Temperature and Gas Composition Measurements Flashcards
5 types of temp. measurement
Expansion on heating Thermoelectric effects
Changes in thermal radiation levels
Change of state
Speed of sound
Factors affecting choice of instrument
- temp range
- response time
- accuracy
- spatial resolution
- application/environment
Low T applications
- expansion
- change of state
- thermoelectric (thermistors)
High T applications
- thermoelectric (elec. resistance, thermocouples)
- pyrometers
Solid expansion
Used for indicating process temp and control switching of thermostats Thermostat metals, processed in strip form, take advantage of bimetal effect (diff. coefficients of expansion) T range (40-560 celcius) Uses: boilers, oven thermostat, water heaters, AC Thinner strip = quicker response Advantages: - low cost - good accuracy for cost - easy to install + maintain - fairly wide T range Disadvantages: - overall not very accurate - limited to local temp - liquid + gas = more accurate - calibration can change over time
Liquid expansion
Slow response time (glass is slow conductor), but measurement is accurate
Not applicable for industry but used in meteorology
Gas expansion
T range 50-1100 celcius
Good for high T work e.g. exhaust gas temp
Rapid response time
PV=NRT (V&N constant, T proportional to P)
Bulb containing gas is connected to pressure measurement by capillary
Affected by altitude and ambient air temp
Widely used in process industry
Vapour pressure
Vapour: gas that can be liquefied by compression without cooling
If always liquid and vapour present in capsule, saturated vapour pressure of liquid depends only on temp. not container size
Pressure sensing device records temp.
Very simple, non-powered, very reliable
T range: 0-250 celcius
Liquid used: from methyl chloride to toluene
More sensitive to temp. than gas type, they can be physically smaller, but smaller temp span
Two processes = liquid evaporation and gas condensation are in eq
Crayon and paint thermometers
Relatively inexpensive
Main use: quick check of desired temp or when a max temp has been exceeded
Accuracy of paint = 8.5 celcius margin of error
Temp range: 120-1150 celcius
Group 1 = MC paints: lower temps, 4 or less colour changes, max 590 degrees
Group 2 = TP paints: high temp, multi change paints (min 5 changes), range 500-1300
D-liquid Crystals
In normal liquid properties are isotropic ( same in all directions)
In liquid crystal they aren’t, strongly depend on direction
Partly ordered materials, between solid and liquid phases
Molecules often shaped like rods or plates to encourage alignment
Order of liquid crystals can be manipulated with mech, magnetic or elec forces
Liquid crystals are temp sensitive: liquid if too hot, solid if too cold
Nematic phase: close to liquid phase, molecules float but are still ordered
Cholesteric: chiral + nematic = arranged in twisted structure, reflects visible light in bright colours depending on temp
T range: -30-120 celcius
self adhesive reversible temp indicator labels
Electrical resistance thermometers (RTD)
Electrical resistance of pure metallic conductors is a function of temp
R = R0 (1 + a1T + a2T^2 + a3T^3 +…)
over limited temp range R = R0 (1+a1T)
Follows polynomial function of temp
Metals used: copper (223-523K), nickel (73-623K)(also Ni-Fe alloy), platinum (15-1073K)
a1 to a3 gets smaller
- thin film Pt: cylinder with film deposited, wire wound cased in ceramic sheath to protect from moisture
Pt most common, Ni cheap but variable, Cu at lower temps
T coefficient of resistance v sensitive to internal strains so wire must be annealed
degree of precision depends on Pt purity
(Alpha platinum wire): temp coefficient of resistance of conductor called alpha value (0.003900 as per BS 26 148)
Advantages:
- stable for years
- fast response
- high accuracy and reproducible(Pt)
- some parts cheaper than thermocouple (no extra extension leadwires needed)
Disadvantages:
- expensive compared to thermocouple
- sensor size larger than thermocouple
- self-heating and vibrations can be issue
-can become fragile above 320 celcius
- tolerance + accuracy decreases as temp increases
Thermistors
Solid state device of metallic oxide whose resistance decreases or increases as temp increases
NTC thermistors = decrease with body temp increase
PTC = increase with increase
NTC have neg temp coefficient about 10x greater than Cu or Pt RTDs
But relationship = non-linear
R = a exp(b/T)
best for narrow temp ranges. recent thermistors can measure up to 1200K, extreme care required in calibration (drift)
Used in labs for low T measurements
Less stable than metals
Resistance measured using Wheatstone bridge
Thermocouples: Peiter-Seebeck effect
If 2 conductors of same metal connected together with one junction exposed to a higher temp, thermal energy transferred to hot junction, converted to electrical energy, establishes emf between the 2 junctions and a current along the circuit
Thermal gradient causes electrons at the hot junction to diffuse towards cold junction, setting up current
Emf not affected by length and diameter of wires
Emf difference between 2 junctions related to temp difference between them
