Class Notes

1. AET 314 INSTRUMENTATION
HND 1 AGRIC & BIO- ENGINEERING
(FARM POWER & SOIL + WATER)
R R Dinrifo, Ph.D
MODULE 111
• Temperature Measurement.
• Relative Humidity.
• Density Measurement.

2. Temperature Measurement.
• The accurate measurement of temperature is vital across a broad
spectrum of human activities, including industrial processes (e.g.
making steel), manufacturing; monitoring (in food transport and
storage), and in health and safety. In fact, in almost every sector,
temperature is one of the key parameters to be measured.
• Temperature can be measured via a diverse array of sensors. All of
them infer temperature by sensing some change in a physical
characteristic. Six types with which the engineer is likely to come into
contact are: thermocouples, resistive temperature devices (RTDs and
thermistors), infrared radiators, bimetallic devices, liquid expansion
devices, and change-of-state devices.

3. • Liquid in glass - in particular mercury, thermometers have
been used for almost 300 years in science, medicine,
metrology and in industry. They rely on the expansion of a
fluid with temperature. The fluid is contained in a sealed
glass bulb and the temperature is read using a scale etched
along the stem of the thermometer.
• Thermocouple Temperature Measurement Sensors
Thermocouples consist essentially of two strips or wires
made of different metals and joined at one end. Changes in
the temperature at that juncture induce a change in
electromotive force (emf) between the other ends. As
temperature goes up, this output emf of the thermocouple
rises, though not necessarily linearly

4. • Resistance Temperature Devices(RTD)
Resistive temperature devices capitalize on the fact
that the electrical resistance of a material changes as
its temperature changes. Two key types are the metallic
devices (commonly referred to as RTDs), and
thermistors. As their name indicates, RTDs rely on
resistance change in a metal, with the resistance rising
more or less linearly with temperature. Thermistors are
based on resistance change in a ceramic
semiconductor; the resistance drops nonlinearly with
temperature rise.

5. • Infrared Temperature Measurement Devices
Infrared sensors are non-contacting devices. They infer
temperature by measuring the thermal radiation
emitted by a material
• Bimetallic Temperature Measurement Devices
take advantage of the difference in rate of thermal
expansion between different metals. Strips of two
metals are bonded together. When heated, one side
will expand more than the other, and the resulting
bending is translated into a temperature reading by
mechanical linkage to a pointer.

6. Thermocouples
• Thermocouples are a widely used type of temperature
sensor and can also be used as a means to convert thermal
potential difference into electric potential difference. They
are cheap, interchangeable, have standard connectors, and
can measure a wide range of temperatures. The main
limitation is accuracy; system errors of less than 1 °C can be
difficult to achieve

7. LIQUID LEVEL INDICATORS:
• HYDROSTATIC:
The simplest form of hydrostatic measurement is that in an
open vessel. The pressure at the bottom of a vessel is
directly proportional to the liquid height. By connecting one
end of a pressure sensing instrument to the lowest liquid
level in the vessel and the other end to a point above the
liquid where vapour is guaranteed the difference in pressure
can be obtained. Once the difference in pressure is known a
simple calculation can be performed between density of
liquid, height and pressure, which will indicate the height of
the level of the liquid.

8. Bubble tube systems:

9. • pH measurement
• In the process world, pH is an important parameter to be measured and
controlled. The pH of a solution indicates how acidic or basic (alkaline)
it is. The formal mathematical definition of pH is the negative logarithm
of hydrogen ion activity. In most cases, hydrogen ion activity can be
approximated by the hydrogen ion concentration, and the formula
becomes pH = - log10 [H+].
PH
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14
[H+]

10. Density Measurement.
• The density of a sample of a material is defined as the quantity of
mass of the material to a given volume of the sample. The material
becomes denser as the amount of material per unit volume rises. It
can be said that the molecules of a dense material are "packed" more
tightly together than the equivalent weight of a lighter material.

11. • Several methods are used to measure density. As with any
instrument, all factors must be considered before deciding upon the
method most suited for the application. Among these factors are
Capacitance
It is possible to measure the density of a liquid by using a large
capacitor. The capacitor is usually some arrangement of concentric
tubes made of conducting material. As the liquid fills the annular
space, the dielectric constant of the capacitor changes, causing a
change in the capacitance of the sensor. The capacitance is
proportional to the dielectric constant, which is proportional to the
density.

12. Hydrometers
• Hydrometers represent the simplest and most economical
method of measuring density. A hydrometer is a calibrated
glass float, which incorporates a scale for direct reading.
Hydrometers may be calibrated in one of several specialized
scales:
• Alcoholometer: This hydrometer is used for testing alcoholic
solutions. The scale is calibrated is percent of alcohol by
volume
• Ammoniameter: For testing Ammonia solutions. The scale is
calibrated in 0°-40°.
• Brix: Used in the sugar industry for determining the percentage
of sugar in solution. Can also be used in determining fruit
maturity (Brix is percent sugar).

13. Quiz
1. The types of thermometers which the engineer is likely to come into contact
are includes all except:
a. thermocouples, bimetallic devices
b. resistive temperature devices (RTDs and thermistors),
c. infrared radiators, liquid expansion devices.
d. clinometers
2. The type used to screen a person’s temperature during COVID 19 is
a. Infrared temperature sensor.
b. thermocouple
c. Liquid expansion device
d. thermistor
3. Hydrometers measure relative density and the following, except one, are
classes of hydrometers
a. Ammoniameter
b. Alcoholometer
c. Brix
d. Permeammeter