Chapter 2 - Temperature Measurement Flashcards
What are the two major types of resistive temperature transducers?
- Metals
- Semiconductors
What are the most common metals used as temperature transducers?
- Platinum
- Nickel
- Copper
- Tungsten
What is common about all the metal temperature transducers?
They have a positive temperature coefficient
What are the common semiconductor temperature transducer materials?
Oxides of:
- Chromium
- Manganese
- Iron
- Cobalt
- Nickel
What are semiconductor temperature transducers called?
Thermistors
What is the temperature coefficient of thermistors?
Negative temperature coefficient
What are the common types of metallic temperature transducers?
- Resistance bulbs
- Thin film resistance temperature device
What is the construction of a resistance bulb?
Spiral of resistive wire wound onto an insulating rod
What is the construction of a thin film RTD?
Thin film of metal deposited onto a ceramic substrate
Why is the platinum type of RTD one of the most useful?
- Chemically inert
- Resistant to contamination
- High melting point
What is the most commonly used metal for RTD?
Nickel
What are the temperature ranges of metal RTDs?
-200°C to over 800°C
How is the thermal conductivity between the resistance wire element and the medium being measured improved in a tube type RTD?
Filled with an inert gas or a powder
What is the problem of gas filled RTDs as compared to powder filled ones?
More susceptible to vibration damage
What is the fundamental interval of a metallic temperature transducer?
The classification of the metallic temperature transducer according to their resistance at 0°C and 100°C
What are the features of thermistors?
- 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
In an unbalanced wheatstone bridge for temperature measurement, how is heating of the wires compensated for?
Extra wires are used with a negative temperature coefficient
What are two methods by which the non-linearity of an out-of-balance voltage in a bridge circuit can be resolved?
- Having a correspondingly non-linear scale on the meter
- Dividing the desired temperature range into smaller ranges by switching in different values of R3
Why is a ratiometer type indicator used in a Wheatstone bridge used for temperature measurement?
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
How does a ‘NULL’ balance instrument work?
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
What are the two methods of bridge balancing?
- Apex method
- Parallel method
