Dew Point Measurement
Dew Point Measurement in Compressed Air
Dew Point is the Temperature to Which Air Can be Cooled Down without Condensation. Dew Point is Pressure Dependent and Would Change when Air is Compressed.
The Term “pressure dew point” is encountered when measuring the dew point temperature of gases at pressures higher than atmospheric pressure. It refers to the dew point temperature of a gas under pressure. This is important because changing the pressure of a gas increases the dew point temperature of the gas. Consider an example of air at atmospheric pressure of 1013.3 mbar with a dew point temperature of -10C. The partial pressure of water vapour (designated by the symbol “e”) in this case is 2.8 mbar. If this air is compressed and the total pressure is doubled to 2026.6 mbar, then according to Dalton’s law, the partial pressure of water vapour, e, is also doubled to the value of 5.6 mbar. The dew point temperature corresponding to 5.6 mbar is approximately -1C, so it is clear that increasing the pressure of the air has also increased the dew point temperature of the air. Conversely, expanding a compressed gas to atmospheric pressure decreases the partial pressures of all of the component gases, including water vapour, and therefore decreases the dew point temperature of the gas. The relationship of total pressure to the partial pressure of water vapour, can be expressed as follows,
P1/P2 – e1/e2
By converting dew point temperature to the corresponding saturation vapour pressure, it is easy to calculate the effect of changing total pressure on the saturation vapour pressure. The New saturation vapour pressure value can then be converted back to the corresponding dew point temperature. These calculations can be done manually using tables, or performed by various kinds of software.
How is dew point in compressed air reliably measured?
- Select an instrument with the correct measuring range: Some instruments are suitable for measuring high dew points, but not low dew points. Similarly, some instruments are suitable for very low dew points but are compromised when exposed to high dew points.
- Understand the pressure characteristics of the dew point instrument: Some instruments are not suitable for use at process pressure. They can be installed to measure compressed air after it is expanded to atmospheric pressure, but the measured dew point value will have to be corrected if pressure dew point is the desired measurement parameter.
- Install the sensor correctly: Follow instructions from the manufacturer. Do not install dew point sensors at the end of stubs or other “dead end” pieces of pipe where there is no airflow.
Importance Of Monitoring Dew Point
Effect Of Pressure On Dew Point
Increasing the pressure of a gas increases the dew point temperature of the gas. Consider an example of air at atmospheric pressure of 1013.3 mbar with a dew point temperature of -10 °C (14 °F). From the table above, the partial pressure of water vapour (designated by the symbol “e”) is 2.8 mbar. If this air is compressed and the total pressure is doubled to 2026.6 mbar, then according to Dalton’s law, the partial pressure of water vapour, e, is also doubled to the value of 5.6 mbar. The dew point temperature corresponding to 5.6 mbar is approximately -1 °C (30 °F), so it is clear that increasing the pressure of the air has also increased the dew point temperature of the air. Conversely, expanding a compressed gas to atmospheric pressure decreases the partial pressures of all of the component gases, including water vapour, and therefore decreases the dew point temperature of the gas. The relationship of total pressure to the partial pressure of water vapour, e, can be expressed as follows:
P1/P2 – e1/e2
By converting dew point temperature to the corresponding saturation vapour pressure, it is easy to calculate the effect of changing total pressure on the saturation vapour pressure. The new saturation vapour pressure value can then be converted back to the corresponding dew point temperature. These calculations can be done manually using tables, or performed by various kinds of software.
Why Is Knowledge Of Dew Point In Compressed Air Important?
The importance of dew point temperature in compressed air depends on the intended use of the air. In many cases dew point is not critical (portable compressors for pneumatic tools, gas station tire filling systems, etc.). In some cases, dew point is important only because the pipes that carry the air are exposed to freezing temperatures, where a high dew point could result in freezing and blockage of the pipes. In many modern factories, compressed air is used to operate a variety of equipment, some of which may malfunction if condensation forms on internal parts. Certain water sensitive processes (e.g.) paint spraying) that require compressed air may have specific dryness specifications. Finally, medical and pharmaceutical processes may treat water vapour and other gases as contaminants, requiring a very high level of purity.
Typical Range Of Dew Point Temperatures To Be Found In Compressed Air
Dew point temperatures in compressed air range from ambient down to -80 °C (-112 °F), sometimes lower in special cases. Compressor systems without air drying capability tend to produce compressed air that is saturated at ambient temperature. Systems with refrigerant dryers pass the compressed air through some sort of cooled heat exchanger, causing water to condense out of the air stream. These systems typically produce air with a dew point no lower than 5 °C (41°F). Desiccant drying systems absorb water vapour from the air stream and can produce air with a dew point of -40 °C (-40 °F) and drier if required.
Which WADS dew point sensor do you need?
