Electrical Engineering Technology EE306 Lecture 8 Northrop : 7.7 pgs 479 - 494
Temperature Measurement Many physical and chemical phenomena and physical constants are found to be functions of temperature and thus, can be used to measure temperature. Temperature dependent properties and constants include resistance, dielectric constant, and the magnetic permeability and susceptibility (of paramagnetic salts), linear and volume expansion of solids and gases, generation of thermoelectric EMF by thermocouples and the generation of white noise (thermal noise) by resistors.
Scientific temperature measurements are generally made using the Celsius or Kelvin scales. Absolute zero (thermodynamic zero) occurs at 0 K, or 273.15C. That is, K= C + 273.15. The nominal boiling and freezing temperatures of water were originally taken as the two calibration points for linear temperature scales; 100C and 0 or 212F and 32F are boiling and freezing in the Fahrenheit scale. C = 0.55556 (F – 32)
Temperature Standard The SI primary standard for temperature is the triple point of pure water. The triple point of a pure substance is defined as that temperature and pressure at which all three phases (solid, liquid, vapor) are in equilibrium in a closed vessel. The triple point of pure water occurs at +0.0098C and 4.58mmHg pressure.
Temperature Measurement Mechanical temperature sensor – Bimetallic strip - one of the more commonly encountered mechanical temperature sensing systems. This device is used in many common household thermostats and thermometers. At reference temperature, To, the strip is perfectly straight and has a length, L. At some higher (or lower) temperature, T, the strip is seen to bend in a section of an arc with radius R.
A and B are the coefficients of linear expansion of the top and bottom strips, respectively. We assume A B and the strips have the same thickness In general, R will be large compared to L. From the system geometry and a knowledge of R(T), we can predict the deflection of the tip of the bimetallic strip, and the angle the tip makes with the equilibrium (horizontal) position of the strip. At equilibrium position, If 15 then, The linear tip deflection and the deflection angle are linear functions of the temperature difference, T - To.
Another commonly encountered mechanical temperature sensor is the mercury or colored alcohol filled, sealed glass reservoir and capillary tube. In this common design, the filling liquid expands at a greater rate than the glass, so the column of liquid rises. The space above the expanding liquid in the sealed capillary tube is often filled with an inert gas, or is under vacuum. Most mercury filled, glass laboratory thermometers are designed for 76mm immersion in the medium whose temperature is being measured.
Electrical and Electronic Temperature Sensors Resistance Temperature Detectors or Sensors(RTDs)/Resistance Thermometers/thermistor The electrical resistance of all metals and alloys increases with temperature. This increase can be modeled by a power series equation of the form: 25C is taken as the reference temperature.
Tempco = temperature coefficient Platinum is the preferred RTD metal because of several factors: It has a high melting point (1775.5C), it does not oxidize in air at high temperatures, it is relatively chemically inert, and its R(T) characteristic is quite linear from - 190 C to + 400C, and has a slight negative second derivative above 400C.
The power dissipation of a thermistor must be kept low enough so that self-heating does not disturb the temperature measurement. In the biological temperature range, platinum RTDs are very precise and a resolution of 0.0001C is not uncommon. However, accuracy decreases with increasing temperature.
Another type of RTD material which has a much higher tempco than pure metals or metallic alloys is the thermistor, which is made from amorphous semiconductor material, generally sintered mixtures of oxides, sulfides and silicates of elements such as Al, C, Co, Cu, Fe, Mg, Mn, Ni, Ti, U and Zn. The resistance of negative tempco (NTC) thermistors generally follows the rule: T and To are kelvin temperatures and To is customarily taken to be 298. The NTC thermistor tempco is easily calculated to be: When T = 298K, = 4000, is - 0.045.
Thermocouples and thermopiles Thermocouples are traditionally used to measure temperatures. A thermocouple consists of a junction, often spot welded, between two dissimilar metal wires. A basic thermocouple system must contain two couples (Figure 6.18A). However, thermocouple systems may contain three or more couples.
In a simple, two couple system, one couple junction is maintained at a reference temperature, typically 0C, by melting ice in water. The temperature difference between the two couples will create EMF and current will flow if the circuit is closed and hence current flows through the millivoltmeter. The thermocouple system (measuring and reference junctions), consisting of two different metals must be joined to a third metal, such as copper, to connect to a millivoltmeter. The joining of the two thermocouple metals to copper produces one additional junction. The addition of the third metal to the system will have no effect on the performance of the basic two-metal thermocouple system, as long as the two junctions with the third metal are at the same (reference) temperature. This property is called the Law of Intermediate Metals.
The net EMF in a thermocouple system is not a linear function of the temperature difference between the reference and the measuring junction. The net EMF can be expressed as a power series: T is the temperature difference above the reference junction temperature, usually at 0C. The thermoelectric sensitivity of a thermocouple pair is defined as ST = dEo/dT = A + B(T) + C(T)2 and is usually expressed in V/C.
Thermocouple EMFs were traditionally measured using accurate potentiometers (calibrated with standard cells). Modern thermocouple measurement makes use of high input impedance, solid state, electronic micro-voltmeters. Electronically regulated, solid state temperature references for thermocouple systems are also used, replacing melting ice and water in a thermos flask.
A vacuum thermocouple is used in conjunction with a sensitive dc microammeter to measure the true RMS value of the current in its heater resistance element. A thermocouple junction is bonded to the heater wire and thus generates an EMF proportional to the temperature of the heater resistor. A reference junction is at room (ambient) temperature, so the net EMF of the thermocouple is proportional to T, the temperature of the heater above ambient temperature.
Thermopiles are close arrays of thermocouples, arranged in series to have great sensitivity and resolution. They are generally used to measure the energy of electromagnetic radiation at optical frequencies, such as from lasers. The total absorption of the radiation causes a minute temperature rise, which is measured by the series thermocouples.
Resistance Noise Thermometer – This is a system which uses the fact that the power density spectrum of Johnson (thermal) noise from a resistor is proportional to the Kelvin temperature of the resistor. The resistance noise thermometer should be useful at temperatures as low as 0.5 K. A noise thermometer made with fine tungsten wire has been used for temperatures as high as 1700K with an accuracy of 0.1%.
Electronic IC Temperature Sensors – These are specialized integrated circuits used for sensing temperatures in the - 55C to +150C range. The AD sensors operate from a 4–30V dc supply, and provide an output current which is a linear function of the sensor’s kelvin temperature. The AD temperature-to-current sensors are generally used with an op-amp current-to-voltage amplifier, which allows generation of a voltage output proportional to the Fahrenheit or Celsius scales. IC temperature sensors are ideally suited for environmental monitoring and control applications within their temperature ranges.
Optical Pyrometer – These instruments provide a no-touch means of estimating the surface temperatures of hot objects in the range of 775–4200C, such as metals being hot-worked, molten metals, gas plasmas and furnace interiors. Optical pyrometers make use of the fact that all objects at temperatures above 0 K radiate heat in the form of broadband, electromagnetic energy. The range of the electromagnetic spectrum, generally considered to be thermal radiation, lies in the range 0.01–100 mm wavelength.
Objects which are radiating heat are characterized by three parameters which describe what happens to long wave, electromagnetic radiation (heat) at their surfaces. e is the surface emissivity, which is always equal to its absorbtivity, a. r is the reflectivity of the surface. An ideal blackbody radiator has e = a = 1 and r = 0 (that is, all radiant energy striking its surface is absorbed, and none is reflected).