The formation of condensation water in the compressed air system often produces moisture on the forehead and even tears in the eyes of those in charge of quality assurance. This is because the consequences for process reliability can be quite dramatic, with the worst case scenario being the loss of the entire production output. It is therefore crucial to identify any possible risks or existing disturbances at an early stage.

And this is where pressure dewpoint measurement comes in.

Moisture in the air is natural, it is a physical necessity and essential to life.

However, under certain conditions moisture is only acceptable when it is controlled and suitably reduced.

For instance in compressed air systems.

Here, moisture can cause serious damage: premature tool wear, mechanical defects due to disrupted lubricating films, blistering of paints and plastics, sticking together of transported bulk goods such as sugar or granulates, increased bacteria formation in medical applications, etc As a result, the process engineer is faced with extensive malfunctions in the production plant, expensive standstills and higher reject rates - all factors pushing up the costs.

Quite clearly these are situations that a company operating in line with modern cost management and observing today's quality standards can neither afford nor tolerate.

It is therefore rather surprising that many companies are not yet fully aware that continuous, comprehensive and reliable monitoring of the compressed air treatment is a major element determining process reliability within a production chain.

To this end, the precise measurement of the pressure dewpoint is indispensable for success.

* When air humidity 'has had enough' - it is well-known that the air can only take up a certain amount of water vapour.

How much exactly will depend on the actual air pressure and temperature.

If the moist room air cools down, for instance on a window pane, the water vapour will condense in the form of droplets and the glass will mist up - a result of condensation familiar even to children.

The point at which condensation commences - under atmospheric air pressure - is referred to as the dewpoint, or pressure dewpoint (PDP) in the case of compressed air.

It is when the humidity of the air, under the given conditions, has reached 100 percent, so that the air cannot retain any more water vapour.

As a general rule: the higher the temperature, the more water vapour can be taken up by the air, but the higher the air pressure, the lower the possible quantity of moisture in the air.

Therefore, the dewpoint is always determined by the factors 'temperature', 'pressure' and 'relative air humidity'.

The relative air humidity is the proportion of the actual water vapour in the air in relation to the maximum possible quantity of water vapour in the air, i.e to the degree of saturation of the air.

* And what is a pressure dewpoint (PDP)?

How does it differ from the 'normal', the atmospheric dewpoint?

- As explained, the amount of moisture remaining in the air, will depend on the air temperature and the pressure.

With compressed air systems, it is a fundamental rule that a cubic metre of compressed air cannot hold more water vapour than a cubic metre of air under atmospheric conditions.

For instance, if eight cubic metres of atmospheric air are compressed to produce two cubic metres of compressed air, then six parts of water vapour will be 'left over' - assuming the same air temperature.

This excess water vapour will turn into condensate.

If the air cools down during the compression process, it will be even less able to retain moisture - and this leads to the formation of additional condensate.

On the other hand, if the compressed air is expanded to atmospheric pressure, the volume will increase.

At identical temperatures, the dewpoint of expanded air is therefore lower than the pressure dewpoint.

If the compressed air has a pressure dewpoint of, say, +3 deg C, there will be no condensation so long as the corresponding ambient temperature lies above +3 deg C.

However, if the ambient temperature drops below this value, the condensation process will set in immediately.

* A 'detective story' to capture humidity - in view of the complex conditions described above, it is obvious that the dewpoint measurement in compressed air systems must be given top priority.

This is the only way to ensure a maximum of process reliability.

At the same time, the measurement of humidity depends on and is influenced by numerous factors, and this makes it generally more difficult than the measurement of temperature, pressure, filling level or weight.

To realize this, one only needs to think of leaks in the pipe network, the state of the material at the measuring point, impurities in the compressed air or the various flow velocities.

Dewpoint measuring systems have been continuously improved over the years.

One of the instruments used originally was a hair hygrometer of a type that required enormous maintenance expenditure and extremely long regeneration periods.

However, the measurement techniques gradually became more and more refined.

New devices were developed that utilize the principle of the dewpoint mirror method, where a stainless steel mirror is cooled with the aid of a Peltier element until water vapour condenses on the mirror.

An optoelectronic control loop then detects this condensate due to the reduction in the light reflected by the mirror, and determines the measured value via a number of functional steps.

Instruments of this kind are not suitable for the tough operating conditions in industry, since the optical system is very sensitive and gets dirty every time it is wetted and subsequently needs to be cleaned.

Moreover, these measuring instruments, which are employed in calibration laboratories, are very expensive with prices ranging from EUR 15,000 to 30,000, depending on the specific design.

Measuring instruments used in industrial facilities today are typically based on electrical instead of optical methods.

A common procedure up until now has been the measurement via aluminium oxide sensors with a surface-active capacitor open towards the atmosphere.

The deposition of water molecules on the surface of the aluminium oxide changes the capacity of the sensor and this change is used for calculating the measured value.

Problematic with this type of sensor are the drift, the response time and, above all, the stability over time.

Furthermore, these sensors are not suitable for damage monitoring in the event of condensate formation because the aluminium oxide layer of the sensor does not react very well to high humidities.

A number of manufacturers, e.g, state calibration cycles of 6 months for their aluminium oxide sensors.

Consequently, these sensors only have a limited suitability for humidity monitoring under industrial conditions.

With the new generation of dewpoint meters the above problems have been virtually eliminated.

A recent arrival on the scene, the dewpoint meters from the German compressed air specialist Beko Technologies, are even a precision step ahead.

