Temperature measurement Flashcards
What dictates the choice of device
- temperature range
- accuracy and sensitivity
- cost
Two categories of temperature measurement
- contact methods
- radiation methods
based on physical properties responsive to temperature change
These methods are suitable for temperatures up to 2500oC
contact methods
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
Radiation methods
EMF stands for
electromotive force
electromotive force (emf) that develops across two points of an electrically conducting material when there is a temperature difference between them
Seebeck effect
https://en.wikipedia.org/wiki/Thermoelectric_effect
- 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
Thermocouples
measuring element exposed to the process in a thermocouple
measuring junction
A point outside the process of known temperature at which the voltage is measured in a thermocouple
reference junction
________ wires have faster response but are more fragile
thinner
Temperature range of thermocouple
-200 to over 2000oC
Types of thermocouple
- Nickel - chromium copper - nickel
- Iron copper - nickel
- Nickel - chromium nickel - aluminium
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
extension couples
If they do not maych, there are additional cold junctions.
Seebeck effect is _____
non linear
Polynomial equation used to describe Seebeck effect
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
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
Cold junction compensation
Advantages of thermocouples
- simple, small and inexpensive
- rugged
- fast response
Disadvantages of thermocouples
- 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
Resistance increases with temperature
Resistance temperature detector
Resistance falls with temperature increase
Thermistors
Two methods of RTD contruction
- wire wound
- thin film
wire wound arounf a creamic core or within a ceramic sheath
Wire wound RTD
Deposited as coating on to ceramic substrate using sputtering. Patterned using photolithography. Small dimensions. less material hence more responsive, more rugged
Thin film RTD
Advantages of RTDs
- highest accuracy and stability
- most linear of all methods
- defined by standards
- most stable
Disadvantages of RTDs
- highest cost
- less durable than thermocouples
- not as sensitive as thermistors
Advantages of thermistors
- highest sensitivity
- more accurate than thermocouples
Disadvantages of thermistors
- non linear
- lowest temperature range
- less durable than thermocouples
- not as accurate or stable as RTDs
- Non standard
- resistor whose resistance depends on temperature
- negative temperature coefficient
- packaged in a thermally conductive epoxy or glass bead or disc
thermistors
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.
thermowells
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
Exposed thermocouple
junction is welded to the sheath
Faster response than ungrounded
Grounded thermocouple
Junction is electronically separated from the sheath wall
Ungrounded thermocouple
Slower but protected from electrical noise
The ____ the RTD or themistor, the slower the response time and the less susceptible it is to self heating errors
larger
used instead of a thermowell if pigging is required
surface mount
- 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
surface mount
have a negative effect on response time since the thermal mass of the thermowell far exceeds that of the sensor
Good conduction is crucial
Thermowells
T or F? Fast moving media transfer hear and changing temperature better than slow moving media
T
T or F: higher density media are better hear conductors than lower density media
T
- 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
Liquids
- Not good at transferring heat
- Sensor can be affected by external heat sources
Gas
Protocol for sensor replacement
- Remove sensor from thermowell for calibration or replacement
- clean well bore of thermal paste and/or corrosion
- Inspect for corrosion on terminals, enclosures and on thermowell
- Inspect thermowell mounting for evidence of cracking indicating potential failure
- Inspect and tighten flange bolts
- For high temperature applications verify integrity of protection tube
Protocol for sensor replacement for surface mount sensors
- Verify mounting is secure for good thermal contact of the sensor tip with the process vessel or pipe
- Verify that the insulation surrounding the sensor is in place with minimal leakage
- For pipe clamp units ensure clamp bolts are tight and that no corrosion exists
Bi-metallic strips are a … method
contact
change of state devices are a ….. method
contact
Infrared is a …. method
non contact
spot and thermal imaging are …. methods
non contact
- 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
Bi metallic strip
- 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
bi metallic strip
- 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
Change of state temperature indicators
- irreversible temperature labels
- a series of waves with different melting temperatures
- melt causes wax to absorb into backing paper and change strip colour
Change of state devices
Advantage of change of state devices
- low cost and no installation required
Disadvantages of change of state devices
- 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
electromagnetic radiation emitted by a body is … to temperature
proportional
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
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
Advantages of infrared temperature measurement
- 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
Disadvantages of infrared temperature measurement
- 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
Low cost hand help systems
Spot measurement
Higher cost system
stationary pyrometers
- heavy duty stainless steel
- fixed devices
- can output 4-20mA
- process control
A temperature “map” of a large area using same principle as spot radiation devices. A range of detectors are used for different temperature ranges
thermal imaging
Real surfaces emit less radiation than ….
the idealised blackbody
0 perfectly reflecting surface which emits no radiation
