Basic Principles of Chilled Mirror Dew Point Meter and Several Problems That Should Be Noticed in Dew Point Measurement
1.
Basic principle of cold mirror dew point meter
2.
Some problems that should be noticed in dew point measurement
2.1
Effect of mirror pollution on dew point measurement
2.2
Selection of measurement conditions for dew point meter
2.3
Problems in low frost point measurement
1.
Basic principle of cold mirror dew point meter
When
a certain volume of gas is cooled uniformly under a constant pressure, the
partial pressure of the gas and the moisture in the gas remains unchanged until
the moisture in the gas reaches a saturated state, and the temperature in this
state is the dew point of the gas. Usually, the mirror surface and its accessories
are installed in the measurement chamber through which the gas flows, and the
dew point of the gas is determined by measuring the temperature of the mirror
surface when the number of water molecules leaving and returning to the mirror
surface reaches dynamic equilibrium in a unit time. A certain gas humidity
corresponds to a dew point temperature; a dew point temperature corresponds to
a certain gas humidity. Therefore, measuring the dew point temperature of the
gas can determine the humidity of the gas. The maximum humidity can be obtained
from the dew point, and the relative humidity of the gas can be obtained from
the dew point and the temperature of the measured gas. The value directly given
by the dew point meter is the dew point temperature, which should be
"thermodynamic dew point temperature". The definition adopted by the
World Meteorological Organization is the thermodynamic dew point temperature Td
of the wet air with a pressure of P and a mixing ratio of r, which refers to
the temperature when the wet air is saturated with water at a given pressure.
At this temperature, the saturated mixing ratio rw of the wet air is equal to
the given mixing ratio r. The working principle of the photoelectric dew point
meter can be simply described as: the measured gas is passed through a smooth
metal mirror surface cooled with cold nitrogen at a certain flow rate under a
constant pressure, and as the temperature gradually decreases, the mirror
surface reaches a certain temperature Dew (or frost) begins, and the mirror
temperature at this time is the dew point temperature. The instrument measures
the dew point temperature Td through the optical system, temperature
measurement circuit, logic control circuit, and digital display circuit, and
displays it.
2.
Some problems that should be noticed in dew point measurement
2.1
Effect of mirror pollution on dew point measurement
In
the dew point measurement, the specular pollution is a prominent problem, and
its impact is mainly manifested in two aspects: one is the Raoul effect, and
the other is the change of the specular background scattering level. The Raoul
effect is caused by water-soluble substances. If this substance (usually
soluble salts) is carried in the measured gas, dew condensation will occur on
the mirror surface in advance, which will cause a positive deviation in the
measurement result. If the pollutants are water-insoluble particles, such as
dust, it will increase the background scattering level, which will cause the
photoelectric dew point meter to drift zero. In addition, it is needless to say
that the vapour of some condensable substances (such as organic matter) with a
boiling point lower than that of water will interfere with the measurement of
the dew point. Therefore, any type of dewpoint meter should prevent
contamination of the mirror surface. Generally speaking, the impact of
industrial process gas analysis pollution is more serious. However, even in the
measurement of pure gas, the pollution of the mirror surface will accumulate
over time. In order to eliminate the effects of pollution, people have explored
various methods. * The intuitive method is to filter the measured gas. At the
same time, clean the mirror regularly or at any time according to the specific
situation. In addition, the usual method is to heat the mirror surface before
each measurement and ventilate the pollutants. In the case of obvious
pollution, it can also be achieved by repeating the dew condensation and dew
process multiple times. In order to eliminate the interference of easily
condensable hydrocarbons, some people use the double mirror technology to add a
"dry" mirror in the dew point room. The temperature is slightly
higher than that of the dew point. "The mirror is as contaminated as the
exposed mirror, but it does not condense, providing compensation.
2.2
Selection of measurement conditions for dew point meter
In
the design of the dew point instrument, several factors that directly affect
the heat and mass exchange in the dew condensation process must be considered.
This principle is also applicable to the selection of the operating conditions
of the dew point instrument with a low degree of automation. Here we mainly
discuss the problem of mirror cooling rate and sample gas velocity. The
temperature of the measured gas is usually room temperature. Therefore, when
the airflow passes through the dew point chamber, the heat and mass transfer of
the system must be affected. When other conditions are fixed, increasing the
mass density of the flow velocity will facilitate the mass transfer between the
airflow and the mirror surface. Especially when measuring the dew point, the
flow rate should be appropriately increased to speed up the formation of the
dew layer; but the flow rate should not be too large, otherwise it will cause
overheating problems. This is especially true for thermoelectric cooling dew point
meters with relatively low cooling power. Excessive flow rate will also cause
the pressure of the dew point chamber to decrease, and the change of the flow
rate will affect the thermal equilibrium of the system. Therefore, it is
necessary to choose an appropriate flow rate in dew point measurement. The
choice of flow rate should depend on the cooling method and the structure of
the dew point chamber.