- ONLY temp differences detected (have to know one value, usually cold)
Thermocouples: Law of intermediate metals
In thermoelectric circuit of 2 metals (A+B), with junctions at temps Ta and Tb, voltage difference is not changed if one or both of the junctions is open and a third metal placed between A and B provided the new junctions are kept at Ta or Tb
Means a third wire (Cu) found in a measuring device (e.g. voltmeter) will not affect total emf of the circuit, provided its kept at the same temp as the reference junction
Thermocouples: Law of intermediate temperatures
2 circuits with common intermediate temp = single circuit
Uab + Ubc = Uac
Cold junction compensation: temp of cold junction measured and calculated into overall emf signal to obtain accurate hot junction temperature
Thermocouples: Practical aspects
Materials;
- Ni, Cr, W = cheapest base metals
- Pt: precious metal, used for higher temp range or in corrosive environments
- Pt TCs need sheathing in combustion environments to avoid exothermic catalytic reactions artificially raising local temp
- contamination can cause wire breakage (avoided by sheathing)
- sheaths made from steel or ceramics
tables slide 49+50
Extension Leads:
- ideal for TC to connect back to reading instruments with leads of same metal
- impractical because high cost
- so made from cheaper metals with same thermoelectric properties, Cu
Ambient T correction (compensation):
- common that ref junc not at 273K but at ambient T, despite TC calibrated for 273K
- comp. can be needed, ambient T measured by bimetal element or thermistor, they adjust voltage in compensating leads
Suction ‘Pyrometers’ and Aspirated TCs
Simple sheathed TC in a hot gas stream indicated temp between the gas and its surroundings
Error caused by radiant heat exchange between TC and walls, + radiation from gas itself (not in these devices)
- In SP, the tc is protected by an impermeable sheath and put in isolating shields to reduce radiation
- gas also drawn at high velocity through the shields, increases heat transfer from gas to tc, so reduces difference between 2 temps
- cooled with water
Water-cooled Suction Pyrometer:
Safer and more accurate measurement of gas temps at any point inside furnace
Drawbacks:
- high suction rates can modify local aerodynamics
- relatively slow response (can’t follow rapid temp fluctuations T<2100K)
Radiation Pyrometers
Pyrometers: measure amount of thermal radiation leaving a body, function of body’s temp
Used where upper limit of TC exeeded / non-contact measurement required
Principle:
- all bodies emit over whole spectrum of wavelengths
Black body: perfect absorber-emitter (at eq, emits all the energy it absorbs by radiation)
- radiation emitted over whole spectrum Eb follows Stefan-Boltzmann law (slide 60)
- Planck’s formula also states what monochromatic radiation obeys (61)
Eb/wavelength graph (62):
- curves described by Planck’s
- area under curve S-B law
Most bodies = not perfect emitters, emissivity is ratio of radiation emitted at a certain temp and wavelength to radiation emitted by a black body at same temp and wavelength
Grey bodies = same emissivity for all wavelengths
Radiation incident on a body either: absorbed, reflected or transmitted
Kirchhoff Law: at thermal eq, energy absorbed by radiation = energy emitted by radiation (65)
Types of pyrometer: Total Radiation Pyrometer
Measure radiation of all wavelengths from body
Most use lens systems, limit bandwidth (lower sensitivity)
Consists of black metals (absorbent) surface under transparent hemisphere, tc attached underneath
Cold junction of tc connected to white surface of elec. cold junction
Radiation absorbed by black = temp >ambient/ref
Material table (69)
Types of pyrometer: Photoelectric pyrometers
Same as total radiation thermometer except TC replaced with photodiode
- incident photons = release of an electron (measured by change in resistance)
- higher response speed than thermocouple
- higher accuracy and stability
- no upper limit excpet limit by max black body temp, lower limit between 450-700 celcius
- suitable for small hot objects/moving
Types of pyrometer: Partial Radiation Pyrometers
Probe a narrow band of the spectrum Optical Pyrometers: - colour and brightness of an object gives indication of temp (colour temp range pg 73) - e.g. disappearing filament pyrometer Infrared Thermometer: - diagram pg 77 - Limitations: limited to surface measurements, requires emissivity correction, altered by moisture and particulates - non-IR layers become new target - fast changes can affect reading
Pyrometer Calibration
Usually calibrated using a black body enclosure at a known temp, must make corrections for surfaces with emissivity <1
Emissivity Corrections
Total radiation pyrometers:
correction for grey bodies (equation pg 82) T always in K
- correction depends on emissivity of solid, lower emissivity value = larger correction factor
Partial radiation pyrometers:
equations (pg 85)
Comparison:
- total pyrometer only better at very high temperatures
-over wide range partial errors significantly less than total
- if wavelength increased, errors increased in proportion
- most commercial pyrometers operate on wavelength > 0.52 micrometres
