Temperature-Sensing and Control Devices

Question 1

What are the three basic bi-metal type temperature sensing devices?

Answer 1

Strip.
Spiral.
Disc.

Question 2

Why are disc type bi-metal elements more suitable for high current loads?

Answer 2

The snap action of their contacts.

Question 3

Which type of bi-metal element has the highest level of sensitivity?

Answer 3

Spiral type.

Question 4

What kind of fluid is often inside bellows type temperature sensing devices?

Answer 4

Ether.

When heated it vaporizes to expand the bellows greatly.

Question 5

What is the most suitable application of liquid bulb temperature sensors?

Answer 5

To measure the temperature of fluids in a sealed tank or pipe.

Question 6

How is a thermopile constructed?

Answer 6

Many thermocouples connected in series.

Question 7

What is a thermistor made of?

Answer 7

Semiconductive material, with a negative temperature coefficient.

Question 8

What material is commonly used in RTDs?

Answer 8

Platinum.

It is chemically stable and has a high melting point.

Question 9

What would be used to measure the temperature of a blast furnace safely?

Answer 9

A pyrometer.

It can measure up to 4000 degrees, they measure at a distance.

Question 10

What kind of furnace always has more than one high limit control?

Answer 10

Counter flow furnaces.

Question 11

What is a typical high limit set point?

Answer 11

80-120 degrees C, 121 being the maximum allowable for forced air furnaces. This is increased to 149 degrees C for gravity furnaces.

Question 12

What does DIP mean?

Answer 12

Dual Inline Package.

Question 13

What are the three primary considerations when setting a fan control?

Answer 13

Comfort,
Proper cycling,
Economy.

Question 14

What will happen if Fan -on is set too high?

Answer 14

The registers will blow air that is too hot.

Question 15

What will happen if fan -off is set too high?

Answer 15

The inrush of cool air when the fan starts may trigger the fan to cycle instantly.
It may also simply reduce efficiency as residual heat is wasted to the flue.

Question 16

What is the typical maximum fan on setting and the differential between on and off?

Answer 16

60 degrees C and from 17 to 22 degrees of differential.

Question 17

When does high limit cycling often occur?

Answer 17

When the fan fails.

Question 18

Why must a combination high limit fan control switch the primary side of the transformer?

Answer 18

Because class 2 wiring may not exist in the same enclosure as the power circuit.

Question 19

What are the three common types of thermostat contacts?

Answer 19

Mercury,
Spring-activated, snap-action exposed,
Glass-enclosed magnet and armature snap-acting.

Question 20

What is a major difference with Spring-activated, snap-action exposed contacts, and the other two types?

Answer 20

They have a wiping function, rather than being enclosed.

Question 21

What is a green wire used for in a heat/cool thermostat?

Answer 21

Fan control.

Question 22

How is an anticipator attached to a cooling thermostat?

Answer 22

In parallel with the contacts, as they are normally open.

Question 23

If a heating thermostat is not working, what is a likely cause of the problem?

Answer 23

Burnt out anticipator, as it is connected in series.

Question 24

In a dual stage heating system, will the thermostat have 2 set points?

Answer 24

Yes.

Question 25

Can you use a 24v thermostat on a 120v circuit?

Answer 25

No, it will burn out the anticipator.

Question 26

What is an important consideration when installing mercury thermostats?

Answer 26

That it is mounted level. That is how it is calibrated.

Question 27

What happens if a thermostat is not mounted in a location of regular air flow?

Answer 27

The temperature will swing wildly.