Temperature and Pressure Sensors Seth Price Department of Chemical Engineering New Mexico Tech Rev. 10/27/14

Temperature Sensors Thermal expansion –Thermometer –Bimetallic strip Thermoelectrical –Thermocouple –RTD –Thermistor Thermal emission –Optical pyrometry

Bimetallic Strip Metallic strip can be used to close a switch Often found in dial thermometers and thermostats

Thermocouples Seebeck Effect: –Two different metals in direct contact at one end will generate a voltage –This voltage linearly relates to temperature

Cold Junction Compensation Voltage arises from difference in temperature at T 1 and T 2 (relative temperature) T 2 must be known to know T 1. Ways to do this: –Hold T 2 constant –Use another thermocouple to measure T 2 –Handled through software/hardware configuration (NI uses this)

Thermopiles Thermocouple voltage is small Signal must be amplified Connect multiple thermocouples in series (thermopile) –VT = V1 + V2 + … + Vn Caveat: Thermocouples should be at the same temperature

Resistance Temperature Detectors (RTD) Resistance in metals increase with increasing temperature Resistance also changes with strain –Metal in RTDs should have minimal residual strain Much slower response than other temperature sensors Wire lead resistance is a problem

Resistance of Metals

Thermistors Semiconductor that changes resistance with temperature –Similar to RTDs Has much higher resistance –Wire lead resistance becomes insignificant Fast response times

Optical Pyrometer Uses emissions from the object to determine its temperature

Optical Pyrometer Receives radiation and can calculate temperature Typically used for higher temperatures Used to determine if UV shielding is needed

Bourdon Tube A coiled tube uncoils with respect to pressure Linked to a needle on a gauge Really common

Gas Cylinders Safety –Transport with cap in place (not regulator!) –Chain to wall when regulator is in place Regulators –2 Stage regulator keeps constant pressure –Different gases use different regulators –Open or close all the way (except acetylene)

Piezoelectric Transducer Piezoelectric: generate voltage when pressure is applied –Barium Titanate BaTiO3 –Quartz (SiO2)

Strain Gauges A diaphragm is deflected by increased pressure, causing a strain Strain measured with a small strain gauge

Parallel Plate Capacitor Sensor Pressure moves one plate of the capacitor to change capacitance Depends on dielectric constant Can be used to measure pressure or vacuum C = (Є*Є 0 *A)/dC = (Є*Є 0 *A)/(d+Δ)

Thermocouple Pressure Sensing A heater is placed near a thermocouple junction Convective heat transfer is proportional to gas pressure 1 to 0.01 torr

Other Uses for Pressure Gauges Level gauge –Pressure gauge at the bottom of a tank –Calibrate voltage output to % full Flow meter –Pressure gauge (measuring differentially) across an orifice plate Proximity sensor –Pressure steps from baseline to positive value

Calibration Compare voltage (or current) measured to a standard (ex. thermometer) Change environment, recheck voltage and standard Plot voltage (or current) vs. physical phenomenon

Ex: Thermocouple Calibration Volts, V Temperature, F

In LabVIEW Create a sub-VI to convert from voltage to temperature Document when you did this calibration –If possible, include serial numbers

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References 000A_240x225.jpg nmuscle/momuscle2.htm