Electronic Instrumentation Lecturer Touseef Yaqoob PART TWO Measurement of Temperature Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Learning objectives To understand basic principles of temperature measurement To learn different temperature measurement techniques To be able to determine the operating range of different temperature sensors. Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Glass thermometers Electronic Instrumentation Lecturer Touseef Yaqoob

Pressure Thermometers Electronic Instrumentation Lecturer Touseef Yaqoob

Thermal expansion methods: Bimetallic sensors This type of sensors is based on the observation that different materials can have different thermal expansion properties. These properties are mainly mechanical in nature. Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Bimetallic sensors Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Bimetallic sensors Bimetallic sensors can be directly used in temperature control applications. Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Bimetallic sensors Helical Coiled Bimetal element Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Bimetallic sensors Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Thermocouple Thermocouples are the most popular temperature sensors. They are cheap, interchangeable, have standard connectors and can measure a wide range of temperatures. The main limitation is accuracy, system errors of less than 1°C can be difficult to achieve. Electronic Instrumentation Lecturer Touseef Yaqoob

Thermocouple: Principle Temperature dependent voltage source Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Thermocouple: Types Type K (Chromel / Alumel) Range: -200 °C to +1200 °C. Type E (Chromel / Constantan) High output (68 µV/°C) more suited for low temperature Type J (Iron / Constantan) Range: -40 to +750 °C. Type N (Nicrosil / Nisil) High stability and resistance to high temperature oxidation. Type B (Platinum / Rhodium) Suited for high temperature measurements up to 1800 °C. Low sensitivity (10 µV/°C) Type R (Platinum / Rhodium) Suited for high temperature measurements up to 1600 °C. Low sensitivity (10 µV/°C) Type S (Platinum / Rhodium) Suited for high temperature measurements up to 1600 °C. Low sensitivity (10 µV/°C) Electronic Instrumentation Lecturer Touseef Yaqoob

Thermocouple: Characteristics Electronic Instrumentation Lecturer Touseef Yaqoob

Thermocouple: Connection Volt meter Thermocouple Extension cord Meter leads Care should be taken to compensate the voltages generated at junctions Electronic Instrumentation Lecturer Touseef Yaqoob

Resistive thermometers This type of sensors is based on the observation that different materials can have different resistive profiles at different temperatures. These properties are mainly electrical in nature. Electronic Instrumentation Lecturer Touseef Yaqoob

Resistive thermometers: RTD Electronic Instrumentation Lecturer Touseef Yaqoob

Resistive and temperature profile Electronic Instrumentation Lecturer Touseef Yaqoob

Mathematical relationships For RTD made of platinum, the temperature profile is quite linear and can be represented by For RTD made of copper, the characteristics can be approximated by Electronic Instrumentation Lecturer Touseef Yaqoob

Characteristics and issues of RTD Industrial RTDs are very accurate: the accuracy can be as high as ±0.1°C. The ultra high accurate version of RTD is known as Standard Platinum Resistance Thermometers (SPRTs) having accuracy at ±0.0001°C. Special attention should be given on the wiring of RTD bridge connection as well as self-heating when a current is sent through the RTD. Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob 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. (Note: opposite from RTD) The operating range can be as wide as -200°C to + 1000°C Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Thermistors Electronic Instrumentation Lecturer Touseef Yaqoob

Typical characteristics of Thermistors Electronic Instrumentation Lecturer Touseef Yaqoob

Measurement of temperature with some examples To understand the principle of bridge circuit and be able to perform simple calculations to use bridge to measure low level voltages To be able to determine the temperature from an RTD sensor reading To be able to calculate the temperature from an thermistor reading To be able to calculate the temperature from an thermocouple reading To be able to understand the basic concept of D/A and A/D converters and calculate quantization errors. Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Bridge circuits Bridge circuit is used quite often to measure low level voltages, such as the outputs from RTD, thermister, or thermocouples. In the case of a balanced bridge, there is no voltage drop between, B and C, hence, Ig = 0. We have Electronic Instrumentation Lecturer Touseef Yaqoob

Bridge circuits (Cont) Furthermore Therefore, the following condition is established for a balanced bridge: Any change in one arm of the bridge will destroy this balance condition. However, one can use the measured voltage across the bridge to calculate changes in one arm. Electronic Instrumentation Lecturer Touseef Yaqoob

Bridge circuits (Cont) Suppose that a volt meter of infinite impedance (Ig = 0) is used to measure the voltage across points B and C.Because (Ig = 0), The voltage drop will be Of course, under a balanced condition, Electronic Instrumentation Lecturer Touseef Yaqoob

Bridge circuits (Cont) Under this condition, suppose that there is a change in R1, such that: The corresponding change in voltage across B and C is To simplify the analysis, if we assume that Electronic Instrumentation Lecturer Touseef Yaqoob

Bridge circuits (Cont) Electronic Instrumentation Lecturer Touseef Yaqoob

Bridge circuits (Cont) Example An RTD is connected in a Wheatstone bridge as shown: Electronic Instrumentation Lecturer Touseef Yaqoob

Bridge circuits (Cont) Under a balanced condition, the parameters are given as follows: The temperature constant of the RTD: Electronic Instrumentation Lecturer Touseef Yaqoob

Bridge circuits (Cont) Questions: (a) What is the value of R RTD under the balanced condition ? (b) As temperature changes, it is found that the maintain a new balance, the new value for R1 has to be: Determine the change in temperature. Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Solution: (a) Using the relation: Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Solution: (b) Since the relationship between the resistance of the RTD and the temperature change can be approximated by: At the newly established balance condition with We can conclude that: Electronic Instrumentation Lecturer Touseef Yaqoob

Temperature measurement with RTD Typical characteristics of RTD Electronic Instrumentation Lecturer Touseef Yaqoob

