There are numerous situations in which process engineers need to measure moisture in high-pressure gases.

Now Michell Instruments www.michell-instruments.com has risen to the challenge by increasing the maximum pressure rating for its Easidew and Cermet II ranges of dewpoint instruments (Figs1 and 2)

These on-line hygrometers are already able to withstand 300barg and this has been extended for use in gases with pressure up to 400barg. The company believes that this is the highest pressure rating of any dewpoint or moisture measurement instrument on the market.

All of the company’s hygrometers that utilise its ceramic moisture sensor feature a glass/metal seal capable of withstanding high-pressure gas. This high integrity seal allows the sensor to be installed in the process with no possibility of gas leaking from the system through the instruments sensor.

Dewpoint, or dewpoint temperature, is the most common measurement used to determine the amount of water vapour in a gas and is widely used in industries such as compressed air, natural gas, power generation and industrial/medical gas. Dewpoint is a pressure dependent variable and as the pressure of a gas increases, so does the dewpoint, making condensation more likely to occur and thus heightening the importance of reliable measurement.

Measurement of dewpoint in very high-pressure gas is commonly seen in the following applications:

• Breathing air. Users of high pressure breathing air cylinders appreciate the importance of keeping the dewpoint of their breathing air within specified limits. Failure do this can cause condensation at the exit valve and the liquid formed, combined with the Joules Thompson cooling effect associated with expanding gases, can cause the valve or regulator to ice up. Michell Instruments’ portable dewpoint meters can be used for spot-checking of the dewpoint within cylinders, whilst the filling of such cylinders can be continuously monitored using the Easidew or CermetII product ranges.

• Natural gas filling stations. Recently there has been an increase in the number of road vehicles powered by compressed natural gas (CNG). The gas is taken from a supply of typically 1–1.5barg that has a dewpoint of around -30°C. This gas is compressed to more than 200barg before being used to fill the gas tank on the vehicle and therefore needs to be dried to prevent condensation in the tanks. The dryers used for this application are often supplied with a Michell Instruments intrinsically safe dewpoint transmitter to ensure consistent performance.

• Military. Machines such as jet aircraft, ships and submarines utilise high-pressure gas for various applications including purging of optical systems and pneumatic actuation of guidance systems.

One for the dryers

The new Vaisala (www.vaisala.com) DRYCAP dewpoint and temperature transmitters DMT345 and DMT346 measure dewpoint and mixing ratio directly in high temperature drying processes. Therefore, there is no need for sampling systems and trace heating in pipelines. The DMT345 probe is designed for direct measurement in temperatures up to 180°C. The optional installation flange allows for adjustable installation depth and therefore precise positioning. In addition to measuring dewpoint and mixing ratio, the DMT345 measures temperature and relative humidity.

The DMT346 probe is constructed for humidity measurement in process temperatures between 140°C and 350°C and comes with a cooling set as a standard accessory. System cooling is accomplished without moving parts, additional power or cooling utilities. This removes the risk of damage to the transmitter that may occur due to cooling failures.

The DMT345 and DMT346 incorporate the latest generation of the Vaisala DRYCAP sensor, which provides accurate and reliable measurement with the best long-term stability and fastest response time available in the market. The sensor recovers quickly after getting wet and is immune to particulate contamination, condensation, oil vapour and most chemicals.

Typical applications for the new transmitters include high temperature ovens in the food industry and high temperature drying in the process industry.

The use of hydrogen peroxide (H2O2) as a sterilising agent in healthcare and production processes is extremely important to health and safety and has been growing in recent years due to its success. So much so that it is commonly used in bottle sterilisation plants, clean rooms, isolators and biosafety laboratories.

It is critically important that humidity levels and temperature are controlled for optimum performance and, according to the company, the Testo hygrotest 650 (www.testo.com) is the most technically developed means.

The Testo hygrotest 650, with its patented filter, converts H2O2 vapour into water vapour and oxygen. This novel measuring method enables humidity levels to be accurately monitored throughout the various phases of the H2O2 sterilisation cycle without risk to the sensor. With its fast response time and analogue or digital outputs, the Testo hygrotest 650 may be used in a wide range of monitoring, recording and controlling applications during H2O2 sterilisation applications.

Polymers adsorb moisture

Meanwhile Brownell (www.brownell.co.uk) has developed versatile moisture adsorbing polymers (VMAP) that adsorb moisture, odours and organic volatiles. Using a combination of polymers and adsorbent desiccants this newly developed range of materials offer new and improved opportunities for preventing the deleterious effects of moisture and high humidity including condensation and corrosion (Fig.3).

VMAP maintains a high moisture adsorption capacity during the moulding process with the added advantage of vapour adsorption property of the polymer.

Many instruments and industrial enclosures are sensitive to high humidity and moisture that can lead to malfunction and low reliability performance.

The extended use of electronic systems and instruments in hostile environmental conditions including automobiles, industrial control, marine applications and high voltage switch gear requires protection from the effects of humidity and moisture for reliable operation.

These instruments and products are required to operate for many years in hostile environments with high humidity and wide variations in temperature. For example roadside electronic enclosures and marine instruments are subject to a variety of climatic conditions of driving rain, solar heating and frost through out their operational life.

Specialist and long-term packaging can benefit from the controlled adsorption behaviour of VMAP with the added advantage of adsorbing odours and volatiles.

Conventional methods of protection against these effects include desiccant bags and panel mounting desiccators.

VMAP can be moulded or machined to suit applications where complex shapes are needed to occupy the available space. This also makes it useful for low cost prototyping and high volume production quantities.