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Measuring Devices (Temperature, Pressure, Flow etc.)
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Measuring Devices (Temperature, Pressure, Flow etc.)
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Measuring Devices PRCO031 Semester I_2018The Thermocouple is a thermoelectric temperature sensor which consists of two dissimilar
metallic wires, e.g., one chromel and one constantan, coupled at the probe tip (measurement
junction) and extended to the reference (known temperature) junction.
The temperature difference between the probe tip and the reference junction is detected by
measuring the change in voltage (electromotive force, EMF) at the reference junction.
The absolute temperature reading can then be obtained by combining the information of the
known reference temperature and the difference of temperature between probe tip and the
reference.
Three Wire Layouts of Typical Thermocouples
Thermistor
Various types of sensors are used to measure temperature. One of these is the thermistor, or
temperature-sensitive resistor. Most thermistors have a negative temperature coefficient (NTC),
meaning the resistance goes up as temperature goes down. Of all passive temperature
measurement sensors, thermistors have the highest sensitivity (resistance change per degree of
temperature change). Thermistors do not have a linear temperature/resistance curve.
Pitot tube
A Pitot-static tube can measure the fluid flow velocity by converting the kinetic energy in the
fluid flow into potential energy.
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, Measuring Devices PRCO031 Semester I_2018
The principle is based on the Bernoulli Equation where each term can be interpreted as a form of
pressure
p + 1/2 ρ v2 + γ h = constant along a streamline (1)
where
p = static pressure (relative to the moving fluid) (Pa)
ρ = density (kg/m3)
v = flow velocity (m/s)
γ = ρ g = specific weight (N/m3)
g = acceleration of gravity (m/s2)
h = elevation height (m)
Each term of this equation has the dimension force per unit area - N/m2 or in imperial units psi,
lb/ft2.
Static Pressure
The first term - p - is the static pressure. It is static relative to the moving fluid and can be
measured through an flat opening in parallel to the flow.
Dynamic Pressure
The second term - 1/2 ρ v2 - is called the dynamic pressure.
Hydrostatic Pressure
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