Measuring mugginess

The term 'humidity' is used to describe the quantity of water vapour present in air or other gases. A simple definition; but one that conceals the diversity of the field of humidity measurement, both in the potential number of measurement methods and the wide range of applications requiring measurement.

Specifying a hygrometer incorrectly can be an extremely costly error, both financially and through the loss of control over operating conditions; this is especially true of critical processes where inaccurate or inconsistent measurement of moisture content can have a significant impact on quality, performance and safety.

The first step in any specification procedure is to establish exactly what each measurement is needed for. For example, if a unit is to be operated primarily to confirm that an existing monitoring system is functioning accurately, measurements will need to be taken only periodically and will probably best be achieved using a portable hygrometer. Conversely, if a unit is designed to measure a compressed air stream where humidity levels must remain constant for an application to progress successfully, a hygrometer needed must be capable of monitoring continuously, and with high repeatability and accuracy over a long period.

Where the data will allow an operation to run within certain parameters, or to comply with documented limits, both the hygrometer and method of calibration may already be specified, which will pinpoint the use of certain instruments. If such a specification is not made, however, the likely measurement range for an application will indicate the most appropriate instruments. For example, an application in the semiconductor industry will require measurements in ppb (parts per billion), while those in high temperature, high relative humidity test chamber environments will operate in ppm (parts per million).

Although several instruments may operate within the required range, it is preferable not to use a sensor operating at either extreme of its range, as this is more likely to result in inconsistencies and inaccuracies.

If a measurement parameter is not specified legally as part of site or process controls, the type of application in question can also be used to indicate which parameter is the most appropriate to measure (see Table 1). Relative humidity, for example, is often used to express ambient environmental conditions; while dewpoint, which is practical over a wide range and is independent of temperature, is frequently used in industrial applications.

Once the preferred parameter has been established, this will, in turn, indicate the most appropriate instrument to use. While a variety of equipment and methods can measure a particular parameter, they will not all measure the unit directly. Even those monitors which display a range of analyses will generally calculate the majority of readings from a single measurement. The most accurate and reliable readings, however, will be those which are calculated intrinsically.

Once the preferred unit of measurement has been determined, the next stage of the specification process is to determine which operating principle is best for producing the type of data required. (See Table 2 for summary)

While a wide range of principles are available, two of the most popular for monitoring in an industrial environment are electrical impedance and cooled mirror hygrometers.

ELECTRICAL IMPEDANCE DEVICES

Electrical impedance hygrometers are often portable, compact and easy to use. Their sensors are made from a hygroscopic material, the electrical properties of which alter as it absorbs water. As a result, any change in humidity can be measured by determining a change in the impedance sensor's electrical capacitance, resistance or some combination of the two.

The majority of modern impedance sensors use thin-film technology, with the sensors built up from layers of absorbent materials such as aluminium oxide or silicon oxide. Typically, these give good reliability and accuracy with most common process gases, and are relatively unaffected by contact with moisture.

However, with harsher process gases, some thin film sensors can exhibit weaknesses, such as poor chemical and physical resilience, as well as calibration instability. In these environments, an impedance sensor based on a ceramic moisture sensor is generally a better choice.

Further variations of the impedance sensor are also available. These include resistive, capacitative and dewpoint devices. Capacitative sensors, for example, operate most effectively to measure relative humidity in low-humidity environments. Conversely, resistive impedance sensors are better suited to high humidities, though they will not tolerate the formation of condensation.

Dewpoint impedance sensors are unique in that they measure in absolute, rather than relative, humidity units. They use aluminium or other metal oxides, or a silicon base, as the active element for the sensors, which can be used across a wide measurement range including very dry gases. Typically, they convert the signal produced from measurement of the partial pressure of water vapour into other absolute units, displaying results as dewpoint or ppm.

The sensors of impedance devices should periodically be exchanged for recalibration, so it is important to consider how easy it is to replace sensors when specifying these devices. As it is important that the sensor is calibrated independently, it must be easy to interchange with the measurement electronics. Otherwise, the sensor, its instrumentation, or both, will need to be removed for calibration, causing potentially lengthy down-times.

COOLED MIRROR HYGROMETERS

When a sample of gas is cooled, its dewpoint will be indicated by the temperature at which frost or condensation appears on a 'mirror' or polished surface. This phenomenon is the basis for cooled mirror hygrometers. Early examples of these instruments relied upon the skill of the operator to make a visual detection of this point. Modern versions, by contrast, detect the change in the intensity of scattered light reflected from the mirror surface when condensation or frost begins to form upon it.

In operation, a light source is reflected from a mirrored surface to a light detector. The temperature of the mirrored surface is controlled by, for example, a Peltier effect thermoelectric heatpump, which cools or heats the surface depending on the current applied. Closed-loop control of the heat pump current and, therefore, the mirror temperature, enables the temperature to be held precisely at the point at which condensation occurs. The mirror contains a highly accurate measurement device, such as a platinum resistance thermometer, which provides the measurement medium, while a digital display and/or analogue outputs provide a readout.

Some cooled mirror sensors use a spot measurement technique, while others provide a constant measurement. The latter generally give more comprehensive information, as any variation in moisture content is immediately evident.

As a rule of thumb, cooled mirror devices are regarded as the benchmark instrument, often being used as a transfer standard to link national standards with working instruments. Although expensive, the accuracy and reliability of cooled mirror instruments means that they are increasingly being used in industrial and process applications for on-line measurement and control.

A number of other factors might also influence the choice of instrument, not least the cost of purchase, installation and maintenance; accuracy and repeatability; robustness and lifespan.

The above guidelines should help users establish the purpose for which humidity measurement is needed, the unit or units of measurement required, and the best measurement parameter for achieving these. The last remaining choice, therefore, will be to select the most appropriate manufacturer to meet the needs of a particular operating company or environment, with the best supplier needing to demonstrate equipment that is accurate, traceable, reliable and available at the right price.

Perhaps as importantly, they should also be able to offer excellent technical support and, ideally, facilities for prompt re-calibration of sensor mechanisms. But the most sensible thing to ask potential suppliers is for them to explain exactly why their instrument is the best choice for your application.

Andrew Michell is chairman of Michell Instruments