Thermal measuring and testing – Transformation point determination – Between gaseous and liquid states
Patent
1997-12-02
2000-02-08
Gutierrez, Diego
Thermal measuring and testing
Transformation point determination
Between gaseous and liquid states
G01N 2568
Patent
active
060221380
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to methods of measuring the dew point temperature of a moist gas, and apparatus suitable for carrying out such measurements.
The temperature at which a sample of a moist gas will become saturated and condense to form water droplets or, alternatively, frost, on a surface with which the moist gas is in contact is known as the "dew point" temperature. Generally, where the dew point temperature is above 0.degree. the moist gas condenses to form water droplets at the dew point temperature, and where the dew point temperature is below 0.degree. C. moist gas condenses to form frost. Although dew points below 0.degree. C. are sometimes referred to as frost points, in the present context the term "dew point temperature" is intended to include dew point temperatures both above and below 0.degree..
It is often important to measure the dew point temperature of a moist gas, for example, one constituting the atmosphere inside a furnace such as a metallurgical furnace used in semiconductor or fabrication processes. The dew point temperature can provide a measure of the water content in the gas which must be kept extremely low in order to protect one or more of the product, a catalyst, and equipment used in the production process against damage e.g. in optical fibre manufacture. Dew point temperature measurements are also important in protecting gas pipelines against corrosion from moisture in the gas supply. Additionally, a dew point temperature measurement of the contents of a cylinder of gas provides a measure of the purity of the gas in the cylinder. In fact, the industrial applications for dew point temperature measuring and dew point temperature measuring apparatus are numerous and varied.
One known method of measuring the dew point temperature of a moist gas involves passing a flow of gas over a mirror surface. A light source, such as a light emitting diode (L.E.D.) is directed towards the mirror surface and a photosensitive detector is positioned so as to detect light, emitted by the L.E.D., which is reflected from the mirror surface. A cooling system is used to reduce progressively the temperature of the mirror surface until moisture in the moist gas condenses to form dew or frost on the mirror surface. Dew (or frost) forming on the mirror surface in the path of light emitted from the L.E.D. causes the light to be deflected (or scattered) from its original path such that the amount of light detected by the photodiode is reduced. By providing a servo loop between the photodiode and the cooling system, the temperature of the mirror may be controlled in order to maintain it at an equilibrium temperature where the rates of condensation and evaporation of water molecules onto or from the mirror surface are equal and a constant mass of water (or frost) is maintained on the mirror thereby providing a constant level of scattering and hence of the light level detected. This occurs at the dew point temperature. This type of apparatus is often referred to as a chilled mirror hygrometer.
However, this method has the disadvantage of requiring a sufficient amount of moisture to condense on the mirror in order to provide a reliably detectable scattering of the light emitted by the L.E.D. Where the moist gas being measured is relatively dry, for example having a moisture content of 10 parts per million of water molecules (or below), it can take a considerable time, typically several minutes, for enough condensation to occur in order to achieve a reduced reading from the photodiode detector. As a result, the progressively reducing temperature of the mirror may have decreased to well below the dew point temperature before a sufficient amount of condensation has occurred. This limits the accuracy with which the dew point temperature an be determined over relatively short periods of time due to the relatively large oscillations of the mirror temperature above and below the dew point temperature before an equilibrium state is reached. Thus, measurements of the dew point temperature
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patent: 2106593 (1938-01-01), Deniston et al.
patent: 3177716 (1965-04-01), Warman
patent: 3937059 (1976-02-01), Nisolle
patent: 5165793 (1992-11-01), Rall et al.
patent: 5364185 (1994-11-01), VanZandt et al.
Gutierrez Diego
Verbitsky Gail
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