Slide Nr. 0 of 15 Slides Introduction Measurement of temperature is generally considered to be one of the simplest and most accurate measurements performed in engineering. Some important considerations when measuring temperature are: 1. The contact between the sensor and the substance being measured should be satisfactory 2. The sensor should be small enough not to disturb the temperature conditions or cause too much thermal lag during transient testing. 3. There should be no chemical reactions between the sensor and the substance which will cause heat to be produced or absorbed.
Slide Nr. 1 of 15 Slides Definition of temperature a. According to thermodynamic basis: Temperature is related to heat. If two bodies at different temperatures are in contact, or in view of each other, heat flows from the one at higher temperature to the other at lower temperature. b. According to the kinetic theory of gases: Temperature is related to the average kinetic energy of an ideal gas. The relation is given by pv mr T
Slide Nr. 2 of 15 Slides Temperature scales
Slide Nr. 3 of 15 Slides Considerations of s (1) Thermodynamic equilibrium To measure temperature, it is necessary to establish a thermodynamic equilibrium between the body and the device to sense the temperature; e.g. bulb thermometers or thermocouple wire.
Slide Nr. 4 of 15 Slides Considerations of s (2) Thermal characteristics of temperature sensors Temperature sensors posses thermal characteristic which largely depend upon their size and shape and the materials from which they are made. Disturbance of the medium The introduction of temperature sensors into body tends to modify (change) the temperature conditions at that point. Also the body will be modified by any holes drilled in it to insert the probe, etc. The measured value will therefore be the modified value and not the true value.
Slide Nr. 5 of 15 Slides Considerations of s (3) Error due to signal transmission In most cases, the sensor is connected to a recording instrument by means of an intermediate system. The intermediate system and the recorder may be subjected to temperature or other changes and so compensating device are usually to reduce or eliminate errors. Ways of energy transfer Heat is transferred from place to place by conduction, convection and radiation. It is necessary to take these into account when choosing a temperature measuring method.
Slide Nr. 6 of 15 Slides Classification of Temperature Measuring Devices 1. Non electrical methods: a. Expansion thermometers: Expansion of solids. Expansion of liquids. Perfect gas thermometers b. Refractory cones, paints and crayons. 2. Electrical methods: a. Electrical resistance. b. Thermocouples. 3. Radiation and optical: a. Total radiation pyrometer. b. Photoelectric pyrometer. c. Optical pyrometer.
Slide Nr. 7 of 15 Slides Non-Electrical Temperature Measuring Devices Expansion thermometers Most solids and liquids expand when they are subjected to an increase in temperature. The direct observation is an increase in their size. This is used to indicate temperature in many thermometers. 1. Expansion of solids This includes: a. Solid rod thermometers or thermostat. b. Bimetallic thermometers. Solid rod and bimetallic thermometer are designed using the principle that some metals expand more than others when they are subjected to the same rise in temperature.
Slide Nr. 8 of 15 Slides Rod Thermostat The change (DL) of the original length of a solid due to a change (DT) of temperature is given by: DL L DT The displacement of rod A is (x) which is given by: x DT A B
Slide Nr. 9 of 15 Slides Bimetallic Thermometer (1) L L B A r r d L L 0 0 1 1 B A 1 1 B A r d 1 A B A If Invar is used for strip A, then α A is virtually zero r d B It is seen that for thinner strips, r is smaller, i.e. more bending occurs.
Slide Nr. 10 of 15 Slides Bimetallic Thermometer (2) The following are essential characteristics of the bimetal strip thermometer: Inexpensive: often used instead of liquid in glass thermometers. Compact: the volume of metal used in the protective sheath is small and thus of low thermal capacity. This of course improved its response. Robust: good resistance to mechanical shock due freely floating spindle. Close linearity: throughout the temperature range. Range of application: Accuracy: low temperature -30 C to 200 C high temperature 0 C to 550 C low temperature 1% of scale range high temperature 2% of scale range To increase the sensitivity, bimetal are coiled in helical form.
Slide Nr. 11 of 15 Slides Liquid in Glass Thermometer (1) The increase in temperature causes the liquid and the container to expand. The change (DV 1 ) of the volume (V 1 ) of a liquid due to change (DT) of its temperature is given by DV V1 1 1 D T The container will also have a volume changes (DV c ), and its temperature changes is the same (DT), then DV c V c c DT But V 1 = V c. Hence the apparent volume changes (DV) DV DV DV V T 1 c 1 c c D
Slide Nr. 12 of 15 Slides Liquid in Glass Thermometer (2) The advantages of liquid in glass thermometers are: 1. Inexpensive 2. Simple in design 3. Portable 4. No additional indication instruments are required The disadvantages of liquid in glass thermometers are: 1. Fragile and can be easily braked. 2. Not suitable for distant readings 3. Not suitable for surface temperature measurements 4. Not suitable for dynamic measurements because it possesses high heat capacity
Slide Nr. 13 of 15 Slides Immersion Errors (1) Complete immersion: By definition, if the complete bulb and stem are immersed at the same temperature, the thermometer is completely immersed. Total immersion: Total immersion applies to the situation where all the thermometric liquid, i.e., all the mercury in the bulb, the contraction chamber, and the stem, is at the temperature of interest. Partial immersion: One way around the problem of scale visibility and the need to move the thermometer is to immerse the thermometer to some fixed depth so that most, but not all, of the mercury is at the temperature of interest.
Slide Nr. 14 of 15 Slides Immersion Errors (2) For partial immersion thermometers, the true temperature reading t is given by: t t i N t 2 t1 K where t i is the indicated temperature, N is the length of emergent column expressed in degrees, as determined by the thermometer scale, t 2 is the mean temperature of the emergent column when calibrated (i.e., the stem temperature on a certificate for partial immersion or the thermometer reading for a total-immersion certificate), t 1 is the mean temperature of the emergent column in use and k is a coefficient of apparent expansion of the thermometric liquid used in the glass of which the thermometer stem is made
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