A SURVEY OF WATER QUALITY MEASUREMENT SENSORS
A SURVEY OF WATER QUALITY MEASUREMENT SENSORS
Water is essential for the survival of
all life. Humans depend on water to fulfill a variety of needs and wants. These
include water for drinking, health and sanitation, recreation and as part of spiritual
and cultural traditions.
Water quality is measure of suitability
of water for particular use. It depends on various physical, chemical and
biological parameters. Generally measured water quality parameters are temperature,
turbidity, pH, conductivity, dissolved oxygen (DO) and total dissolved solid
(TDS).These parameters are measured routinely in order to maintain the good
water quality.
Temperature:
Water temperature is one of the five
important factors for water quality testing. It controls the rate of metabolic
and reproductive activities and hence aquatic life cycle. If water temperature
increases, decreases or fluctuate, these activities may speed up, slow down or
stop. Thermoelectric power and heat resistance temperature sensor are most
commonly used to detect
water temperature. Water temperature is generally measured in Celsius (ºC).
Conductivity:
Conductivity defines the ability of
water to conduct electricity. This type of measurement accesses the concentration
of ions in a solution. More the ions higher will be the conductivity. For water
to be pure its conductivity should be poor. Generally there are two types of conductivity
sensors: two electrodes and multiple electrodes from which two electrodes
sensor is commonly used. It is made by using two platinum plates deposited on
two parallel glass or inner wall of glass tube. Conductivity of water measures
in µs/cm or mA.
Turbidity:
Turbidity is a
measure of suspended particles present in water also called as cloudiness of
water. It is mainly caused by suspended solids like soil particles and plankton
(microscopic plants). These particles block the incident light and scatter or
diffuse it. Therefore photoelectricity sensor is used to detect the turbidity
of water. Turbidity sensor measures the turbidity level of water by measuring
the amount of light scattered at 90 degree. Moderately low levels of turbidity
indicate a healthy and safe water to drink while higher level of turbidity
poses several problems. Higher turbidity of drinking water can provide food
shelter to microbes and thus pathogens are more likely to present in such
water. More turbid water can also raise the water temperature above normal as
suspended particles in water absorb the heat from sunlight. Turbidity of water measured
in unit called „NTU‟ i.e. nephelometric turbidity unit. For water to be pure
its turbidity should lie in range of 0 to 5 NTU.
pH:
pH is an
important chemical factor of water. It is measure of acidity or basicity of a
solution. It is based on concentration of H+ or OH- ions present in water. pH
scale is a logarithmic scale in which value changes in factor of 10. The pH
scale ranges from 0 to 14.If pH of a solution is less than 7 then the solution
is acidic on the other hand if pH value is greater than 7 then the solution is
basic or alkaline and if pH of a solution is 7 then the solution is neutral. Normal
water generally has pH value in between 6 to 9. The most common way of
measuring pH of solution involves the use of pH sensitive glass electrode, a
reference electrode and a pH meter which measures the difference in electrical potential
between the pH electrode and the reference electrode and display the result
converted into equivalent pH value.
Water
Quality Sensors
Temperature
Sensor
LM 35
LM 35 is
commonly used temperature sensor whose output voltage is linearly proportional
to Celsius temperature. It is more accurate than thermistor. It does not
require any external calibration or trimming to provide typical accuracies at
room temperature over range -55ºC to 150º C. It possesses low self-heating and
does not cause more than 0.1º C temperature rise or fall. The output varies by
10mV with respect to every ºC rise or fall in ambient temperature which means
its scale factor is 0.01V/ºC.
DS18B20
It is a
digital temperature sensor, using single bus protocol. Operating temperature
range is -55ºC to +125ºC and accuracy between -10ºC~+85ºC is ±0.5º.
PT100
PT100 sensor
is a temperature dependent component. Its resistance rises linearly with the
temperature. Its operating temperature range is -50ºC to +230ºC. It is low
weight precise temperature measuring device with ±0.5 ºC accuracy. Its nominal
resistance is 100Ω at 0ºC and has no self-heating. It is available in 2 wire, 3
wire and 4 wire package.
Turbidity
Sensor
Photo-electricity sensor
It works on phenomenon that, when light
rays passed through waterthe amount of light scattered is equal to the
turbidity of water. More the suspended particles more will be light scattered
that means higher the turbidity of water. Photoelectricity sensor consists of
LED at transmitter side and LDR at receiver side. When LED emits light the rays
passes through water and scattered due to suspended particles present in water.
LDR detect the scattered light rays and likewise measures the turbidity of
water.
pH Sensor
A pH glass probe is made up of special
glass that can conduct electricity and allow only hydrogen ion. When glass
probe touches the hydrogen ion the potential is produced. Likewise different pH
in water generates corresponding potential. It has pH value ranging from 0 to
14. It operates on temperature range of 5 to 60ºC.The combined electrodes
(glass and reference) are fitted with BNC type connector as in pH sensor
module. Below the cap a hole is provided for filling the solution in internal
reference electrode. It operates on 5V DC supply and current of 5 to 10mA. Its
response time is 5 sec. and stability time is 60 sec. It has service life of 3
years and has analog voltage signal output.
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