This parameter is inherently related to the moisture content, which defines the amount of water molecules as a fraction of the total.
[4] In this technique, a sample of material is weighed, heated in an oven for an appropriate period, cooled in the dry atmosphere of a desiccator, and then reweighed.
[5] Because the manual laboratory method is relatively slow, automated moisture analysers have been developed that can reduce the time necessary for a test from a couple of hours to just a few minutes.
[6] An accurate method for determining the amount of water is the Karl Fischer titration, developed in 1935 by the German chemist, whose name it bears.
[7][5] Natural gas poses a unique problem in terms of moisture content analysis because it can contain very high levels of solid and liquid contaminants, as well as corrosives in varying concentrations.
A manufacturer calibrates the tubes, but since the measurement is directly related to exposure time, the flow rate, and the extractive technique, it is susceptible to error.
The exact point at which condensation begins to occur is not discernible to the unaided eye, so modern manually operated instruments use a microscope to enhance the accuracy of measurements taken using this method.
With proper filtration and gas analysis preparation systems, other condensable liquids such as heavy hydrocarbons, alcohol, and glycol will not distort the results provided by these devices.
On the other hand, chilled-mirror devices are not subject to drift, and are not influenced by fluctuations in gas composition or changes in moisture content.
In this method, a transparent inert material is cooled as an infrared (IR) beam is directed through it at an angle to the exterior surface.
Large amounts of water in the pipeline (called slugs) will wet the surface and require tens of minutes or hours to “dry-down.” Effective sample conditioning and removal of liquids are essential when using an electrolytic sensor.
The labor for frequent replacement of desiccant dryers, permeation components, and sensor heads greatly increases the operational costs.
In the first type (optical) when light is reflected through the substrate, a wavelength shift can be detected on the output, which can be precisely correlated to the moisture concentration.
Water molecules do take time to enter and exit the pores, so some wet-up and dry down delays will be observed, especially after a slug.
Contaminants and corrosives may damage and clog the pores, causing a “drift” in the calibration, but the sensor heads can be refurbished or replaced and will perform better in very clean gas streams.
Whilst most absorption type devices can be installed at pipe line pressures (up to 130 Barg) traceability to International Standards is compromised.
Operation at near atmospheric pressure do provide traceability and offer other significant benefits, such as enabling the direct validation against known moisture content.
Traditional spectroscopic techniques have not been successful at doing this in natural gas because methane absorbs light in the same wavelength regions as water.