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Instrumentation, Measurement and Control > Temperature measurement > Flashcards

Temperature measurement Flashcards

1
Q

What dictates the choice of device

A
  • temperature range
  • accuracy and sensitivity
  • cost
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2
Q

Two categories of temperature measurement

A
  • contact methods
  • radiation methods
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3
Q

based on physical properties responsive to temperature change

These methods are suitable for temperatures up to 2500oC

A

contact methods

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4
Q

use different types of instrument. These methods are suitable for all temperatures but are typically used for high temperatures and difficult to get to areas

A

Radiation methods

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5
Q

EMF stands for

A

electromotive force

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6
Q

electromotive force (emf) that develops across two points of an electrically conducting material when there is a temperature difference between them

A

Seebeck effect

https://en.wikipedia.org/wiki/Thermoelectric_effect

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7
Q
  • uses the Seebeck effect
  • closed circuit thermoelectric temperature sensing device
  • two wires of dissimilar metals joined at both ends
  • voltage (emf) is created between junctions when the temperature is applied to one end or junction differents from the other end
  • Higher temperature mean higher voltage
  • lowest cost off the three devices
A

Thermocouples

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8
Q

measuring element exposed to the process in a thermocouple

A

measuring junction

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9
Q

A point outside the process of known temperature at which the voltage is measured in a thermocouple

A

reference junction

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10
Q

________ wires have faster response but are more fragile

A

thinner

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11
Q

Temperature range of thermocouple

A

-200 to over 2000oC

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12
Q

Types of thermocouple

A
  • Nickel - chromium copper - nickel
  • Iron copper - nickel
  • Nickel - chromium nickel - aluminium
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13
Q

can sometimes be required to carry the signal to a measurement device away from the measurement location, typically using the same metals as the thermocouple wires

A

extension couples

If they do not maych, there are additional cold junctions.

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14
Q

Seebeck effect is _____

A

non linear

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15
Q

Polynomial equation used to describe Seebeck effect

A

𝐸𝑋𝑌
𝜃1𝜃2 = 𝛽1(𝜃1 − 𝜃2) + 𝛽2(𝜃1
2 − 𝜃2
2).

XY refer to the types of metal
θ1, θ2 are temperatures of measuring and ref junctions

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16
Q

he voltage measured at the cold junction correlates with
temperature difference between hot and cold junctions and the cold junction temperature has to be known in order to calculate hot junction temperature. This can be achieved by using and isothermal terminal block which consists of a metal block with an
imbedded high precision thermometer (such as an RTD). This can then be used for an
array of thermocouples

A

Cold junction compensation

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17
Q

Advantages of thermocouples

A
  • simple, small and inexpensive
  • rugged
  • fast response
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18
Q

Disadvantages of thermocouples

A
  • lowest accuracy
  • small voltages (sensitivity depends on the ability of electronics to detect the small voltage changes, 0.5oC resolution is typical but this depends on the choice of thermocouple)
  • prone to drift
  • requires a known reference junction
  • long leads can be problematic
  • not very useful for low temperatures where there is a small difference form reference junction
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19
Q

Resistance increases with temperature

A

Resistance temperature detector

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20
Q

Resistance falls with temperature increase

A

Thermistors

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21
Q

Two methods of RTD contruction

A
  • wire wound
  • thin film
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22
Q

wire wound arounf a creamic core or within a ceramic sheath

A

Wire wound RTD

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23
Q

Deposited as coating on to ceramic substrate using sputtering. Patterned using photolithography. Small dimensions. less material hence more responsive, more rugged

A

Thin film RTD

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24
Q

Advantages of RTDs

A
  • highest accuracy and stability
  • most linear of all methods
  • defined by standards
  • most stable
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25
Q

Disadvantages of RTDs

A
  • highest cost
  • less durable than thermocouples
  • not as sensitive as thermistors
26
Q

Advantages of thermistors

A
  • highest sensitivity
  • more accurate than thermocouples
27
Q

Disadvantages of thermistors

A
  • non linear
  • lowest temperature range
  • less durable than thermocouples
  • not as accurate or stable as RTDs
  • Non standard
28
Q
  • resistor whose resistance depends on temperature
  • negative temperature coefficient
  • packaged in a thermally conductive epoxy or glass bead or disc
A

thermistors

29
Q

closed end metal tubes installed in the vessel or pipe where thermometers are placed

Can be cylindrical tapered or stepped.
Thinner profile means faster response, less drag, vibration etc.

