Compressed

air dew point

sampling
Application Note
 Measurement in the process pressure

Chamber Valve
for DMP74

Sample gas Sample gas

inlet outlet

DSC74

Chamber Valve Removable

for DMP74 flowlimiter

Sample gas Sample gas

Compressed air
inlet outlet

dew point sampling

DSC74B

Measurement in the atmospheric pressure

Sample gas
Sampling is needed when direct measurement of outlet
the air is undesirable or is simply not feasible. This Removable Valve Chamber
may be due to a high process temperature, the Why Vaisala? flowlimiter for DMP74

need to protect the sensor from water spikes, the

added convenience of installing and removing the Vaisala is a global leader in measurement Sample gas
inlet
instrument from a pressurized process without instruments and intelligence for climate
shutting down the line or wanting to make the action. We equip our customers with devices
measurement in a more convenient location. and data to improve resource efficiency, drive
energy transition, and care for the safety and Diffusion coil
DSC74B
To get a representative sample of the process well-being of people and societies worldwide.
gas and avoid potential sources of error caused
DSC74C
by incorrect sampling practices, the following With almost 90 years of innovation and
aspects should be considered. expertise, we employ a team of over 2,300
experts committed to taking every measure
for the planet.

» Visit vaisala.com/compressedair for more » See technical note Dewpoint sampling cells
information about the products. for DRYCAP® products for details

1 2
01 03
Pressure Leak tightness
It is important to keep in mind that changing the
gas pressure changes the dew point temperature
& materials
of the gas. If the sensor is at a different pressure Due to the very low level of humidity typically
than the process itself, an error of several tens of found in compressed air and gas systems, dew
degrees dew point may be apparent. Generally, point measurements are highly sensitive to even
the measurement should be taken at the actual the smallest leaks in the system. Consequently,
system pressure to avoid pressure drops in the the importance of having a leak-tight sampling
sampling system. Exceptions can sometimes be system is critical. All connections should be tight
made when standards require reporting the dew and sealed correctly. For tapered threads like NPT,
point at ambient pressure, thus excluding the use of Teflon tape is recommended. For straight
effect of pressure fluctuations. thread connections such as G 1/2”, the sealing
washer provided with the instrument should be

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installed between the probe and the sample cell.

Careful consideration should always be given to

the sampling system’s material of construction
since water vapor diffusion through the pipe

Condensing water or tubing walls can occur. The piping should

preferably be made of metal, e.g., stainless steel Flow rate
with a good surface finish. Hygroscopic material
When the need for sampling is due to a high
process gas temperature prohibiting direct
such as rubber hose and plastics is undesirable
and should be avoided. PTFE or Teflon is one
considerations
measurement, it is important to ensure that the exception that is a rather vapor tight plastic and
dew point level of the process is lower than the can be used down to a dew point level of about – Stagnant air can be a problem for a few reasons: Using a dew point sensor that is not dependent
ambient temperature to which the sample gas is 40 °C (-40 °F). on flow rate is strongly recommended. Although
cooled in the sampling line. This prevents water 1. A representative sample of the process air may higher flow rates generally improve the sensor
vapor in the sample from reaching saturation Sample tubing should be as short in length as not be obtained. response time, flows of 1-2 l/min (0.035-
causing liquid water to form in the sample line. possible and “dead ends” should be avoided. Also 2. Response time may be dramatically impaired. 0.7 ft3/min) are typically enough for high-
minimizing the number of connections helps in 3. The risk of ambient air leaking or diffusing quality sensors.
In cases where the dew point is higher avoiding leaks. If possible, prepare the sampling through sampling materials increases.
than ambient temperature, either a direct system for the measurement by purging it with 4. Back diffusion of ambient water vapor through
measurement or a heated sampling line the dry process gas sufficiently to ensure a faster the outlet port of the sample cell can occur.
is required. stabilization and response time. •

3 4
 Our sample cells are compatible with several of our dew
point measurement instruments that are meant for
different purposes:

• Spot-checking: Vaisala Indigo80 handheld indicator in

combination with DMP80
• Inline process measurement: Vaisala DRYCAP® dew point
probe DMP7 (requires SWG12ISO12)
• Original equipment manufacturing: Vaisala DRYCAP®
dew point probes DMT132, DMT143, DMT143L, DPT146,
and DMT152
• The sample cells can also be used with various previous
generation Vaisala DRYCAP® dew point instruments
such as Vaisala DRYCAP® dew point instruments DM70,
DMT142, DMT242, DMT347 (SWG12ISO12 required),
» See how to spot-check a compressed air system: and DMT348.
vaisala.com/compressedair

05 Indigo-compatible probes
and sample cells

DMT242SC & SC-025NPT DMT242SC2 DSC74B DSC74 DSC74CSP DMCOIL

basic models Swagelok® connectors for 1/4” tubing two-pressure model for elevated and quick connector, leak screw and two-pressure sampling cell with coil stainless steel (aisi316l)
Basic sample cells consisting of just the main Same body as DMT242SC, but with welded atmospheric pressure thread adapters Designed for the most critical Coil accessory – DMT242SC2 required.
sampling body. Swagelok® connectors at both the inlet and Includes inlet/outlet valve and inlet/outlet Contains an adjustable leak screw that measurements at atmospheric pressure Used as a cooling coil in high
Suitable for users requiring only a outlet that fit directly to 1/4” tubing. connection for gas tubing. maintains the compressed air system The diffusion coil is connected to the temperature gas processes to cool gas
sample cell for a probe and doing further To fit 6 mm tubing to the connectors, Limits flow rate with a fixed leak screw. pressure at the cell (the leak screw is sampling cell outlet to protect the sensor temperature down to a suitable level for
assembly themselves an adapter such as Swagelok® Reducer Flow is optimized for pressures 3 ... 10 bar. opened and closed with a screwdriver from ambient humidity disturbing measurement sensors.
(piping into inlet and outlet, valves and SS-6M0-R-4 (not supplied by Vaisala) can Fixed leak screw eliminates the risk and should be turned 1/2 turn which the measurement For applications where gas pressure is
possible flow meter). be used. of opening the screw completely and approximates a 1 l/min flow rate.) Used with a ISO1/2” Swagelok connector reduced to ambient pressure prior the
The inlet and outlet are female threaded Can be used with DMCOIL - Shown below. emptying a gas vessel. Comes with a quick connector that fits sample cell, it can act as venting coil in gas
sample connectors. Maximum flow can be increased, if needed, industry standard compressed air line outlet to prevent ambient humidity from
DMT242SC: inlet G3/8” ISO, outlet G1/4” ISO by removing the leak screw and adjusting connectors. This allows for easy installation disturbing low dew point measurement.
SC-025NPT: inlet 1/4”NPT, outlet 1/4”NPT the flow manually with the valve. and detachment of the dew point
Reducing parts supplied (G3/8” -G1/2” or transmitter without having to shut down
G3/8” - G1/4”) on the outlet side help to the process.
protect the sensor from ambient humidity Alternative ways to connect are through
leaking in. the two different thread adapters - R3/8”
to R1/2” and G3/8” to G1/4” ISO – that are » Order sample cells, spare parts and selected products via
supplied with each DSC74 cell. store.vaisala.com or contact our sales experts.

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Ref. B211229EN-E ©Vaisala 2024
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