Unit 9 Optoelectronic and Miscellaneous Devices PDF

Unit 9 Optoelectronic and Miscellaneous Devices - 5 1. Discuss the operation of three types of temperature- measuring circuits. 2. Describe methods of measuring strain, pressure, and flow rate 1. Temperature-measuring circuits Thermocouple - is formed by joining two dissimilar...

Unit 9 Optoelectronic and Miscellaneous Devices - 5 1. Discuss the operation of three types of temperature- measuring circuits. 2. Describe methods of measuring strain, pressure, and flow rate 1. Temperature-measuring circuits Thermocouple - is formed by joining two dissimilar metals. A small voltage, called the Seebeck voltage is produced across the junction of the two metals when heated. – The amount of voltage produced is dependent on the types of metals and is directly proportional to the temperature of the junction. – The voltage produced by the junction is generally much less than 100 mV. Thermocouple Voltage vs. Temperature Curves - Output of some common thermocouples with 0ºC as the reference temperature. When a thermocouple is connected to a signal- conditioning circuit, an unwanted thermocouple is effectively created at the 350 Unit 9 F2014 IC 1 point(s) where the thermocouple wires connect to the circuit terminals. The unwanted thermocouple junction is sometimes referred to as a cold junction in some references because it is normally at a significantly lower temperature than that being measured by the measuring thermocouple. One method for eliminating an unwanted thermocouple effect is to add a reference thermocouple at a constant known temperature (usually 00C) Another approach is to compensate for the unwanted thermocouple effect by adding a compensation circuit consisting of a resistor and an integrated circuit temperature sensor. This is sometimes referred to as cold-junction compensation. Resistance Temperature Detector (RTD) A resistive device with resistance that changes directly with temperature. (positive temperature coefficient) 350 Unit 9 F2014 IC 2 - The second major type of temperature transducer. - The RTD is more nearly linear than the thermocouple. – RTDs are constructed in either a wire-wound configuration or by a metal-film technique. - The most common RTDs are made of either platinum, nickel, or nickel alloys. Basic RTD Temperature - Measuring Circuits Thermistor A third major type of temperature transducer. - Is a resistive device made from a semiconductive material such as nickel oxide or cobalt oxide. – The resistance of a thermistor changes inversely with temperature (negative temperature coefficient). – The temperature characteristic for thermistors is more nonlinear than that for thermocouples or RTDs – The temperature range of a thermistor is more limited than that of a thermocouple. – Thermistors have greater sensitivity than RTDS or thermocouples. General comparison of the responses of a thermistor circuit to a similar RTD circuit: 350 Unit 9 F2014 IC 3 2. Strain, Pressure, and Flow Rate Measurement Strain - is the deformation, either expansion or compression, of a material due to a force acting on it. If a metal plate is bent, there is an expansion on the upper surface, called tensile strain, and a compression on the lower surface, called compressive strain. A strain gauge is basically a long very thin strip of resistive material that is bonded to the surface of an object on which strain is to be measured Gage factor: The ratio of the fractional change in resistance to the fractional change in length long the axis of the gage. R / R GF  L / L Strain gauges exhibit a resistance change when deformed, therefore, strain gauges are usually applied in bridge circuits or in constant-current-driven circuits. 350 Unit 9 F2014 IC 4 Basic Strain-Measuring Circuits Pressure transducers - Are devices that exhibit a change in resistance proportional to a change in pressure. - Pressure sensing is accomplished using a strain gauge bonded to a flexible diaphragm. A Simplified Pressure Sensor: Measuring Flow Rate The flow rate of a fluid through a pipe may be measured using differential pressure. – A flow restriction is placed in the flow stream (such as a Venturi section). – The volume of fluid through the restriction remains constant, so the velocity and pressure increase Although the velocity of the fluid increases as it flows through the narrow channel, the volume of fluid per minute (volumetric flow rate) is constant throughout the pipe. 350 Unit 9 F2014 IC 5