Temperature measurement with RTD For RTD made of platinum, the temperature profile is quite linear and can be represented by For RTD made of copper, the characteristics can be approximated by Electronic Instrumentation Lecturer Touseef Yaqoob

Temperature measurement with RTD A platinum RTD is connected as one arm of a Wheatstone bridge as shown The fixed resistors, R2 and R3 are 25Ω. The RTD has a resistance of 25Ω at 0oC, and the coefficient of resistance of the RTD is Electronic Instrumentation Lecturer Touseef Yaqoob

Temperature measurement with RTD A temperature measurement is made by placing the RTD in the measuring environment and balancing the bridge by adjusting R1 to a new value of 37.36 Ω. Determine the temperature of the measuring environment Solution At balanced condition, we have From We can find that the temperature is 126o C. Electronic Instrumentation Lecturer Touseef Yaqoob

Temperature measurement with thermistors The mathematical relationship describing a thermistor can often be expressed as As temperature increases, the resistance decreases. R0 is the resistance at T0, and β is a parameter ranging from 3500 to 4600. Electronic Instrumentation Lecturer Touseef Yaqoob

Temperature measurement with thermistors Example A thermistor is placed in a 100oC environment, and its resistance is measured at 20,000Ω. The material constant, β, for this thermistor is 4000. If the thermistor is then used to measure a particular temperature, and its resistance is measured as 500 Ω, determine the environmental temperature being measured. Electronic Instrumentation Lecturer Touseef Yaqoob

Temperature measurement with thermistors Solution: From the basic equation of thermistor, we can have Further Electronic Instrumentation Lecturer Touseef Yaqoob

Temperature measurement with thermocouples Electronic Instrumentation Lecturer Touseef Yaqoob

Temperature measurement with thermocouples Electronic Instrumentation Lecturer Touseef Yaqoob

Thermocouples: Example A type-J thermocouple circuit below is used to measure the temperature T1 . The thermocouple junction # 2 is maintained by 32o F. The voltage output is measured to be 15 mV. Determine the temperature T1. Solution The temperature T can be read off from the graph for type-J thermocouple to be 530o F. Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Quartz Thermometers Quartz thermometer works on a principle that the resonant frequency of a material/quartz is a function of time. The instrument has a very linear output characteristic over the range between -40 and +230 degree centigrade. The characteristics of the instrument are generally very stable over long periods of time and therefore less frequent calibration is required. Electronic Instrumentation Lecturer Touseef Yaqoob

Example of quartz thermometer Electronic Instrumentation Lecturer Touseef Yaqoob

Radiation Thermometers Radiation thermometer deals with principle that all bodies emit electromagnetic radiation as a function of their temperature. The power spectral density of this emission varies with the temperature. Measurement of radiation emitted from the body allows the temperature of the body to be calculated. Radiation thermometers have one major advantage that they do not require to be in contact with the body. Electronic Instrumentation Lecturer Touseef Yaqoob

Radiation Thermometers Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Optical pyrometer Optical pyrometers are non-contact temperature measurement devices. They work by using the human eye to match the brightness of a hot object to the brightness of a calibrated lamp inside the device. Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Practical pyrometer Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Radiation Pyrometer Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Thermography Thermography , or thermal imaging, involves scanning an infrared radiation detector across an object. Electronic Instrumentation Lecturer Touseef Yaqoob

How Thermal-Imaging Works The basic operation of a thermal imaging device is a five-step process: A special lens focuses the incoming infrared radiation (heat given off from all objects) of the objects in the view. The focused radiation is scanned by a “phased array” of infrared detectors. Thousands of points and heat readings for the field of view are collected in only one thirtieth of a second. The detector elements create a very detailed “temperature map” called a thermogram. Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Continued The thermogram created by the infrared detector elements is translated into electric impulses. The electric impulses are sent to a circuit board, called a signal-processing unit, which has a dedicated chip for translating the electric impulses into data for the display. The signal-processing unit sends the data to the display, where it appears as various colors or shades depending on the temperature of the infrared emission. The image is created from the combination of all the impulses from all of the elements. Electronic Instrumentation Lecturer Touseef Yaqoob

Electronic Instrumentation Lecturer Touseef Yaqoob Thermography device Electronic Instrumentation Lecturer Touseef Yaqoob

Thermography device result Electronic Instrumentation Lecturer Touseef Yaqoob

Acoustic Thermometers Acoustic thermometer relies on the principle that the sound velocity in substances depends on temperature. In gases, the velocity is proportional to the square root of the absolute temperature. In solids and liquids, the velocity decreases as the temperature increases. Two categories of acoustic sensing systems are used in temperature measurements: a) The system measures the acoustic characteristics of the medium whose temperature is measured. b) The system measures the acoustic characteristics of the object that is in thermal equilibrium with the measured medium. Electronic Instrumentation Lecturer Touseef Yaqoob

Fiber optic temperature sensors There are a wide number of devices that utilize fiber optics to aid in measuring temperature. Most are actually slight variation of radiation thermometers, but not all. Most all depend upon a temperature sensing component being placed on the tip of the fiber optic's "free end". The other end is attached to a measuring system that collects the desired radiation and processes it into a temperature value. Electronic Instrumentation Lecturer Touseef Yaqoob

Intelligent instrument Intelligent temperature transmitters bring all the usual benefits associated with intelligent instruments, like the transmitters now available include adjustable damping, noise rejection, self adjustment for zero sensitivity drifts and expanded measurement range. All the intelligent transmitters presently available have no-volatile memories. Such transmitters are separate boxes designed for use with transducers with have either a d.c. voltage output in the millivolt range or an output in the form of a resistance change. Electronic Instrumentation Lecturer Touseef Yaqoob