Mechanical Measurement and Instrumentation MECN 4600 Professor: Dr. Omar E. Meza Castillo omeza@bayamon.inter.edu http://www.bc.inter.edu/facultad/omeza Department of Mechanical Engineering Inter American University of Puerto Rico Bayamon Campus

Tentative Lecture Schedule Topic Lecture Basic Principles of Measurements Response of Measuring Systems, System Dynamics Error & Uncertainty Analysis 1, 2 and 3 Sensor & Transducers Basic Electronics, Signal Processing 4 Measurement of Pressure 5 Measurement of Temperature 6 Measurement of Fluid Flow 7 Measurement of Level 8 Measurement of Stress-Strain 9 Measurement of Time Constant 10

Topic 6: Measurement of Temperature Thermocouple

Course Objectives Understand the main operating characteristics of temperature measurement devices such as resistor temperature detectors, thermistors, and thermocouples Be able to experimentally determine temperature curves (Equations) for temperature measurement devices.

Temperature Temperature is the most common used and measured variable. Actually, the definition of temperature and scale is not well understood. Temperature is described as the property of an object that describes its hotness and coldness. The zeroth law of thermodynamics states that two systems in thermal equilibrium with a third system are in thermal equilibrium with each other. The conversion equations for the four modern temperature scales are: oC = 5/9 (oF - 32) oF = 9/5 oC + 32 K = oC + 273.15 oR = oF + 459.67

Temperature Temperature can be measured via a diverse array of sensors. All of them infer temperature by sensing some change in a physical characteristic. Six types with which the engineer is likely to come into contact are: THERMOCOUPLES, RESISTIVE TEMPERATURE DEVICES (RTD and THERMISTORS), INFRARED RADIATORS, BIMETALLIC DEVICES, LIQUID EXPANSION DEVICES, AND CHANGE-OF-STATE DEVICES. The most common method of measuring and controlling temperature uses an electrical circuit called a THERMOCOUPLE.

Devices to Measure Temperature Thermocouple RTD Thermistor

Thermocouple Thermocouples utilize the so-called SEEBECK effect (1821) in order to transform a temperature difference to a voltage difference (electromotive force emf). A THERMOCOUPLE consists of two electrical conductors that are made of dissimilar metallic materials and have at least one electrical connection. This electrical connection is referred to as a junction. A thermocouple junction may be created by welding, or by any method that provides good electrical contact between the two conductors, such as twisting the wires around one another. The output of a thermocouple circuit is a voltage, and there is a definite relationship between this voltage, and temperatures of the junctions that make up the thermocouple circuit.

Thermocouple Thermocouple Circuit To demonstrate the errors introduced in this procedure, introduced the junction of a type T thermocouple in boiling water (known to be at 100ºC) and read the voltage across the leads. The reading was 3.634 mV, which corresponds to 86.1ºC. emf Copper Constantan T Probe junction Thermocouple Type T

Thermocouple This temperature error arises because the connection of the thermocouple leads to the voltimeter constitutes two additional thermoelectric junctions that substract voltage from the signal being measured. This problem can be remedied using the arrangement show in this figure: emf Copper Constantan T Ice bath Reference junction Probe junction T To

Thermocouple One thermocouple junction is held in an ice bath at 0ºC. This called THE REFERENCE JUNCTION. The other thermocouple junction is THE TEMPERATURE PROBE. If the probe is at 0ºC, then there is no thermoelectric voltage across the leads because the thermoelectric voltage created by each junction cancel each other out. The general form of the electromotive force emf as a function of temperature T for all thermocouples is:

Type of Thermocouple

Type of Thermocouple

Type of Thermocouple

Experiment Setup https://www.youtube.com/watch?v=ETsyJR_p49M

Experiment using a Thermocouple Type K Temperature (oC) emf (mV) Experimental emf (mV) Standard 1 0.039 21 0.886 0.839 30 1.252 1.204 40 1.680 1.612 50 2.105 2.023 65 2.770 2.644 75 3.217 3.059 85 3.621 3.474 92 3.918 3.765

Laboratory 3  WebPage Omar E. Meza Castillo Ph.D.