purpose of the RTD is to detect, or sense, temperature and in turn produce a change in resistance signal that is precisely representative of the sensor’s environmental temperature.

S

Set point
An input variable which sets the desired value of the controlled variable. Set point is written in the same units as the controlled variable. Characterized set point is positioning on a non-linear scale.

Span
The difference between the upper and lower range values.

T

Temperature Deviation Indication
A temperature readout meter which has a center null scale with plus and minus deflection increments of 10 or so. The meter pegs until the process temperature l within the deviation scale limits and then provides accurate indication of actual temperature vs. desired.

Thermal Gradient
The rate of change in temperature within a system, being highest at or near the heating element and lowest away from the heating element. Proper heat input and output alleviate excessive thermal gradients and ensure greater control.

Thermistor
A semiconductor device whose electrical resistance varies as a function of its temperature. It is very sensitive in terms of ohmic change per degree of temperature change and has a useful upper range of
550°F.

Thermocouple
Two dissimilar metals joined together at two points, one being the temperature sensing junction and the other within the temperature measuring Instrument. An electrical potential is developed between the two metals that is proportional to the temperature of the junction.

Thermocouple Break Protection
A circuit in an instrument which shuts down the controlled process when a thermocouple breaks, the circuit simulates maximum thermocouple signal; in a reverse-acting controller, this causes the controller to shut off the heat.

Thyristor
Thermally sensitive, semi-conductor resistance bead whose resistance varies drastically but uniformly with temperature change.

Time Proportioning
Produces a varying ratio of time-on to time-off during a base reference cycle. Thus, if the base reference cycle is 15 seconds and the controller remains on for 7.5 seconds, it produces the same effect as operating heaters at half power.

Type J Thermocouple
Comprised of iron and constantan wire that form a temperature measuring junction, usually welded, brazed or soldered to a metal surface.

Type K Thermocouple
Chromel and Alumel wire joined in the same manner as above.