When direct measurement in the process is not possible or practical, proper sampling enables precise and reliable dew point measurements in compressed air and gas systems. This is especially useful at high process temperatures, to protect the sensor, or when measurements need to be taken at a more accessible location.
For accurate results, the correct system pressure, leak-tight tubing, suitable materials, and sufficient gas flow are essential. These factors help prevent condensation, moisture ingress, and measurement errors.
Vaisala offers suitable sampling cells and DRYCAP® dew point instruments for everything from portable spot checks to continuous process monitoring, ensuring stable and reliable measurements even in demanding applications.
Sampling is needed when direct measurement of the air is undesirable or is simply not feasible. This may be due to a high process temperature, the need to protect the sensor from water spikes, the added convenience of installing and removing the instrument from a pressurized process without shutting down the line or wanting to make the measurement in a more convenient location.
To get a representative sample of the process gas and avoid potential sources of error caused by incorrect sampling practices, the following aspects should be considered.
| Basic Models | Swagelok® Connections for 1/4" Tubing | Dual-Pressure Model for Elevated and Atmospheric Pressure |
|---|---|---|
| • Basic sampling cells consisting only of the main sampling cell body. • Suitable for users who only need a sampling cell for a probe and want to add the remaining components themselves, such as inlet/outlet tubing, valves or a flowmeter. • The inlet and outlet are internally threaded sample connections. • DMT242SC: G3/8" ISO inlet, G1/4" ISO outlet. SC-025NPT: 1/4" NPT inlet, 1/4" NPT outlet. | • Same housing as the DMT242SC, but with welded Swagelok® connections at the inlet and outlet for direct connection to 1/4" tubing. • To connect 6 mm tubing, an adapter such as the Swagelok reducer SS-6MO-R-4 can be used (not available from Vaisala). • Can be used with DMCOIL – see below. | • Includes an inlet/outlet valve and connections for gas lines. • Limits the flow rate with a fixed bleed screw. • Flow is optimized for pressures of 3–10 bar. • The fixed bleed screw prevents complete opening and accidental emptying of the gas container. • The maximum flow rate can be increased by removing the bleed screw and manually adjusting the flow using the valve. • Reducers on the outlet side protect the sensor from ambient moisture. |
| Quick Connector, Bleed Screw and Thread Adapters | Dual-Pressure Sampling Cell with Coil | Stainless Steel (AISI 316L) |
|---|
| • Includes an adjustable bleed screw that maintains the compressed-air system pressure inside the cell. Half a turn corresponds to a flow rate of approximately 1 l/min. • Supplied with a quick connector suitable for standard industrial compressed-air connections. This allows the dew point transmitter to be installed or removed without shutting down the process. • Two thread adapters are also supplied with each DSC74 sampling cell: R3/8" to R1/2" and G3/8" to G1/4" ISO. | • Designed especially for measurements at atmospheric pressure. • The diffusion coil is connected to the sampling cell outlet to protect the sensor from ambient moisture that could affect the measurement. • Used with an ISO 1/2" Swagelok fitting. | • Coil accessory – DMT242SC2 required. • Used as a cooling coil in high-temperature processes to reduce the gas temperature to a suitable level for the measurement sensor. • It can also serve as a venting coil at the gas outlet, helping prevent ambient moisture from affecting low dew point measurements when the gas pressure has been reduced to atmospheric pressure before the sampling cell. |
The Right Solution for Every Dew Point Measurement
Vaisala sampling cells can be flexibly combined with a range of dew point instruments. For portable spot-check measurements, the Indigo80 with DMP80 is an ideal choice, while the DMP7 is designed for continuous inline measurements. For OEM applications, compatible DRYCAP® probes include the DMT132, DMT143, DMT143L, DPT146, and DMT152.
Various previous-generation DRYCAP® instruments are also compatible with the sampling cells, making it easy to select the right solution for reliable dew point measurement in compressed air systems.
