Liquid-in-glass thermometers
These
thermometers are used for temperature measurements from -200 to 750 °C.
They are contact-type thermometers. Fig.
1 shows the principle of their design.
Figure 1.: Liquid-in-glass thermometer
This
thermometer consists of a glass bulb 1, which is connected with a glass
capillary tube 2. A scale 3 in degrees of Celsius or
Fahrenheit is placed behind the capillary tube. The bulb, the capillary tube
and the scale are placed in a glass tube 4 to protect them against the
damage. A thermometric liquid 5 fills the bulb and a part of the
capillary tube. The operational principle of these thermometers is based on the
difference between the volume expansion of liquids and glass with temperature.
The relationship that governs the operation of this device is
where,
VT -
volume of liquid at temperature T, m3;
VT -
volume of liquid at temperature T0, m3;
∆T= T-T0 -
difference of temperatures, K;
ᵦ -
volumetric thermal expansion coefficient, 1/K.
The
volumetric thermal expansion coefficient of glass is much less than that of
liquids. The variation of temperature (up and down) of the bulb causes liquid
in the system to expand or decrease its volume, respectively. As a result of
such changes (the internal volume of the glass bulb and the glass capillary
varies negligible), the length of the liquid column in the capillary tube goes
up or down proportionally to the variation of temperature.
The
type of thermometric liquid depends on the lower and upper limits of the
measuring temperature range. Table presents the most common types of liquids used in these types of thermometer.
Table : Types of thermometric liquids.
Liquid
|
Temperature
range, °C
|
|
From
|
To
|
|
Mercury
|
-35
|
750
|
Toluene
|
-90
|
200
|
Ethanol
|
-80
|
70
|
Kerosene
|
-60
|
300
|
Petroleum
Ether
|
-120
|
25
|
Pentane
|
-200
|
20
|
Among
these liquids mercury is the
most widely used, because:
•
mercury is easy obtainable with high chemical purity;
•
mercury does not wet glass (this increases the accuracy of measurement/
reading);
•
mercury remains in liquid state in a wide temperature range.
Among
disadvantages inherent
to mercury-in-glass thermometers we can mention the following:
•
mercury is a poisonous element, which affects the central and peripheral
nervous system, its vapour is the most toxic;
•
small volumetric thermal expansion coefficient for mercury, therefore, mercury
is used in thermometers with capillaries of small internal diameter;
The
solidifying point of mercury, ie 38 °C,
limits the lowest temperature that can be measured by mercury-in-glass
thermometers. The upper temperature is determined by the temperature at which
glass still retains its solid properties. This temperature is equal about 600 °C
for glass, and about 750 °C
for silicon glass.
When
air above mercury in the capillary is removed, a mercury-in-glass thermometer
can be used at temperatures below 300 °C,
because the boiling temperature of mercury at atmospheric pressure is equal
356.9 °C.
In order to increase this temperature range it is necessary to increase the
boiling temperature of mercury (saturation temperature). This can be achieved
by increasing pressure in the capillary. Usually, the space above mercury in
the capillary is filled by inert gas (such as nitrogen, argon) under pressure.
Liquid-in-glass
thermometers with organic thermometric liquids are used for temperature
measurements from -200 to 200 °C.
One advantage of these thermometers is:
One advantage of these thermometers is:
• a higher volume thermal
expansion coefficient comparing with that for mercury (six times higher in
average).
Disadvantage
of thermometers with organic liquids
is:
•
these liquids wet glass, therefore, in order to increase the accuracy of
measurement/reading, glass capillaries with bigger internal diameters (up to 1
mm) are used.
Advantages
of liquid-in-glass thermometers are as
follows:
•
they are simple in design;
•
they are relatively highly accurate in temperature measurement.
There
are several disadvantages inherent
to liquid-in-glass thermometers
•
they are fragile;
•
it is difficult to perform readings due to low visibility of the scale;
•
they are not capable of distance transmission of a measuring signal, therefore,
they are used as locally placed devices;
•
impossibility to repair;
•
high values of time lag;
•
low visibility of mercury in the capillary.
Article Source:: Dr. Alexander Badalyan, University of South Australia
Article Source:: Dr. Alexander Badalyan, University of South Australia
