Experimental Methods of HCDP Determination
In the "experimental" methods, one actually cools a surface on which gas condenses and then measures the temperature at which the condensation takes place. The experimental methods can be divided into manual and automated systems. Manual systems, such as the Bureau of Mines dewpoint meter, depend on an operator to manually cool the chilled mirror slowly and to visually detect the onset of condensation. The automated methods use automatic mirror chilling controls and sensors to detect the amount of light reflected by the mirror and detect when condensation occurs through a drop in measured light. The chilled mirror technique is a first principle measurement which is not subject to "calculation" or "choice of model" errors. However, some liquids must condense before the detection of condensation on the mirror can occur, and therefore results in values lower than the theoretical methods.
The experimental method also suffers from two sources of error. The first error is in the detection of condensation. A manual chilled mirror device relies on the operator to determine when a mist has formed on the mirror, and is highly subjective. In most cases, the operator under-reports the dew point because by the time condensation has accumulated enough to be visible to an operator, the dew point has already been reached and passed. The automated chilled mirror devices are significantly more repeatable but can be affected by contaminants that condense on the mirror before the hydrocarbons. The second source of error is the speed of the cooling of the mirror and the measurement of the temperature of the mirror when the condensation is detected. This error can be minimized by controlling the cooling speed, or having a fast condensation detection system.
The experimental methods only provide a HCDP at the pressure at which the measurement is taken, and cannot provide the cricondentherm or the HCDP at other pressures. However, in any event, most pipeline operators like to know the HCDP at their current line pressure. Given that the input pressure of the experimental systems can be adjusted by a regulator, one can also measure the HCDP at other pressures by adjusting the input pressure.
Companies who offer an automated chilled mirror system include: Vympel, Ametek, Michell Instruments, and ZEGAZ Instruments
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