Filled thermal systems
Another class of
thermometers that utilise the principle of expansion of substances with
temperature is called filled thermal
systems. Depending on the phase of the substance, which fills these devices,
these systems are sub-categorised into gas-,
liquid- and vapour-filled systems.
Gas-filled systems are
based on a basic law of gases. If a gas is kept in a metallic bulb (or a container)
at a constant volume, then if the temperature varies, so does the pressure
according to the relationship
(1)
where:
P1and T1 - absolute pressure (Pa)
and temperature (K) at state 1;
P2and T2 -
absolute pressure (Pa) and temperature (K) at state 2;
β - thermal coefficient of pressure,
equal to the volumetric thermal expansion coefficient,
K-1
Figure
1:
Gas- or liquid-filled thermometer.
Figure
1
schematically shows the design of a gas-filled thermometer. Gas (nitrogen or
helium) 1 fills the thermal bulb 2, capillary tube 3
and Bourdon tube of a manometer 4. The thermal bulb (usually made of
a stainless steel) is immersed in the measuring media. Variation of its
temperature causes change in pressure of the gas in the system. The manometer
measures this variation of pressure. The scale of the manometer is graduated in
°C, but not in Pa.
• a thermal bimetallic
temperature compensator is used in the manometer;
• an internal volume of the
thermal bulb should be greater than that of the capillary tube, the ratio Vb/Vc
(where Vb and Vc are volumes of the thermal bulb and of
the capillary, respectively) may vary from 40 to 60; this can be achieved by
reducing the internal diameter of the capillary tube or increasing the internal
volume of the thermal bulb. The longer the capillary tube, the bigger the
thermal bulb should be.
Therefore, gas-filled
thermometers are not widely used in practice.
Depending on the measured
temperature range, the system may be filled with a gas under pressure higher
than atmospheric. That is why variations in atmospheric pressure have no effect
on the indications of gas-filled thermometers.
Gas-filled thermometers have
several advantages:
• they have the widest
temperature range of all filled systems;
• as follows from the
equation (1) these thermometers have
uniform scales;
• they have the longest
capillary length compared with other filled systems.
These thermometers are
usually used for temperature measurement in the range from
-200 to 600 °C.
Figure 2: Vapour-pressure system.
Liquid-filled systems have
similar design with gas-filled thermometers (see Fig. 1). Organosilicone liquids, propanol and mercury are used as
thermometric liquids, which fill the entire system. Since the total volume of
the thermal system is constant, then variation of temperature of the media,
where the thermal bulb is immersed, causes variation in the pressure of the
thermometric liquid. This variation in pressure is proportional to the
variation of temperature. Therefore, scales of liquid-filled thermometers are
uniform.
Several factors influence
the accuracy during temperature measurements, namely:
• variation in ambient
temperature;
• variation in pressure head;
• variation in atmospheric
pressure.
In order to compensate the
influence of variation of an ambient temperature it is necessary to increase
the ratio internal volume of the
thermal bulb/internal volume of the capillary tube, and employ thermal
bimetallic compensators (see gas-filled thermometers). The error due to
variation of an ambient temperature is bigger in the case of liquid-filled
systems, compared to gas-filled systems. Therefore, the capillary length for
liquid-filled systems can not exceed 10 m.
When the thermal bulb is
placed below or above the manometer, results of such temperature measurements
will not be correct. This is because of different pressure head of the liquid
column compared with the case when this thermometer was calibrated (the
manometer and the thermal bulb were placed on the same level). In this case the
error can be eliminated by zero correction of manometer. The ultimate elevation
distance between the thermal bulb and the manometer are given in the
calibration certificate supplied with the liquid-filled thermometer.
To reduce influence of
variation of atmospheric pressure, the system is filled with liquid under
pressure from 0.5 to 2.0 MPa.
Here are the advantages of
liquid-filled thermometers:
• small time lag;
• small dimensions of thermal
bulb.
These thermometers are used
for temperature measurement in the range from -150 to 300 °C.
Vapour/pressure systems (see Fig. 2) are filled by 2/3 of the volume
of the thermal bulb 1 by liquid 2 which has a low boiling
temperature, for example, freon (refrigerant), propylene, acetone,
ethylbenzene, methyl chloride, etc. Another (upper) part of the thermal bulb
and the capillary tube 3 is occupied by saturated vapour 4 of
this liquid. Vapour pressure depends only on the temperature of saturated
liquid in the thermal bulb, and therefore, does not depend on the variation of
the ambient temperature (this is an advantage).
Relationship between saturation pressure and temperature for liquids is
non-linear (see Fig. 3). Hence, the
scales of these thermometers are non-uniform, with more widely spaced
increments at high temperatures. The length of the capillary tube usually does
not exceed 25 m.
Disadvantages of
vapour-pressure thermometers are as follows:
• narrow temperature range,
from -50 to 300 °C;
• slow response time (time
lag) of about 20 seconds;
• non-uniformity of the
temperature scale.
Figure
3:.
Saturation (vapour pressure) curve for methyl chloride.
Article Source:: Dr. Alexander Badalyan, University of South Australia
