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