Compressed air dew point sampling

» See technical note Dewpoint sampling cells for DRYCAP® products for details

01 Pressure

It is important to keep in mind that changing the gas pressure changes the dew point temperature of the gas. If the sensor is at a different pressure than the process itself, an error of several tens of degrees dew point may be apparent. Generally, the measurement should be taken at the actual system pressure to avoid pressure drops in the sampling system. Exceptions can sometimes be made when standards require reporting the dew point at ambient pressure, thus excluding the effect of pressure fluctuations.

02 Condensing water

When the need for sampling is due to a high process gas temperature prohibiting direct measurement, it is important to ensure that the dew point level of the process is lower than the ambient temperature to which the sample gas is cooled in the sampling line. This prevents water vapor in the sample from reaching saturation causing liquid water to form in the sample line.

In cases where the dew point is higher than ambient temperature, either a direct measurement or a heated sampling line is required.

03 Leak tightness & materials

Due to the very low level of humidity typically found in compressed air and gas systems, dew point measurements are highly sensitive to even the smallest leaks in the system. Consequently, the importance of having a leak-tight sampling system is critical. All connections should be tight and sealed correctly. For tapered threads like NPT, use of Teflon tape is recommended. For straight thread connections such as G 1/2”, the sealing washer provided with the instrument should be 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 or tubing walls can occur. The piping should preferably be made of metal, e.g., stainless steel with a good surface finish. Hygroscopic material such as rubber hose and plastics is undesirable and should be avoided. PTFE or Teflon is one exception that is a rather vapor tight plastic and can be used down to a dew point level of about – 40 °C (-40 °F).

Sample tubing should be as short in length as possible and “dead ends” should be avoided. Also minimizing the number of connections helps in avoiding leaks. If possible, prepare the sampling system for the measurement by purging it with the dry process gas sufficiently to ensure a faster stabilization and response time.

04 Flow rate considerations

Stagnant air can be a problem for a few reasons:

1. A representative sample of the process air may not be obtained.
2. Response time may be dramatically impaired. 
3. The risk of ambient air leaking or diffusing through sampling materials increases. 
4. Back diffusion of ambient water vapor through the outlet port of the sample cell can occur.

Using a dew point sensor that is not dependent on flow rate is strongly recommended. Although higher flow rates generally improve the sensor response time, flows of 1-2 l/min (0.035- 0.7 ft3/min) are typically enough for high-quality sensors.

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), and DMT348.

» See how to spot-check a compressed air system: vaisala.com/compressedair

Indigo-compatible probes and sample cells

DMT242SC & SC-025NPT

Basic models

• Basic sample cells consisting of just the main sampling body.
• Suitable for users requiring only a sample cell for a probe and doing further assembly themselves (piping into inlet and outlet, valves and possible flow meter).
• The inlet and outlet are female threaded sample connectors.
• DMT242SC: inlet G3/8” ISO, outlet G1/4” ISO
  SC-025NPT: inlet 1/4”NPT, outlet 1/4”NPT

DMT242SC2

Swagelok® connectors for 1/4” tubing

• Same body as DMT242SC, but with 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.
• Can be used with DMCOIL - Shown below

DSC74B

Two-pressure model for elevated and atmospheric pressure

• Includes inlet/outlet valve and inlet/outlet connection for gas tubing.
• Limits flow rate with a fixed leak screw.
• Flow is optimized for pressures 3 ... 10 bar.
• Fixed leak screw eliminates the risk of opening the screw completely and emptying a gas vessel.
• Maximum flow can be increased, if needed, by removing the leak screw and adjusting the flow manually with the valve. 
• Reducing parts supplied (G3/8” -G1/2” or G3/8” - G1/4”) on the outlet side help to protect the sensor from ambient humidity leaking in.

DSC74

Quick connector, leak screw and thread adapters

• Contains an adjustable leak screw that maintains the compressed air system pressure at the cell (the leak screw is opened and closed with a screwdriver and should be turned 1/2 turn which approximates a 1 l/min flow rate.)
• Comes with a quick connector that fits industry standard compressed air line connectors. This allows for easy installation and detachment of the dew point transmitter without having to shut down the process.
• Alternative ways to connect are through the two different thread adapters - R3/8” to R1/2” and G3/8” to G1/4” ISO – that are supplied with each DSC74 cell.

DSC74CSP

Two-pressure sampling cell with coil

• Designed for the most critical measurements at atmospheric pressure
• The diffusion coil is connected to the sampling cell outlet to protect the sensor from ambient humidity disturbing the measurement
• Used with a ISO1/2” Swagelok connector

DMCOIL

Stainless steel (aisi316l)

• Coil accessory – DMT242SC2 required.
• Used as a cooling coil in high temperature gas processes to cool gas temperature down to a suitable level for measurement sensors.
• For applications where gas pressure is reduced to ambient pressure prior the sample cell, it can act as venting coil in gas outlet to prevent ambient humidity from disturbing low dew point measurement.