Vaisala DMT143 - Manual (Page 28)

USER'S GUIDE ____________________________________________________________________
26 ___________________________________________________________________ M211435EN-E
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.
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.
Make sure there is sufficient flow of gas to the sensor (for example,
1 l/min) to give a representative sample.
You can use the Vaisala Humidity Calculator to simulate the effect of
pressure change to dewpoint. The Humidity Calculator can be found at:
www.vaisala.com/humiditycalculator .
Sampling Accessories
DMT242SC Sampling Cell
The DMT242SC is a basic sampling cell with only the main sampling
cell body. The inlet and outlet are female threaded sample connectors
(inlet G3/8", outlet G1/4" ISO). Refer to Figure 13 on page 27.
The DMT242SC is suitable for users requiring only a sampling cell to fit
the transmitter into and doing further assembly (piping into inlet &
outlet, valves, possible flow meter) by themselves.
DMT242SC2 Sampling Cell with
Swagelok Connectors
The DMT242SC2 is similar to DMT242SC, but connections are made
easy. The sampling cell includes welded swagelok connectors at both the
inlet and outlet that fit directly to 1/4" tubing.
To fit 6 mm tubing to the connectors, an adapter such as Swagelok
®
Reducer SS-6M0-R-4 (not supplied by Vaisala) can be used.
DMT242SC2 is the suitable choice in, for example, plastics drying
systems, where the measurement is made by tapping off the dryer system
and bringing a small air stream to the sensor.