Scope:
Prescribes method for the measurement of temperature of water &
waste water.
Principle:
Temperature measurements may be
made with any mercury-in-glass thermometer, provided it is checked occasionally
against a precision thermometer certified by a competent agency. Depth temperature may be
obtained with a protected reversing thermometer or a thermister. Measurements
of temperature are, usually, more conveniently done using thermistors.
Procedure:
Make measurement with the
thermometer immersed directly in the water body, after a period of time
sufficient to permit constant reading. If the measurement of water temperature
cannot be carried out directly, carry it out in a sampling bottle. The bottle
should have a volume of at least one litre. Adjust its temperature to that of
the sample water before the measurement. Do not expose it to heat or direct
solar radiation. Measure temperature of tap water in a bottle through the water
flows until a constant reading is obtained. Make measurement of the
temperature of a water body at a particular depth with the thermometer or
thermistor immersed directly in the water body. After sufficient time has
elapsed to allow the thermometer or thermistor to come to the exact temperature
of the water, take a reading. In the case of the thermistor make a direct
measurement of its resistance & obtain the temperature of the water body
from the calibration curve supplied with the thermistor.
In the case of reversing
thermometer, obtain the reading by dropping a messenger weight along the wire
to which is attached the reversing thermometer in a reversing frame on a water
sampling bottle. This weight normally drops at a speed of about 150 meters per
minute except when the wire is extremely inclined to the vertical. After
sufficient time has passed for the messenger weight to trip the thermometer,
haul up the wire & keep the water bottle with the thermometer carefully in
a vertical position away from direct sunlight in order to prevent accidental
reversing before reading the temperature to be measured. Allow about 10 to 15
minutes for the thermometers to reach the air temperature, after they are
brought up from the water. At this stage the auxiliary thermometer records the
atmospheric temperature & the main thermometer, the approximate temperature
of the water body.
Calculations: Calculate the exact temperature of the water body, in the case
of the reversing thermometer, from the following formula:
Tw = T΄+C+l
& C = (T΄-Vo) (T΄-T1) / K – 100
Where
Tw
= The corrected value, that is, the true value of the water
temperature, °C
T’ – the reading of the main
thermometer , °C
I - the index correction given on a
calibration sheet supplied with the thermometer
C-
correction for thermal expansion
V0 – volume of mercury
below 0°C mark given on the calibration graph
K
– reciprocal thermal expansion coefficient given on the calibration graph, and
T1
– temperature reading of the auxiliary thermometer , °C
If
an unprotected reversing thermometer is used along with the protected
thermometer the corrected temperature Tu can be similarly obtained.
The actual depth of reversal of the thermometers can be obtained using the
following equation
Z = Tu – T w
Pm – Q
Where,
Z = depth in metres
Tu
= corrected reading of the protected thermometer, °C
Tw
= corrected reading of the protected thermometer , °C
Pm
= mean density of the water column
Q = pressure coefficient of the unprotected thermometer given on
the
calibration graph
Report:
Report the temperature of water to the nearest 0.01 , 0.1 or 0.5°C ,
depending on the accuracy required and the thermometer used.