A

thermowells

30
Q

junction protudes out of the sheath tip and is exposed to the process fluid

Best reponse time but limited to low pressure and low corrosive

A

Exposed thermocouple

31
Q

junction is welded to the sheath

Faster response than ungrounded

A

Grounded thermocouple

32
Q

Junction is electronically separated from the sheath wall

A

Ungrounded thermocouple

Slower but protected from electrical noise

33
Q

The ____ the RTD or themistor, the slower the response time and the less susceptible it is to self heating errors

A

larger

34
Q

used instead of a thermowell if pigging is required

A

surface mount

35
Q
  • can fit the sensor flush with the vessel or pipe wall
  • reduced accuracy since you are away from the bulk of the fluid
  • need to insulate around the sensor to prevent hear loss and ensure temperature is representative
A

surface mount

36
Q

have a negative effect on response time since the thermal mass of the thermowell far exceeds that of the sensor

Good conduction is crucial

A

Thermowells

37
Q

T or F? Fast moving media transfer hear and changing temperature better than slow moving media

A

T

38
Q

T or F: higher density media are better hear conductors than lower density media

A

T

39
Q
  • Very good at making thermal contact
  • high heat capacity and good heat tranfer
  • this is improved if there is liquid flow around the hear sensor
A

Liquids

40
Q
  • Not good at transferring heat
  • Sensor can be affected by external heat sources
A

Gas

41
Q

Protocol for sensor replacement

A
  1. Remove sensor from thermowell for calibration or replacement
  2. clean well bore of thermal paste and/or corrosion
  3. Inspect for corrosion on terminals, enclosures and on thermowell
  4. Inspect thermowell mounting for evidence of cracking indicating potential failure
  5. Inspect and tighten flange bolts
  6. For high temperature applications verify integrity of protection tube
42
Q

Protocol for sensor replacement for surface mount sensors

A
  1. Verify mounting is secure for good thermal contact of the sensor tip with the process vessel or pipe
  2. Verify that the insulation surrounding the sensor is in place with minimal leakage
  3. For pipe clamp units ensure clamp bolts are tight and that no corrosion exists
43
Q

Bi-metallic strips are a … method

A

contact

44
Q

change of state devices are a ….. method

A

contact

45
Q

Infrared is a …. method

A

non contact

46
Q

spot and thermal imaging are …. methods

A

non contact

47
Q
  • two dissimilar metals bonded together
  • each has different coefficient of thermal expansion
  • as temperature rises, one metal expands more than the otherr which causes the bi metallic strip to curl
A

Bi metallic strip

48
Q
  • wound into a coil/helix
  • indicator needle attached to coil
  • motion moves the indicator
  • no power needed
  • the entire strip or coil has to be exposed to the temperature
A

bi metallic strip

49
Q
  • appearance changes when a certain temperature is reached
  • can be used to prove an item has not been subjected to an adverse temperature such as food items during shipment for health and legal reasons
  • on chemical plants labels are commonly placed on sections of pipe or ranks that require high temperature or steam sterilisation
  • confirmation that the specifified temperature has been reached for a set period of time
A

Change of state temperature indicators

50
Q
  • irreversible temperature labels
  • a series of waves with different melting temperatures
  • melt causes wax to absorb into backing paper and change strip colour
A

Change of state devices

51
Q

Advantage of change of state devices

A
  • low cost and no installation required
52
Q

Disadvantages of change of state devices

A
  • Response time not always instantatenous
  • the change is irreversible (single use) (except for liquid-crystal displays)
  • these systems only detect surface temperatures
  • only a visible indication
53
Q

electromagnetic radiation emitted by a body is … to temperature

A

proportional

54
Q

optics focus this radiation on to a sensor via a filter to select the appropriate wavelength band
These is in the infrared part of the electromagnetic spectrum

A

Infrared temperature measurement

The detector can be in the
form of a photodiode, in which photons generate, current or a photoresistor where there
is a temperature rise caused by energy absorption

55
Q

Advantages of infrared temperature measurement

A
  • non-contact (no energy removed)
  • can measure moving objects
  • goof for very hot objects
  • good for hazardous areas (works from a distance)
  • fast response
  • cannot be damaged by process
56
Q

Disadvantages of infrared temperature measurement

A
  • relies on line of sight
  • surface measurement only
  • affected by reflection, emssivity variation etc.
  • your measurement is only as good as these factors will allow - less “absolute” than other systems
57
Q

Low cost hand help systems

A

Spot measurement

58
Q

Higher cost system

A

stationary pyrometers
- heavy duty stainless steel
- fixed devices
- can output 4-20mA
- process control

59
Q

A temperature “map” of a large area using same principle as spot radiation devices. A range of detectors are used for different temperature ranges

A

thermal imaging

60
Q

Real surfaces emit less radiation than ….

A

the idealised blackbody

0 perfectly reflecting surface which emits no radiation

61
Q
A

Instrumentation, Measurement and Control (12 decks)

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