Proper temperature level in an appliance is maintained by
a thermostat, an automatic control which directly or indirectly turns on or turns
off the energy flow. Thermostats control temperature using a hydraulic, rod and
tube, bimetal, or electronic type of sensor. Range oven thermostats are a typical
application of the hydraulic sensor. Rod and tube thermostats are commonly found
in storage water heaters. Room thermostats that control space heating and cooling
typically use the bimetal and electronic sensors. All thermostats operate on the
principle that certain metals, liquids or gases expand and contract in response
to more or less heat or pressure. This expansion and contraction is used to regulate
the flow of natural gas in gas appliances.
For food service facility operators, temperature control in
range ovens is often critical to the quality of the end product. Even slight temperature
variations can produce less-than-desirable results and some dishes require very
low oven temperatures.
The range oven thermostat consists of two parts: the sensor
and an automatic control valve. In a gas oven, the automatic control valve increases
or decreases the flow of gas to the burner in response to a signal from the sensor
for more or less heat to maintain the set temperature. Oven thermostats can be
the modulating type, cycling type or a combination of both. The modulating control
maintains the set temperature by varying the flow of gas to the burner. Since
the modulating control always maintains some flow of gas to the burner, the minimum
oven temperature is determined by the minimum gas flow --usually 250° to 300°F.
In contrast, the burner turns on and off in thermostats utilizing
a cycling control, allowing for oven temperatures as low as 140°F. Usually
the cycling control relies on a standing pilot to light a heater pilot which in
turn ignites the oven burner, resulting in a short delay between the call for
heat and ignition of the burner.
The combination control operates in cycling mode below 325°F and in modulating
mode at higher and broiling temperatures.