Sampling from a process, Pressure, Leak tightness and materials – Vaisala DMT152 User Manual
Page 25: Flow
Chapter 4 ________________________________________________________________ Installation
VAISALA ________________________________________________________________________ 23
Sampling from a Process
Sampling is needed when direct measurement of the air or gas is not
possible or desirable. This may be related to, for example, a high process
temperature, protecting the sensor from water spikes, the need to easily
detach the instrument from a pressurized process without running the
whole process down, or wanting to make the measurement at a more
convenient location. To get a representative sample of the process gas
and avoid error sources caused by the sampling system, the following
aspects should be considered:
Pressure
It is important to keep in mind that changing the pressure of a gas
changes the dewpoint temperature of the gas. If the sensor is at a
different pressure than the process itself, there may be an error of several
tens of degrees in the dewpoint measurement.
You can use the Vaisala Humidity Calculator to simulate the effect of
pressure change to dewpoint. The Humidity Calculator can be found at
Leak Tightness and Materials
Due to the typically very low level of humidity, dewpoint measurements
are sensitive to ambient moisture leaks. It is therefore important to have a
leak tight sampling system construction.
The system is likewise sensitive to diffusion of water molecules through
the materials, as well as the absorption of moisture into the materials.
The piping should preferably be made of metal, for example, stainless
steel with a good surface finish. Hygroscopic materials such as plastics
are undesirable and should be avoided.
Sample tubing should be as short in length as possible. "Dead ends"
should be avoided. Also minimizing the number of connections helps in
avoiding leaks. If possible, prepare for measurements by flushing the
sampling lines with dry gas.
Flow
Vaisala DRYCAP
®
sensor is not flow-dependent as such. For this reason,
the flow of the system is not critical to the measurement, although a
higher flow improves the response time. Typically, a flow of
1 ... 2 l/min (0.035 ... 0.7 ft
3
/min) is enough.