This company - with headquarters in the German town of Neuss on the river Rhine and a worldwide sales network - has chosen the future-oriented technology of capacitive polymer sensors.

* New capacities - in contrast to open-pored aluminium oxide sensors, the capacitive polymer sensors of the Beko instruments are equipped with a closed capacitor.

The cover electrode of this capacitor is penetrated solely by water vapour that diffuses through it, and any other particles contained in the air are excluded.

The vapour attaches itself reversibly to the polymer, thereby changing the sensor's capacity.

The crucial point here is that dust, dirt, oil or other substances can, at most, settle on the smooth surface of the capacitive sensor, but they cannot penetrate into the sensor.

This reduces misinterpretations to an absolute minimum, because it is only the water vapour entering by diffusion that can influence the measuring effect.

Different to the usual sensors whose rough surface makes them difficult to clean, often leaving dirt pockets, Beko's capacitive sensors can be cleaned easily and completely.

They are also suitable for application across the entire measuring range: from 0 to 100% relative air humidity.

It is even recommended to clean the sensor, if required, with water or alcohol.

This results in another advantage: an extremely fast response time compared with aluminium oxide sensors.

In the case of polymer sensors, there is no droplet formation in the first place, because the water enters and escapes merely in the form of vapour.

The adaptation period therefore only amounts to minutes, with complete wetting and subsequent drying.

Assuming an oncoming flow of more than three litres per minute, the response time will only be about 15s.

In practice, the fast reaction of these sensors is ideal, for instance when a dryer fault occurs at a dewpoint of -20 deg C.

Under such conditions, the sensors register a rise in the dewpoint within seconds and give an alarm signal before the condensate spreads throughout the pipe network.

This is why the dewpoint meters should be installed directly downstream of the dryer.

The flow through the measuring chamber of these innovative Beko instruments even takes place, under pressure, when compressed air is not being withdrawn.

In other words, volumetric flow is not a prerequisite for obtaining exact measuring results within the shortest possible time.

The measuring chamber, which has a purge air outlet, can easily be subjected to pressures of up to 16 bar.

And that with an accuracy of +/-0.5 deg C at pressure dewpoints between -10 and +40 deg C.

With pressure dewpoints between -10 deg C and -40 deg C, the accuracy still amounts to an impressive +/-2 deg C.

* Stationary and portable - with three different versions to choose from, two stationary meters and a portable one, the Beko instruments cover a dewpoint measuring range from -80 to +50 deg C.

While the first stationary system is designed for a PDP range of -10 to +50 deg C - making it particularly suitable downstream of refrigeration dryers - the second permanently installed device offers a PDP measuring range between -80 and +20 deg C, and is therefore the preferred option for combination with membrane and absorption dryers.

The portable dewpoint meter, which has a practical pistol handle, can be used right across the above temperature range.

A snap coupling mechanism ensures safe and easy connection to any desired point in the pipe network.

The display shows the measured values directly in situ, including min./max.

values, but the data acquired can also be called up from the internal long-time memory via a PC interface.

Additionally, the optional software of the stationary pressure dewpoint meters allows online integration of the evaluation and alarm functions into the existing process control system.

All three variants are supplied ready for immediate use.

The stationary units come ready for wall-mounting and only need to be plugged into the mains.

The portable device is equipped with an internal rechargeable battery that has a very long lifetime.

* Triple health check for the compressed air system - today's compressed air pipe systems are still on the losing side: leaks are quite common and this may waste as much as 30% of flow.

These leakage points - often located at flanged, screwed or plug-in connections - necessarily lead to errors in humidity measurement.

This is because of the large difference in the partial water vapour pressure of the relatively dry compressed air and the moist ambient air, which results in moisture penetrating into the pipe.

Humidity or dewpoint measurement in compressed air systems should therefore, wherever possible, be accompanied by effective checking and locating of leaks.

For this purpose, Beko's new dewpoint meters are complemented by the 'Leak Detector' from the same company.

This hand-held, 17cm long device, weighing only 400g, is able to locate even the smallest leaks with millimetre precision, leaks that can be neither seen nor heard by human beings.

However, there is one type of sound - ultrasound - generated at the leakage points because the air molecules escape at high velocities and cause friction at the material edges.

And this is the speciality of the Leak Detector: it is designed to pick up ultrasound frequencies.

The instrument has such 'sharp ears' that it can even pinpoint a leak at long distances of up to 15m.

This means, for instance, that the operator at floor level can comfortably check compressed air pipes suspended from the high ceiling of a hall.

Similarly, pipes underneath the floor or behind wall panels or pipes otherwise covered up and difficult to access cannot hide from a thorough health check by the Leak Detector.

The third unit in this alliance for securing greater process reliability is the 'Flowmeter', a mass flow measuring instrument from Beko Technologies.

It enables the comprehensive analysis and documentation of the four major areas that are the key for optimizing the economic performance of compressed air systems: leak elimination, cost unit analysis, capacity utilization analysis, and energy contracting monitoring.

Supported by a dewpoint meter, leak detector and flowmeter - three 'watchful assistants', which are functionally independent from each other - the operator can implement a well-founded compressed air concept for the entire plant.

One thing is beyond doubt: quality-oriented compressed air treatment requires more than a functioning compressor, filter and dryer.

The perfect matching of these components, optimum efficiency, high cost-effectiveness and, above all, maximum process reliability are just as important.