Thermals & Sensors
Two unlike metals bonded together that bend when heated or cooled.
What is a bimetal strip?
Adjustable resistor that slightly warms the sensing element to reduce system overshoot.
What is a heat anticipator?
Safety device that opens on a rise to stop the compressor at excessive discharge values.
What is a high-pressure control?
Two-metal sensor commonly used in older gas water heaters for accurate tank sensing.
What is a rod-and-tube control?
Device that opens to protect a motor from excessive current or temperature and then resets after cooling.
What is an overload protector?
Device that converts a sensed pressure into a proportional electrical signal for a controller.
What is a pressure transducer?
A solid-state device whose resistance varies with temperature (often NTC or PTC).
What is a thermistor?
High-resistance device that starts cooling a bit early to reduce capacity lag.
What is a cold anticipator?
Device commonly used for low-charge protection and to prevent vacuum operation.
What is a low-pressure control?
Temperature device that changes state very quickly to open/close contacts.
What is a snap-disc?
Ensures supply pressure is present before burners ignite; should not be bypassed except for troubleshooting.
What is a gas pressure switch?
Modern controller with LCD and anti–short-cycle that accepts a remote sensor for flexible mounting.
What is an electronic pressure control?
Two dissimilar metals joined at one end that generate millivolts when heated; often used to prove flame.
What is a thermocouple?
Older low-voltage switching component that must be mounted level and handled as hazardous waste.
What is a mercury bulb (in a thermostat)?
The difference between the cut-in and cut-out settings.
What is the differential?
A bulb partially filled with liquid sends pressure through a capillary to actuate a remote switch.
What is a remote bulb control?
Proves air movement before electric heat is energized in a duct.
What is a sail switch?
In this popular type of sensor, increasing pressure decreases plate gap and increases capacitance.
What is a capacitive pressure transducer?
This flexible element provides more travel than a diaphragm and is often used with vapor-filled bulbs.
What is a bellows?
Typical residential control voltage provided by a step-down transformer.
What is 24 volts?
A single housing that includes two opposing pressure switches used on refrigeration equipment.
What is a dual-pressure control?
Valve that modulates condenser water flow to maintain head pressure.
What is a water-regulating valve?
Protects larger compressors by monitoring net lubrication force (discharge minus suction).
What is an oil pressure safety control?
A PTC component wired with a run capacitor provides extra inrush for this common motor type.
What is a PSC motor (with PTC thermistor start assist)?
Analog sensing element that straightens as pressure increases and drives a dial needle.
What is a Bourdon tube?
Term for the drop below set point before heat is actually delivered to the space.
What is system lag?
Opens at 400 psig and has a built-in 75-psig reset band: at what pressure does it re-close?
What is 325 psig?
Factory-set device that opens to relieve unsafe system pressure and must never be adjusted.
What is a pressure relief valve?
Operating with insufficient refrigerant can pull a vacuum, draw in air, raise head, and form acids—name the risk.
What is running with noncondensables/low charge leading to contamination?
Mounted on a rack suction header, this sensor helps stage compressors to match case loads.
What is a suction pressure transducer?