Lecture 3:
Flow, Level, Temperature and Light
Measurement
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1- Flow Measurement
• Optimum performance of many processes requires specific flow rates.
• The cost of many liquids and gases are based on the measured flow
through a pipeline, making it necessary for accounting purposes to
accurately measure and control the rate of flow.
• There are many methods for the measurement of liquid flow such as:
– Venturi tube
– Rotameter
– Turbine flowmeter
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The Venturi tube
• The Venturi tube uses a specific reduction in tube size.
• When a fluid flows through a pipe with a constriction, the velocity of the
fluid increases in the constriction while the pressure decreases to satisfy the
principle of conservation of mechanical energy.
• Any gain in kinetic energy a fluid achieves due to its increased velocity is
negated by a drop in pressure according to Bernoulli’s equation.
p
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V 2  gh  constant
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where p is the pressure,  is the density, V is the velocity,
h is the elevation, and g is the gravitational acceleration
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The Venturi tube
• The flow rate is proportional to the square root of the pressure drop.
• Therefore, a D/P cell is a used to measure the differential pressure
before and at the constriction and then a square root extractor is used
to obtain a measure of the flow.
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Rotameter
• The rotameter is a cheap and reliable variable-area flow
meter used for direct visual indication of flow rate.
• The rotameter has a float in a tapered vertical tube with
the fluid flow pushing the float upwards.
• At equilibrium, the weight of the float is equal to the
force on it due to the flowing fluid.
• The distance to which the float rises in column is
proportional to the flow rate.
• The rotameter is purely mechanical device and so there
is no need to external power supply.
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The turbine flow meter
• The turbine rotor is mounted in the
center of the pipe and rotates at a
speed proportional to the rate of flow
of the fluid or gas passing over the
blades. The rate of revolution of the
rotor can be determined by certain
means.
• The turbine is only used with clean
fluids such as gasoline.
• The rotating flow devices are accurate
but expensive.
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2- Level Measurement
There are many methods for the measurement of liquid level
such as:
–
–
–
–
–
–
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Float sensors.
Ultrasonic devices.
Pressure level gauge.
Capacitive level gauge.
Bubbler devices.
Resistive tapes.
Float sensors
• The float material is less dense than the density of the liquid, and floats up
and down on top of the material being measured.
• The float sensor is accurate and have a linear output with level height.
• However, if the surface of the material being monitored is turbulent, the
float reading may vary excessively, some means of damping might be
required, such as a stilling well.
Stilling well arrangement
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Ultrasonic level gauge
• An ultrasonic pulse is transmitted and then reflected by the liquid surface back
to the receiver.
• The time to receive the echo pulse is proportional to the distance from the
surface of the liquid.
• Since there is no contact with the liquid, this method can be used for solids, and
corrosive and volatile liquids.
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Pressure level gauge
• Pressure is often used as an indirect method of
measuring liquid levels.
• The hydrostatic pressure at the bottom of the
container is given by:
p  gh
• where p is the pressure,  is the density, g is the
gravitational acceleration, and h is the height.
Hence, the level can be calculated from the
pressure and the density of the liquid.
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Capacitive probes
•
In the portion of the probe that is out of the liquid, air
serves as the dielectric between the rod and outer shell
(air has dielectric constant of 1).
•
In the section of the probe immersed in the liquid, the
dielectric is the liquid itself, which causes a large
capacitive change (water has dielectric constant of 80).
•
The capacitance change is directly proportional to the
level of the liquid and can be measured using an ac
bridge.
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Bubbler devices
• Bubbler devices require a supply of clean air or inert gas to prevent
interaction with the liquid.
• Gas from a pressure regulator is forced through a tube via a flow regulator,
and the open end of the tube is close to the bottom of the tank.
• The pressure required to force the liquid out of the tube is equal to the
pressure at the end of the tube due to the liquid, which is given by p  gh
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Resistive tapes
• A resistive element is placed in close proximity to a conductive strip in an
easily compressible nonconductive sheath. The pressure of the liquid
pushes the conductive strip against the resistive element, shorting out a
length of the resistive element that is proportional to the depth of the
liquid.
