Temperature measurement. Metrology and Instrumentation Week-15-December 20, 2011 Dr. Belal Gharaibeh

Transcription

1 Temperature measurement Metrology and Instrumentation Week-15-December 20, 2011 Dr. Belal Gharaibeh

2 Introduction to temperature measurement We can measure temperature change rather than absolute (single) temperature at a certain location Previously we explained how that a measurand (input) is measured by comparing to a standard Example: unknown mass is compared to a universal kg prototype Temperature change, unlike all other properties, is measured by observing the changes in another temperature depended physical property Temperature is gauged by its affect on quantities like: volume, pressure, resistance, and radiation energy Example: change in volume of liquid in glass thermometer indicates the temperature change

3 Temperature sensing techniques 1. Change in physical dimensions 1. Liquid in glass thermometer 2. Change in gas pressure or vapor pressure 1. Pressure thermometer 3. Change in electrical properties 1. Resistance thermometer 2. Thermistors 3. Thermocouples 4. Change in emitted thermal radiation 1. Infrared pyrometer 5. Change in chemical phase 1. Liquid crystals

4 temperature units and conversion Kelvin is the basic temperature unit and is defined as the fraction 1/ of the thermodynamic temperature of the triple point of water For the SI (metric) units: Celsius = Kelvin For the imperial (English) units: Fahrenheit = Rankin Conversion from SI to English units F= 9/5*( C) +32 Conversion from English to SI units C = 5/9*[( F) 32]

5 Liquid in glass thermometer This temperature measuring devise consists of: Large bulb at lower end A capillary tube with scale Liquid filling bulb and portion of the tube (before the scale) Small safety bulb at the end of the tube if temperature range is exceeded Safety bulb will be on top Capillary tube Liquid bulb

6 Operation method of liquid in glass thermometer If temperature is increased then greater expansion in liquid will happen As the liquid expands it will rise in the capillary tube Height of liquid rise is a measure of temperature, note here that the rise in liquid is a volume change due to temperature effect

7 Liquid properties Temperature-dimensional relationship should be linear: the change in volume due to temperature change is linear, this will allow a linear scale on the capillary tube Liquid must have a large coefficient of expansion: small change in temperature results in considerably large change in volume Good temperature range without change in phase ( from liquid to ice or vapor) Visible liquid or add dies if not visible Liquid should not adhere (stick) to the capillary tube walls Example for liquids: Alcohol: has large expansion, needs die because it is not visible and has low boiling point so it cannot be used for high temperatures (higher than 60 C) and it adheres to walls Mercury: lower expansion coefficient but can withstand high temperature and has a natural silver color and does not adhere to walls

8 Resistance Temperature Detector- RTD RTD (Resistance Temperature Detector) is a temperature sensitive resistor. It is a positive temperature coefficient device, which means that the resistance increases with temperature. The resistive property of the metal is called its resistivity. Stable linear change in resistance with temperature change ranging: -260 to 1000 degrees celsius The industry standard is the platinum wire RTD (Pt100) whose base resistance is exactly ohms at 0.0 C. and range of to C platinum has good fabrication which allows various design sizes as shown in pictures

9 Resistance Temperature Detector- RTD R T 0 R( T) R T : Resistance at a reference reference temperature 2 0 [1 A( T T0 ) B( T 0) temperature (T A and B:Temperature coefficien tsof resistance depending on material 0 0 )

10 Thermistors Thermistor, a word formed by combining thermal with resistor, is a temperature-sensitive resistor fabricated from semiconducting materials. The resistance of thermistors decreases proportionally with increases in temperature. The operating range can be -100 C to C

11 Thermistors The thermistors can be in the shape of a rod, bead or disc. Manufactured from oxides of nickel, manganese, iron, cobalt, magnesium, titanium and other metals.

12 Thermistors R 1 1 R0 exp T T0 constant depending on thermistor grade or formulation

13 Thermocouples (TC) When two dissimilar metals are joined together to form a junction, an electromotive force (emf) is produced which is proportional to the temperature being sensed. The magnitude of emf depends on the junction temperature.

14 Thermocouples The electric potential is coming from two sources 1. The Peltier effect: electric potential from the contact of the two dissimilar metals 2. The Thomson effect: Temperature gradient (variation ) along the length of the Conducting wire in the circuit Thomson effect is much less than Peltier effect and can be neglected Net emf results from the difference between the two Peltier emf s at the p, and q junctions If T1=T2 the net emf is zero It is common to have a (hot) junction for which we want to measure T and a (cold) junction Which is usually a known reference temperature

15 Laws of thermocouples Law of intermediate material Insertion of intermediate metal into a TC circuit will not affect the net emf provided that the new junctions are at identical temperatures This is useful for two applications: 1. To connect the TC to a circuit and measure the emf output-figure 1 2. To weld the junctions together with a third welding metal figure 2 A T1 A B C B C Figure 2: metal (C) is a welding metal Figure 1: metal (C) is a connecting metal

16 Law of intermediate temperature If a single TC circuit develops and emf of e1 at T1 and T2, and same circuit but with emf2 between T2 and T3, then it will develop an emf of (e1+e2) when its junctions are at T1 and T3. T1 A T2 A T3 T1 A T3 B B B Emf=e1 Emf=e2 Emf=e1+e2

17 Thermocouple Metal Combinations TC Type Colours Range C Positive Lead (Coloured) J White/Red -210 to 1200 Iron Constantan E Purple/Red -270 to1000 Chromel Constantan T Blue/Red 0 to 400 Copper Constantan K Yellow/Red -270 to1372 Chromel Alumel R Black/Red -50 to 1768 Platinum-13% rhodium S Black/Red -50 to 1768 Platinum-10% rhodium B Grey/Red 0 to 1700 Platinum-30% rhodium C White- Red/Red 0 to 2320 Tungsten/5% rhenium Platinum Platinum Negative Lead (all Red) Platinum-6% rhodium Tungsten 26% rhenium Chromel = Nickel-chromium Alumel = Nickel-aluminum Constantan = Copper-nickel Note that each TC type has a positive and negative lead

18 Thermocouple Tables Voltage to Temperature Conversion Type T Thermocouple (Blue & Red) Reference Junction 0 C C mv equal to temperature 36 C..

19 Example: if a thermocouple circuit at T1 and reference temperature of T2=24 C is designed to measure T1. the output emf from the circuit was E =0.756 mv, what is T1. Solution: E3=E1+E2, E1 = is between T1 and T2=24 C E2 is between temperature 204 and reference 0, from table in previous slide the emf value for T2=24 is equal to mv, E3= = mv, From the table the emf value of corresponds to Temperature of T1= by interpolation T1 A T2=24 A T3=0 T1 A T3=0 B B B Emf=e1 Emf=e2 Emf=e1+e2

20 thermistors are the most sensitive but for narrow temperature range Thermocouples has the widest range of detection RTD are the most linear with temperature changes

21 TC connected in series (thermopiles) and TC connected in parallel When (n) number of TCs are connected in series it is called a thermopile connection and gives better sensitivity to the total emf reading The total emf for thermopiles are the sum of n all individual TC in the circuit emf total emf n 1 If the emfs are equal for each TC then the measured temperature is: T measured n*t 1

22 Thermopiles T1 T2