Transcript Data Acquisition - New Mexico Institute of Mining and Technology

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 T1
and T2 (relative temperature)
• T2 must be known to know T1. Ways to do this:
– Hold T2 constant
– Use another thermocouple to measure T2
– 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
C = (Є*Є0*A)/d
• Can be used to
measure pressure or
vacuum
C = (Є*Є0*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
Temperature, F
Ex: Thermocouple Calibration
Volts, V
In LabVIEW
• Create a sub-VI to convert from voltage
to temperature
• Document when you did this calibration
– If possible, include serial numbers
References
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http://en.wikipedia.org/wiki/Pressure_measurement
http://www.freepatentsonline.com/6604428-0-large.jpg
http://en.wikipedia.org/wiki/Pressure_regulator
http://www.stanford.edu/class/me220/data/lectures/lect08/lect_4
.html
http://zone.ni.com/cms/images/devzone/tut/a/d11edcae1720.jpg
http://www.pchemlabs.com/files/images/531_gauge_tube.jpg
http://www.lesker.com/newweb/Gauges/gauges_technicalnotes
_1.cfm
http://www.varianinc.com/image/vimage/docs/products/vacuum/
pumps/ion/vacion150/shared/vacion150-180.gif
http://www.plasmainstrument.com/images/duo5pump.jpg
http://commons.wikimedia.org/wiki/File:Cut_through_turbomolec
ular_pump.jpg
http://www.medivactech.com/images/cryotorr_6.jpg
References
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http://www.thermoworks.com/products/logger/images/logmaster/RFTC4
000A_240x225.jpg
http://z.about.com/d/homerepair/1/0/0/5/-/-/mercury_tstat_1.jpg
http://img.alibaba.com/photo/11295842/Thermistor_Sensors.jpg
http://www.cih-group.co.uk/reheating.htm
http://www.yearone.com/enthusiast/restoarchives/spring98/sp98motow
nmuscle/momuscle2.htm
http://www.lumasenseinc.com/files/Image/technology/temperature.jpg