Quick answer: A compressed air dew point meter checks that the dryer is doing its job: it reports the pressure dew point (PDP), the temperature at which water would condense in the air at line pressure. Choose a sensor for your target range: about +3 °C PDP after a refrigerant dryer, −40 °C or −70 °C after a desiccant dryer. Install it after the dryer (and at critical points of use) with a proper sampling cell, set an alarm a few degrees above the target, and recalibrate about once a year. ISO 8573-1 defines humidity classes from Class 1 (≤ −70 °C PDP) to Class 6 (≤ +10 °C PDP).
Quick answer
- Refrigerant dryer: about +3 °C PDP (ISO 8573-1 Class 4).
- Desiccant dryer: −40 °C (Class 2) or −70 °C (Class 1).
- Pressure dew point ≠ atmospheric dew point: +3 °C at 7 bar(g) ≈ −20 °C at atmospheric pressure.
ISO 8573-1 humidity classes
| Class | Pressure dew point | Typical dryer |
|---|---|---|
| 1 | ≤ −70 °C | Desiccant (heatless or heated) |
| 2 | ≤ −40 °C | Desiccant |
| 3 | ≤ −20 °C | Desiccant or membrane |
| 4 | ≤ +3 °C | Refrigerant |
| 5 | ≤ +7 °C | Refrigerant |
| 6 | ≤ +10 °C | Refrigerant |
ISO 8573-1 sets purity classes for particles, water and oil; ISO 8573-3 covers the measurement of humidity (ISO 8573-1; ISO 8573-3).
Pressure dew point vs atmospheric dew point
Compressing air raises the partial pressure of its water vapour, so water condenses at a higher temperature. When the air is expanded to atmospheric pressure, its dew point drops.
Example: air with +3 °C PDP at 7 bar(g) (8 bar absolute) has a dew point of roughly −20 °C after expansion to atmospheric pressure. Always state which one a reading is, and the pressure.
Choosing a sensor
| Sensor type | Typical range | Best for |
|---|---|---|
| Polymer capacitive | Down to about −40 °C or −60 °C on good models | Refrigerant dryers and general monitoring |
| Aluminium oxide / ceramic | Down to −80 °C or lower | Desiccant dryers, low dew points |
| Chilled mirror | Very wide; high accuracy | Reference and calibration |
Specify: range, accuracy at your target, pressure rating, response time, output (4–20 mA, Modbus, relay), and how often it needs calibration. NIST maintains humidity measurement standards and hygrometer calibration (NIST).
Installation and sampling
- After the dryer, to monitor dryer performance; at critical points of use, where condensation would damage product or instruments.
- Measuring cell: install in a sampling block with a small, controlled bleed, not directly in a high-velocity main where liquid slugs can hit it.
- Line pressure: measure at line pressure for PDP, or record the pressure if measuring a depressurised sample.
- Avoid dead legs and liquid water: never place the sensor downstream of a drain point that floods.
- Allow response time: at low dew points, sensors can take tens of minutes to settle after installation.
Alarms and actions
| Event | Likely cause | Action |
|---|---|---|
| PDP rising slowly | Desiccant aging, refrigerant dryer fouled, high inlet temperature | Service dryer; check inlet temperature |
| PDP spike | Drain failure, bypass open, dryer switching fault | Check drains, valves, cycle |
| PDP high only at peak demand | Dryer undersized for flow | Check capacity at actual flow and inlet conditions |
The US DOE's compressed air sourcebook covers dryer selection and system efficiency (DOE, Improving compressed air system performance).
Drying processes and dry rooms
The same instruments monitor dry rooms and drying processes (battery manufacturing, pharmaceuticals, plastics drying), where room dew points of −20 to −50 °C may be required. Those rooms are usually supplied by desiccant dehumidifiers; see low dew point desiccant dehumidifier selection and our desiccant dehumidifiers.
FAQ
Frequently asked questions
What dew point should compressed air be?
It depends on use: about +3 °C PDP for general plant air after a refrigerant dryer; −40 °C or lower for instrument air in cold areas, sensitive processes or outdoor lines.
What is pressure dew point?
The temperature at which water vapour in compressed air would condense at the air's actual pressure.
How often should a dew point sensor be calibrated?
Typically once a year, or as the maker specifies.
Where should a compressed air dew point meter be installed?
After the dryer and at critical points of use, in a sampling cell at line pressure.
Sources
- ISO 8573-1, Compressed air: contaminants and purity classes; ISO 8573-3, Test methods for measurement of humidity.
- US DOE, Improving Compressed Air System Performance: A Sourcebook for Industry.
- NIST, Hygrometers.