All three families measure pressure dew point in compressed air to the same accuracy of ±2 °C, in the same IP65 housing, with the same choice of −60 to +60 °C Td or −80 to +20 °C Td measuring ranges. What separates them is where the reading has to appear and what the signal has to do once it leaves the sensor.
| Model | Form factor | Reading appears | Signal out | Choose it when |
|---|---|---|---|---|
| WADS 201–204 | Compact in-line transmitter | Remote only | 4–20 mA and Modbus RTU | The reading goes to a PLC, SCADA or the WiseAir dashboard and nobody needs to read it at the pipe. The compact body suits tight dryer installations. |
| WADS 205 / 206 | Wall-mounting enclosure | Local display on the wall | Pressure dew point and atmospheric dew point, optional red/green lamp and buzzer | An operator has to see the value and be alerted without opening a control panel. The MEMS pressure sensor reports line pressure at the same time. |
| WADS 207 / 208 | In-line with built-in display | On the sensor itself | Alarm output, optional integrated pressure sensor, dual dew point display | You want the number readable at the pipe without running cable to a separate panel. |
If you are unsure, the deciding question is usually not the sensor at all — it is whether anyone will act on the reading. A 4–20 mA transmitter feeding a dashboard that nobody watches catches a desiccant breakthrough only in hindsight. A local display with an audible alarm catches it while the operator is still on the floor.
What dew point does your application actually require?
Specify the sensor range from the ISO 8573-1 humidity class your process has to hold, not from what the dryer is rated at. A sensor whose range stops at +20 °C Td is useless for proving Class 4 compliance, and a −80 °C range sitting on general plant air is money spent on resolution you will never use.
| Class | Pressure dew point | Typical application | Suitable range |
|---|---|---|---|
| Class 1 | ≤ −70 °C | Ultra-dry process air, semiconductor | −80 to +20 °C Td |
| Class 2 | ≤ −40 °C | Pharmaceutical and GMP product contact, food and beverage, instrument air, electronics, PET blowing | −80 to +20 °C Td |
| Class 3 | ≤ −20 °C | Outdoor and unheated piping where freezing is the risk | −60 to +60 °C Td |
| Class 4 | ≤ +3 °C | General plant air, pneumatic tools, paint spraying, textile air-jet looms | −60 to +60 °C Td |
Not sure which class you currently meet? Enter your measured dew point and line pressure into our free pressure dew point calculator. It converts between pressure and atmospheric dew point, returns ppmV and g/Nm³, and tells you which ISO 8573-1 class the reading corresponds to.
Getting a reading you can trust
More dew point complaints come from the installation than from the instrument. Four things decide whether the number on the display means anything:
Install downstream of the dryer and its after-filter
Upstream of the filter you are measuring wet air plus whatever the filter is about to remove. Downstream of the after-filter you are measuring the air your process will actually receive.
Measure at full line pressure
Pressure dew point and atmospheric dew point are not the same number, and at 7 bar(g) they differ by roughly 17 to 25 °C. Fitting a sensor downstream of a regulator and then comparing the reading to a pressure dew point specification is one of the most common errors we see on audits.
Give the sensor continuous flow, not a dead leg
A sampling point with no flow past the sensor will still eventually reach equilibrium, but it can take hours instead of seconds. If your readings drift slowly and never seem to respond to dryer changes, suspect the sampling arrangement before you suspect the sensor.
Allow for dry-down time
Any sensor taken from ambient air into a −40 °C stream needs time to release the moisture it has absorbed. A reading that starts wet and falls over the first hours of operation is normal behaviour, not a fault.
Frequently asked questions
What is the difference between a dew point sensor and a dew point transmitter?
In practice the terms are used interchangeably. Strictly, a transmitter is a sensor that also conditions the signal and outputs it in a standard form such as 4–20 mA or Modbus RTU, which is what the WADS 201–204 range does. A sensor with only a local display and no signal output would not usually be called a transmitter.
Can one instrument measure both dew point and pressure?
Yes. All three WADS families offer an integrated pressure sensor as an option, which lets a single instrument report pressure dew point, atmospheric dew point and line pressure. This matters more than it sounds: without line pressure the conversion between pressure and atmospheric dew point has to be assumed rather than calculated.
How often does a dew point sensor need calibration?
Annually is the common interval and it is what most auditors expect to see documented. Sensors running continuously in the −40 °C and drier range drift more than those sitting near ambient, so verify rather than assume. WiseAir offers calibration services for our own instruments and for other manufacturers.
My refrigerant dryer is rated +3 °C. Do I still need a sensor?
That rating is a design-point figure measured under ideal inlet and ambient conditions. Real plants run hotter, dirtier and at higher flow than the design point, and a fouled condenser or a failed drain will push the actual dew point well above the rating without any other symptom. The rating tells you what the dryer can do; only a sensor tells you what it is doing.
Which range should I order if my requirement might tighten later?
Order the −80 to +20 °C Td variant. Moving from Class 4 to Class 2 usually means changing the dryer, and if the sensor also has to be replaced the cost of the upgrade rises for no engineering reason. The wider range costs little more at the point of purchase.
Not sure what to measure, or where?
Most plants measure the wrong point or the wrong parameter and then distrust the data. Our engineers will tell you which measurements actually answer your question, and where in the system to install them.
Prefer to talk? Call +91 90477 78715. Free tools that use these measurements: leak cost, pressure dew point and savings and ROI.
WISEAIR TECHNOLOGIES INDIA LLP
# 12 Sri Venkatalakshmi Nagar, Singanallur,
Coimbatore – 641005 India.
WiseAir Technologies India LLP is part of the Systel Asia group — specialists in compressed air management and smart monitoring solutions across Asia since 2002.
© Copyright 2026. WiseAir Technologies India LLP
Not sure what to measure, or where?
Most plants measure the wrong point or the wrong parameter and then distrust the data. Our engineers will tell you which measurements actually answer your question, and where in the system to install them.
Prefer to talk? Call +91 90477 78715. Free tools that use these measurements: leak cost, pressure dew point and savings and ROI.