• The sensor can be used in corrosive liquids.
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3- Temperature Measurement
• Temperature is without doubt the most widely measured
variable.
• In chemical reactions, temperature control is of major
importance, since chemical reactions are temperaturedependent.
• All physical parameters are temperature-dependent, making it
necessary in most cases to measure temperature along with
the physical parameter, so that temperature corrections can
be made to achieve accurate parameter measurements.
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Temperature sensors
• There are many properties that change with temperature and
can be used as basis for temperature sensors.
• These are some of the commonly used temperature sensors:






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Bimetallic strips
Resistor-temperature detectors (RTD)
Thermistors;
Thermocouples;
Pyrometer; and
Semiconductors.
Bimetallic strips
• Many materials expand when heated. This forms the basis for the
bimetallic strip
• A bimetallic strip consists of two different metal strips of the same length
bonded together.
• These devices operate on the principle that different metals have different
coefficients of expansion.
• It can be used to open or close electric circuits, as in the simple
thermostat commonly used with domestic heating systems.
• They are robust, relatively cheap, but are fairly slow.
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Resistance temperature detector (RTD)
• The resistance of most metals increases in a reasonably linear way with
temperature according to the following equation:
RT  R0 (1  T )
where RT is the resistance at a temperature T °C, R0 the resistance at 0°C
and α is termed the temperature coefficient of resistance and is
constant for the metal.
• Resistance temperature detectors (RTDs) are resistive elements made
from metals such as platinum, nickel or copper alloys.
• Resistance devices are normally measured using a Wheatstone bridge, or
supplied from a constant current source. Care should be taken to prevent
the electrical current from heating the device and causing erroneous
readings.
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Example
What is the resistance of a platinum resistor at
480°C, if its resistance at 0°C is 110Ω? The
temperature coefficient of resistance for the
platinum is 0.00385 /°C.
Resistance at 480C  110 [ 1  0.00385 (480)] 
 110 (1  1.848) 
 313.28 
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Thermistors
• Thermistors are semiconductor material that
typically has a high negative temperature coefficient
of resistance.
• They have high sensitivity, which can be up to a 10%
change per degree Celsius, making it the most
sensitive temperature element available.
• However, they have very nonlinear characteristics
which make the device difficult to use as an
accurate measuring device without compensation,
but its sensitivity and low cost makes it useful in
many applications.
• The typical response time is from 0.5 to 5 seconds.
• When in use, care has to be taken to minimize the
effects of internal heating.
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Thermocouple
• When any conductor is subjected to a thermal gradient, it
will generate a voltage. This is known as the thermoelectric
effect or Seebeck effect.
• This effect is the basis for the thermocouple device.
• The thermocouple, as the name suggests, involves two
different metals.
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Thermocouple
• One of the ends of the two metals
are joined together and put at
temperature Th (the hot junction).
• The other end of both metals are put
at a temperature Tc (the cold
junction). The potential difference is
measured between these two ends.
• This potential difference depends on
the two metals used and the
temperature difference between the
two junctions.
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Thermocouple
• The Figure shows how the e.m.f varies with
temperature for a number of commonly used
pairs of metals (usually given reference letters).
• Usually, the cold junction is held at 0 °C.
• Thermocouples are generally mounted in a
sheath to give them mechanical and chemical
protection.
• The response time of an unsheathed
thermocouple is very fast. With a sheath this
may be increased to as much as a few seconds
if a large sheath is used.
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IC temperature sensors
• LM35: the output is a linear function of
temperature, of 10 mV/°C when the
supply voltage is 5 V.
• DS18B20: is a digital thermometer
provides 9 to 12-bit (configurable)
temperature readings which indicate the
temperature of the device.
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4- Light Measurement
The detection and conversion of light intensity into
electrical signals can be done using photocells such as:
Photo-resistors: devices change their resistance with light
intensity. As the light intensity increases, their resistance
decreases. Examples of these materials are selenium,
zirconium oxide, and cadmium sulfide.
Photo-transistor: Light generates hole-electron pairs, which
cause base current in phototransistors.
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