Comprehensive measurement of moisture levels

The application of modern microelectronics means that systems are now available that make it economically viable to monitor moisture concentration at all points of use where it is of importance

There are many and varied reasons why compressed air needs to be dry at the point of use.

In some cases, such as breathing air, the consequences of excess moisture in the air supply could lead to the loss of life.

In others, such as the purging of chemical and nuclear plant or maintaining air blast switchgear free from moisture, the failure to maintain dry air supplies will give rise to millions of pounds worth of losses due to plant downtime or damage, frequently both.

In less critical applications, such as factory air supplies for pneumatic tools and control systems, excess moisture in the air is likely to increase the cost of ownership of the system due to premature failure and replacement of components due to corrosion.

Dry air systems where the moisture content of the air is of significance, are usually supplied with a moisture indicator as original equipment.

There are two common pitfalls that are frequently overlooked, however.

The first is that, although the air may be dry at the point of exit from the drier, it is frequently distributed around the factory through a delivery system that may consist of various rings and branches.

The quality of the air at points of use can only be as good as the integrity of this system will allow.

For example, it is not commonly understood that moisture from the atmosphere can diffuse into a dry air supply against a flow of air out of the system.

This can mean that problems may be encountered at a critical usage point because of moisture ingress at some other location where the moisture level may be of no consequence.

Secondly, the moisture indicator, like any analytical instrument, needs to be calibrated routinely to ensure its continued accuracy.

As with any item of electronic measuring equipment the properties of its components may vary with time.

In addition, the moisture sensor itself may be exposed to contamination, such as desiccant dust, within the system.

There are many instances where catastrophic system failures have occurred because the moisture indicators have not identified deterioration in drier performance in order that remedial action could be taken.

The first of these can be addressed by monitoring the moisture levels at points of use, rather than relying solely on the measurement at the drier.

The application of modern microelectronics means that systems are now available that make it economically viable to monitor the moisture concentration at all points of use where it is of importance.

An ideal way of doing this is to install a distributed system based on smart sensors, such as the Alpha Moisture Systems Dewsmart ADHT.

Alternatively discrete measuring units, such as the model DS1000, could be installed wherever the moisture level is critical.

If the user prefers to confirm the integrity of the air at a number of usage points using a portable dewpoint hygrometer, the Alpha Moisture Systems DSP-Ex portable unit with the advantage of the Alpha Moisture Systems desiccant dry down head technology to facilitate rapid spot check measurements, is the ideal instrument.

There are a number of ways of addressing the calibration issue.

The main thing is to recognise that the OEM unit will need regular attention to confirm that it will be able to identify any problem with the drier.

It may well, in fact, need more attention to keep it in good order than would be the case with a higher quality instrument such as the ones mentioned above.

Perhaps the obvious way is to send the unit to a moisture calibration laboratory routinely.

Alpha Moisture Systems Calibration Laboratories are traceable to the UK National Humidity Standard at NPL and are accredited to ISO9002.

All makes of hygrometer can be calibrated and full service and repair facilities are available for both Alpha and Shaw instruments.

An on site calibration service is also available where this is appropriate.

Where several units are involved it may not be convenient to have to keep sending them away at the frequency needed to be sure of their integrity.

A simple way to ensure results are correct at the normal operating level is to initiate a programme of regular checking using a portable instrument such as the Alpha Moisture Systems DSP-Ex mentioned above.

They could then be sent to a calibration laboratory only when the spot checks identified a need.

In addition to the rapid dry down facility already mentioned the model DSP-Ex boasts features such as full intrinsic safety and the ability to measure in a variety of different units.

Where it is desirable to check the calibration at different levels, for example at a critical alarm point as well as the normal operating level, a portable calibrator capable of delivering air conditioned to various levels that can then be passed through the instrument under investigation may be the solution.

Alpha Moisture Systems can offer both standard and custom designed units and would be happy to discuss any particular application.

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