The
general flow rate range is from 0.4L / min to 0.7L / min. In order to reduce
the effect of heat transfer, pre-cooling treatment can be considered before the
measured gas enters the dew point chamber. The control of the cooling rate of
the mirror surface is an important issue in dew point measurement. For
automatic photoelectric dew point meters, it is determined by design, and for
manual dew point meters, it is a problem in operation. Because there is a
process and speed between the cooling point of the cold source, the temperature
measurement point, and the heat transfer between the mirrors, it brings errors
to the measurement results. In this case, the temperature gradient between the
temperature measurement point and the mirror surface used is relatively large,
and the heat conduction speed is relatively slow, so that the temperature
measurement and dew condensation cannot be performed simultaneously. Moreover,
the thickness of the exposed layer cannot be controlled. This will produce a
negative error for visual inspection. Another problem is that cooling too
quickly may cause "overcooling." We know that under certain
conditions, when the water vapor reaches a saturated state, the liquid phase
still does not appear, or the water does not freeze when the temperature is below
zero, this phenomenon is called super saturation or "super cooling".
For the condensation (or frost) process, this phenomenon, is often caused by
the measured gas and mirror surface being very clean, or even a lack of a
sufficient number of condensation cores. Under cooling is short-lived, and its
duration is related to the dew point or frost point temperature. This
phenomenon can be observed through a microscope; one solution is to repeat the
operation of heating and cooling the mirror surface until the phenomenon is
eliminated. Another solution is to directly use the water vapor pressure data
of the subcooled water. And this is exactly in line with the definition of
relative humidity when the meteorological system is below zero. It can be seen
from the above that whether considering the thermal inertia or super cooling
phenomenon, the cooling rate should not be too fast. If it exceeds a reasonable
range, the faster the cooling rate is, the greater the thermal inertia is, and
the larger the dew point measurement error is, the more Prone to overcooling. *
The best cooling rate is generally determined through experiments.
2.3
Problems in low frost point measurement
Frost
point meter or low frost point hygrometer is one of the few * effective methods
in trace water measurement. However, some problems must be paid full attention
during the measurement. The first problem is the thickness of the frost layer
that affects the detection. In the case of low water content, the frost layer
is very thin and the change is slow, which increases the difficulty of
detecting frost. For example, when the frost point is lower than -65 ℃, the
mobility of water molecules on the mirror surface decreases, and the
crystallization speed decreases accordingly. It takes time for the layers to
appear relatively stable. The lower the frost point temperature, the greater
the difficulty, and the measurement error increases rapidly. When the frost
point is close to -85 ℃, a thin blue crystal-like frost is formed on the mirror
surface. At this temperature, the mass density of the frost layer is about
10-8gm-2, which is equivalent to the thickness of a molecular layer.
Measurements are difficult at low frost points. Another problem is overcooling.
This phenomenon easily occurs in high-altitude exploration. Under low humidity,
because the crystallization process of ice is relatively slow, the frost layer
often does not appear when the frost point temperature is reached. When the
temperature continues to decrease, water starts to freeze, and a frost layer
quickly forms in a supersaturated state. However, the saturated water vapor
pressure at this time is not ice, but should be the saturated water vapor
pressure of super cooled water. As mentioned above, due to the super cooling
phenomenon, the measurement error of the frost point is sometimes as high as
several degrees. Therefore special care must be taken in low frost point
measurements to maintain a sufficiently long equilibration time.
By
the way, let's talk about other issues that need attention in low frost point
measurement. Because the working environment of the dew point meter, that is,
the moisture content in the atmosphere is tens of thousands × 10-6 (volume
fraction), which causes great difficulties in operation. The measurement results
are often divergent, and the reasons are complex. In order to make the
measurement data accurate and reliable, in addition to ensuring the
instrument's good performance and quality, the following issues must be paid
attention to.
(1)
The air circuit system must be tightly sealed to prevent external environmental
moisture from leaking in.
(2)
If the measured gas is directly discharged into the atmosphere, the problem of
the diffusion of moisture in the atmosphere into the measurement system should
be considered. * Commonly used method is to connect a length of pipe with an
appropriate length to the exhaust port, and its length and pipe diameter should
not affect the pressure of the measuring cavity.
(3)
The sampling pipeline should be as short as possible to minimize the number of
joints and avoid "dead space" to reduce the interference of
background moisture. (4) The inner wall of the sampling pipe and the
measurement chamber should be clean, and the finish should be good. Using
hydrophobic materials, from the experimental results, we can get the following
material selection order: stainless steel * good, followed by
polytetrafluoroethylene, copper and polyethylene ranked third. However, the
difference is that nylon and rubber tubes are not suitable for low frost point
measurement.
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