Chapter 2 - Temperature Measurement

Question 1

What are the two major types of resistive temperature transducers?

Answer 1

• Metals
• Semiconductors

Question 2

What are the most common metals used as temperature transducers?

Answer 2

• Platinum
• Nickel
• Copper
• Tungsten

Question 3

What is common about all the metal temperature transducers?

Answer 3

They have a positive temperature coefficient

Question 4

What are the common semiconductor temperature transducer materials?

Answer 4

Oxides of:

• Chromium
• Manganese
• Iron
• Cobalt
• Nickel

Question 5

What are semiconductor temperature transducers called?

Answer 5

Thermistors

Question 6

What is the temperature coefficient of thermistors?

Answer 6

Negative temperature coefficient

Question 7

What are the common types of metallic temperature transducers?

Answer 7

• Resistance bulbs
• Thin film resistance temperature device

Question 8

What is the construction of a resistance bulb?

Answer 8

Spiral of resistive wire wound onto an insulating rod

Question 9

What is the construction of a thin film RTD?

Answer 9

Thin film of metal deposited onto a ceramic substrate

Question 10

Why is the platinum type of RTD one of the most useful?

Answer 10

• Chemically inert
• Resistant to contamination
• High melting point

Question 11

What is the most commonly used metal for RTD?

Answer 11

Nickel

Question 12

What are the temperature ranges of metal RTDs?

Answer 12

-200°C to over 800°C

Question 13

How is the thermal conductivity between the resistance wire element and the medium being measured improved in a tube type RTD?

Answer 13

Filled with an inert gas or a powder

Question 14

What is the problem of gas filled RTDs as compared to powder filled ones?

Answer 14

More susceptible to vibration damage

Question 15

What is the fundamental interval of a metallic temperature transducer?

Answer 15

The classification of the metallic temperature transducer according to their resistance at 0°C and 100°C

Question 16

What are the features of thermistors?

Answer 16

• Can be very small
• Respond more quickly to temperature changes
• Must be operated at lower current levels
• More sensitive than metallic RTD
• Resistance changes in a non-linear manner
• Non-linearity can be compensated by connecting resistances in parallel and series to give the form of curve required

Question 17

In an unbalanced wheatstone bridge for temperature measurement, how is heating of the wires compensated for?

Answer 17

Extra wires are used with a negative temperature coefficient

Question 18

What are two methods by which the non-linearity of an out-of-balance voltage in a bridge circuit can be resolved?

Answer 18

• Having a correspondingly non-linear scale on the meter
• Dividing the desired temperature range into smaller ranges by switching in different values of R₃

Question 19

Why is a ratiometer type indicator used in a Wheatstone bridge used for temperature measurement?

Answer 19

Deflection of the meter will depend upon the supply voltage as well as on variations of ‘bulb’ resistance

Changes in the transducer current, due to temperature change, will result in meter movement only

Changes in supply voltage will affect both coil currents equally, and so meter movement will remain unchanged

Question 20

How does a ‘NULL’ balance instrument work?

Answer 20

R₃ is an accurate variable resistor adjusted by means of a dial calibrated in degrees of temperature and is adjusted until the sensitive galvanometer reads zero

Question 21

What are the two methods of bridge balancing?

Answer 21

• Apex method
• Parallel method