This is how you ensure high-quality and dry compressed air in your process

Why moisture in compressed air is a problem

Ensuring dry compressed air is not based on dew point values alone. High-quality compressed air requires three things: good measurement practices, best-practice sensor installation, and thoughtful sensor placement. When these factors are in place, you maintain product quality, protect equipment, and improve energy efficiency.

Compressed air used in many industrial applications must be kept dry. This is because moisture in compressed air can cause corrosion, clogged valves, frozen pipes, product contamination and microbial growth. If compressed air is not dry, it can quickly go from being a clean and reliable ”fourth commodity” to a quality and maintenance risk.

The dryness of compressed air can be measured in different ways, but not all methods are equally reliable.

Relative humidity (RH) is the percentage of water vapor that can be held in the air at a given temperature. However, RH changes with temperature, so it is not a reliable indicator of the risk of condensation in a compressed air system.

The dew point, on the other hand, indicates the exact temperature at which moisture condenses into liquid water or ice. It depends on the pressure and indicates where and when in the process there is a risk of moisture forming in the compressed air. This is why it is an ideal control variable: if the dew point is above 0 °C, the process is at risk of condensation and corrosion. The lower the dew point, the safer the system.

Typical dew point targets by dryer type:

Refrigeration dryer: approx. +5 °C
Adsorption dryer: approximately -40 °C or lower

Correct measurement of dew point

Since the dew point is dependent on pressure, the compression of the air will affect the measurement result. This is important to consider if the air is vented to the atmosphere before measurement, as the dew point at the measurement point will differ from the process dew point. When the pressure dew point is required, either measure at process pressure or ensure that you carefully correct the measurement result.

A low dew point reading is not useful if the measurement does not reflect actual process conditions.

Three simple steps to accurate dew point measurement

Choose a measuring device with the correct measuring range.
Understand the pressure characteristics of the device. For example, some devices on the market cannot measure at process pressure, in which case the measurement must be corrected to determine the pressure dew point.
Install the sensor correctly according to the manufacturer's instructions. Do not install dew point sensors in a closed pipe where there is no airflow.

Where should the dew point be measured?

The placement of the sensor is important. If you only install the sensor after the dryer, the measurement will only confirm that the dryer output meets the set parameters. For best results, it is also a good idea to place sensors at critical points of use and at the end of the network. This will allow you to detect leaks and losses due to cooling and distribution.

For example, Walki Oy Valkeakoski uses a monitoring system with sensors placed after the dryer and at the end of the compressed air network. This enabled real-time alerts when a dryer malfunction increased humidity levels. The system triggered an automatic alarm when the dew point of the compressed air network reached -25 °C. The alarm enabled a quick response before the problem affected the production process.

Pictured are Mikko Nurminen and Petri Parikka from Walki Oy

Five installation basics for accurate dew point measurement

Even the best dew point sensor is only as accurate as its installation allows. Here is a five-point checklist:

Use the right materials. Preferably choose stainless steel with a good surface finish. Avoid hygroscopic plastics and rubber, as they can bind and release moisture. PTFE is an exception down to temperatures of around -40°C, as it has low moisture absorption.
Ensure that the connections are tight. Dew point measurement is very sensitive to even small leaks. All connections must be tight and properly sealed – PTFE tape is recommended for tapered threads such as NPT. For straight threaded connections such as G½", use the gasket supplied with the device.
Keep the flow steady. The sensor requires a small but continuous air flow. Higher flow speeds up response time, but 1–2 l/min is usually sufficient for high-quality sensors. Do not install sensors in a closed vessel without flow, and flush sample lines before measuring.
Avoid condensation in the sample line. The entire sample line, including valves and fittings, must be kept above the process dew point. If necessary, use heated or heat-cabled lines.
Choose the right measurement solution. Direct line installation is suitable if flow and temperature are sufficient. The sample chamber protects the sensor from hot gas or water spikes and allows maintenance under pressure.

In very low dew point measurements such as -40…-70 deg.C, the rubber hose shown in the picture increases the measurement inaccuracy because they bind and release moisture.

Spot measurement or continuous monitoring?

Both have their place – spot tests help with problem solving, continuous monitoring helps with prevention.

Spot checks with a portable dew point meter and recorder are suitable for audits, troubleshooting and calibration checks. Energy savings are often achieved by detecting leaks and optimizing drying. For example, SmartAir And Energy Oy uses the Vaisala Indigo80 to generate energy and cost savings for its compressed air customers.

Pictured are Mikko Nurminen from Walki Oy and SmartAir Sami Uusi-Erkkilä

Continuous monitoring provides real-time situational awareness and enables automatic alerts if the dew point deviates or rises suddenly. Such changes can indicate a dryer failure, tower replacement or sudden load change. At Walki’s Valkeakoski plant, sensors triggered an alarm when the dew point rose to –25°C, enabling rapid corrective action before product quality deteriorated. Continuous monitoring also produces trend data for maintenance and investment planning.

Compressed air quality standards

The key ISO standard for compressed air cleanliness is ISO 8573, which classifies compressed air quality for particles, water and oil. The standard covers nine areas, including impurities, cleanliness classes and test methods for several variables such as oil, gases, water and humidity. Dew point measurement demonstrates compliance with the standard for water.

In hygiene-critical areas where there is a risk of microbial growth, ISO 8573-7 addresses live contaminants. The dryness and purity of compressed air are the primary barriers to microbial growth.

Advantages of modern dew point measuring devices

Modern dew point instruments are significantly different from older chilled mirror hygrometers or slow capacitive sensors. Vaisala DRYCAP® sensors use a highly sensitive thin film polymer that provides:

Quick response from wet to dry and back again
Long-term stability and minimal need for recalibration
High tolerance in condensing conditions
Contamination resistance thanks to automatic chemical cleaning function
Excellent accuracy with low hysteresis
Fast response time and low drift

For engineers who need conversions between atmospheric dew point and pressure dew point or other humidity calculations, Vaisala offers a free and easy-to-use humidity calculator.

Want to learn more about compressed air and dew point measurement? Download Vaisala's Compressed Air eGuide or watch the The essentials with compressed air dew point webinar.

Practical checklist for dew point measurements

✔ Choose the right measurement points, for example after the dryer and at the point of use
✔ Measure at the correct process pressure or correct the measurement appropriately
✔ Use stainless steel sample lines and chambers
✔ Ensure the tightness of the connections
✔ Maintain a small, steady airflow over the sensor
✔ Avoid dead-end installations and flush sample lines before measuring
✔ Keep sample lines above dew point – use heating if necessary
✔ Assess when spot testing is sufficient and when continuous monitoring is necessary

The author of the article is

Juhani Lehto
Product Manager, demanding measuring devices
Vaisala

Juhani Lehto has over ten years of experience in industrial measurements and sensor technology. He holds a Master of Science (Engineering) from Aalto University, Espoo.