Michell Instruments QMA401, User Manual

Page: 50 / 102

QMA401 User’s Manual
40 97450 Issue 2, March 2018
OPERATION
• Use high quality sample pipe fittings
The sample pipework must be capable of withstanding the operating pressure of the
sample line. Wherever possible, always use stainless steel pipework and fittings. This
is particularly important at low dew points since other materials, e.g. nylon, have
hygroscopic characteristics and adsorb moisture on the tube walls, giving rise to
slower measurement response and, under certain circumstances, false dew points. For
temporary applications, or where stainless steel pipework is not practicable, use high
quality, thick-walled PTFE piping, which exhibits similar qualities to stainless steel.
In order to maximize response time, always use the shortest run of pipework and the
smallest bore possible, taking care not to induce pressure differentials by aiming for
too high a flow rate through too small a bore. Michell Instruments supplies a range of
precision pressure fittings suitable for use with the QMA401 instrument. Contact Michell
Instruments for details of the items available.
• Gas samples
Generally, if the sample gas (in conjunction with water vapor) is not corrosive to base
metals, it will be suitable for measurement by the QMA401 instrument. Gases containing
entrained solids should be filtered before application to the instrument.
Care should be taken with gas mixtures containing potentially condensable components
in addition to water vapor, e.g. oil, to ensure that only water vapor is present in
the sample. Once present on the surface of the sensors, oil will not dry out and will
contaminate and damage them.
• Material of construction
All materials are permeable to water vapor, as the water molecule is extremely small
compared to the structure of solids, even when compared to the crystalline structure
of metals.
Many materials contain moisture as part of their structure, particularly organic materials,
salts and anything which has small pores. It is important to ensure that the materials
used are suitable for the application.
If the partial water vapor pressure exerted on the outside of a compressed air line is
higher than on the inside, the atmospheric water vapor will naturally push through
the porous medium against a dry air water vapor pressure. Water will migrate into the
pressurized air line, this effect is called transpiration.
Over a long pipe run water will inevitably migrate into any line even through the most
resistant materials. Moisture on the outlet of the line will be different than on the inlet.
The best material to resist transpiration is 316L stainless steel.
It is also important to note that temperature changes can increase the tendency of
these materials to affect the humidity of the surrounding air. With a given surface and
gas composition, increases of line pressure and decreases in temperature increase
surface adsorption.