Dew point measurement
The pressure dew point tells you how much moisture is still in the compressed air and whether the dryer really works. CS Instruments sensors and portable meters measure down to -80 °Ctd, raise an alarm when limits are exceeded and record the drying process.
Why measure dew point
When compressed air cools below its dew point, moisture separates as condensate. The results are pipe corrosion, failures of pneumatic controls and tools, freezing of outdoor lines and poorer product quality.
A refrigeration dryer cools the air to about +2 to +5 °C, an adsorption dryer dries it to -40 °Ctd or lower. The cooling temperature shown on the dryer is not enough – a faulty condensate drain, an open bypass or an overloaded dryer only show up in a dew point measurement.
CS Instruments models
Stationary sensors for continuous dryer monitoring, portable meters for service and sets with a display ready for immediate use.
Stationary dew point sensor
Pressure dew point sensor for refrigeration, membrane and adsorption dryers, with Modbus-RTU and IO-Link.
Sensor with display
Dew point sensor with integrated display and alarm relay – ideal for all types of dryers.
Robust housing
Sensor in a die-cast aluminium IP 67 housing for outdoor installation and harsh industrial environments.
ATEX version
Dew point sensor for potentially explosive atmospheres and non-aggressive gases, up to 500 bar.
Compact sensor
Long-term stable dew point sensor for OEM applications and adsorption dryers with little space.
Portable meter
Service dew point meter down to -80 °Ctd with data logger and touch screen.
Portable meter
Like the DP 500, with an additional free input for pressure, flow or a current clamp.
Service case
Mobile dew point meter with integrated pressure measurement in a sturdy case.
Dryer set
DS 52 display with FA 510 sensor and two alarm relays for dryer monitoring.
2/4-input data logger
Stationary touch-screen data logger for dew point, flow and pressure – with alarms and memory.
Comparison
| Model | Version | Measuring range | Accuracy | Pressure | Outputs |
|---|---|---|---|---|---|
| FA 510 / FA 515 | Stationary | -80…+20 / -20…+50 °Ctd | ± 1 °Ctd (+50…-20 °Ctd) | -1…50 bar (500 bar) | 4…20 mA, Modbus-RTU, IO-Link |
| FA 500 | Stationary | -80…+20 / -60…+30 / -20…+50 °Ctd | ± 1 °Ctd (+50…-20 °Ctd) | -1…50 bar (500 bar) | 4…20 mA, Modbus-RTU, relay |
| FA 550 | Stationary | -80…+20 / -60…+30 / -20…+50 °Ctd | ± 1 °Ctd (+50…-20 °Ctd) | -1…50 bar (350 / 500 bar) | 4…20 mA, Modbus-RTU, relay |
| FA 515 Ex | Stationary | -80…+20 °Ctd | ± 1 °Ctd (-20…+20 °Ctd) | -1…500 bar | 4…20 mA, 2-wire |
| FA 505 | Stationary | -80…+20 °Ctd | ± 1 °Ctd | 50 bar (350 bar) | 4…20 mA, Modbus-RTU |
| DP 500 | Portable | -80…+50 °Ctd | ± 0.5 °Ctd (-10…+50 °Ctd) | -1…50 bar (350 bar) | – |
| DP 510 | Portable | -80…+50 °Ctd | ± 0.5 °Ctd (-10…+50 °Ctd) | -1…50 bar (350 bar) | – |
| DP 400 mobile | Portable | -80…+50 °Ctd | ± 1 °Ctd (+50…-20 °Ctd) | 0…16 bar ± 0.5 % | – |
| DS 52 | Stationary | – | – | – | 4…20 mA |
| DS 400 | Stationary | – | – | – | USB, Ethernet, RS 485 |
Dryers and dew point
Typical pressure dew points by dryer type and the matching ISO 8573-1 water classes.
Cools the air to 2–5 °C and separates the condensate. Corresponds to water class 4 (≤ +3 °C) or 5. Measurement reveals a faulty condensate drain and bypass.
The desiccant binds water vapour. Corresponds to class 2. Too short or too long regeneration and spent desiccant show up as a rising dew point.
Class 1 for electronics, pharmaceuticals or outdoor lines in winter. Here we measure with a -80…+20 °Ctd range.
The portable DP 500 checks plastic granulate dryers and residual moisture in N₂, O₂ and other gases.
ISO 8573-1:2010
ISO 8573-1 defines limit values for solid particles, water (pressure dew point) and oil. A class is written e.g. as ISO 8573-1 [1:2:1] – particles, water and oil in that order.
A refrigeration dryer reaches about +3 °Ctd (class 4), an adsorption dryer -40 °Ctd (class 2) or -70 °Ctd (class 1). Only a measurement shows whether the dryer really works.
Dew point meters →| Class | Solid particles – number per m³ | Water | Oil (aerosol + vapour) | ||
|---|---|---|---|---|---|
| 0.1–0.5 µm | 0.5–1 µm | 1–5 µm | Pressure dew point | mg/m³ | |
| 0 | As specified by the user – stricter than class 1 | ||||
| 1 | ≤ 20 000 | ≤ 400 | ≤ 10 | ≤ -70 °C | ≤ 0.01 |
| 2 | ≤ 400 000 | ≤ 6 000 | ≤ 100 | ≤ -40 °C | ≤ 0.1 |
| 3 | – | ≤ 90 000 | ≤ 1 000 | ≤ -20 °C | ≤ 1 |
| 4 | – | – | ≤ 10 000 | ≤ +3 °C | ≤ 5 |
| 5 | – | – | ≤ 100 000 | ≤ +7 °C | – |
| 6 | – | – | – | ≤ +10 °C | – |
Source: CS Instruments catalogue 2025/26 (ISO 8573-1:2010).
How to choose
For a refrigeration dryer choose the -20…+50 °Ctd range, for an adsorption dryer -80…+20 °Ctd.
For continuous monitoring a stationary FA 510 or FA 500 sensor, for service and inspections the portable DP 500.
Outdoors or in dust FA 550 (IP 67), in Ex areas FA 515 Ex, above 50 bar the high-pressure version.
Stand-alone alarm (FA 500, DS 52) or connection via 4…20 mA, Modbus or IO-Link to a data logger.
FAQ
The pressure dew point is the temperature at which moisture in compressed air starts to condense at operating pressure. It is given in °Ctd. The lower it is, the drier the air.
Usually directly downstream of the dryer, where we check the dryer's performance. In more demanding systems we add measuring points after filters, at remote parts of the network and at critical consumers.
A refrigeration dryer reaches about +3 °Ctd, so a sensor with a -20…+50 °Ctd range is sufficient. An adsorption dryer reaches -40 °Ctd or lower; for this choose a sensor with a -80…+20 °Ctd range.
Yes. The sensor sits in a measuring chamber with a quick coupling, so it can be removed under pressure. A replacement sensor can be fitted during calibration.
We recommend once a year. CS Instruments calibrates its own sensors and those of other manufacturers in the -80…+20 °Ctd range using DKD/BAM-certified references.
Related measurements
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