Temperature switch and controller testing in the field
Temperature switches and controllers are commonly used in small processes and in control loops where a programmable logic controller (PLC) or larger distributed control system (DCS) are not warranted. Temperature controllers provide both switching capability based on rising and dropping temperatures, as well as a local indication of the measured temperature. Most temperature controllers have some form of tuning, using damping and PID (Proportional, Integral and Derivative values) for smoothing out the measured process temperature, reducing variability. The terminology around switches can be confusing. The set state of the switch is the action the switch takes when an input stimulus above or below a specified value is applied. This stimulus can prompt an action such as closing a switch, which in turn starts or stops a motor, or opens and closes a valve. The reset point is considered the relaxed state of the switch, which is typically referred to as “Normally Open” or “Normally Closed.” This describes the default condition of the switch. Lastly, deadband is the band of temperature equal to the difference between the temperatures where a switch sets, and resets. See illustration at left.
Closed
50 C
Setpoint
Deadband
Reset
Open
High Limit
Process Variable
Open
Reset
Deadband
Setpoint
20 C
Closed
Low Limit
Suggested test tools
Fluke Calibration 7526A Precision Process
Fluke 726 Precision Multifunction Process
Fluke 754 Documenting Process Calibrator
Fluke 712B RTD Temperature
Fluke 714B Thermocouple Temperature
Calibrator See pg 22
Calibrator See pg 8
- HART ® See pg 7
Calibrator See pg 22
Calibrator See pg 7
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Temperature Applications
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