Lab Flashcards
Mechanical principles can withstand?
Extreme or hostile environments
Solid state construction is what then mechanical?
Less robust, are more accurate in temp sensing
Solid state are used for?
For more precise controlled
3 types of bi-metal sensing devices?
1- strip type
2-spiral type
3-disc type
Strip type works by?
Two dissimilar metals welded together, the two metals expand and contract at different rates on cool or warm up, causing them to deflect. And arranged to either open or close on rise of temp.
Spiral type is made/ works?
Made in a shape of spiral or coil
Temperature sensing devices have two different applications. Some operate on…..and…..
-mechanical
And
-solid state construction
What is the differential?
17 to 22degrees
Thermocouple is a?
Flame proving device 26-32, produces voltage when heated. Has enough to hold in but not pull.
A thermopile?
Use as a (TSD) as well as a power source. It is many thermocouples connected in series to make 800mv . Can hold and pull in.
2 other types of temperature sensing devices?
- Bellows type
- liquid bulb type
Bellows type are?
Use a small bellow…
Fluid inside either liquid or vapour.
Temp goes up liquid vaporizes expanding in the bellow and expand with pressure.
Bellow types are commonly used for ?
This large movement is ideal for operating large snap action contacts. Such as those used in line voltage T-stat for unit heaters or electric heaters
Liquid bulb type temp are used for?
For monitoring the temp of water and air in a pipe or tank. Measures temp
Liquid bulb temp device is made of?
A rigid metal bulb attached to one end of a thin metal tube attached on other end. Liquid in diaphragm expands and contracts
Thermistor is?
Solid state device in which resistance of semiconductor changes in response to temp.
Thermistor has a what coefficient?
Negative, so increase in temp mean decrease in resistance. Accurate enough sense air temp in hearing cooling system.
RTD (Resistance temperature detector)? Has what coefficient and is?
Has a positive coefficient. Physical appearance as a thermistor. An increase in temp results in increase in resistance. Platinum most common metal in RTDs chem resistant
RTDS also can sense?
Greater accuracy then thermistor
RTDs used for?
Suitable for industrial thermal processing
Pyrometer?
Hot object emits radiation proportional to temp of a object. Measures radiation by use of focus lens concentrates radiation on sensitive thermocouple.
Pyrometers can measure?
Temp as high as 4000•c can measure temp at a distance
Pyrometer good for?
Measuring temp in industrial kilns and metal refining furnaces
Control and settings are?
Temperature-sensing devices are used to ensure that acceptable limits are maintained.
High limit control are for when?
A faulty blower or blocked cold air return can cause temperature to increase. Purpose is to shut off burner circuit.
High limit control most commonly use ?
Disc type element cheap. Second is spiral.
Set points?
High limit controls are usually set to open the circuit if temp exceeds a pre set value. Typically between 80 to 120•c
High limit set •c
Max •c
Gravity furnace max
80-120•c
121•c
149•c
Fan control purpose?
Start and stop a fan so heated air from the furnace is neither to hot or cold.
What is a summer/ winter switch? Or manual auto switch?
Is a override switch that is connect in parallel with the fan control N.O. Contact. Allows to be independent from fan control