Latest developments in dewpoint measurement

The market is soon likely to see the introduction of a full range of humidity sensors designed to provide the exact level of function, flexibility and manual control required in each application.

Commercial pressures continue to drive process operators to ever greater levels of automation in an ongoing attempt to reduce operating costs, improve process performance and achieve greater profitability, writes Mike Bannister, CEO, Michell Instruments Ltd.

Every aspect of the process and control system has been affected by this trend, including the design of process measurement instruments, such as humidity monitors.

Humidity monitoring has traditionally been carried out using an isolated sensor or meter, which may be connected directly into a compressed air or gas line, or used as a self-contained portable unit, either in smaller process systems where a fixed sensor would be considered over-engineering, or to validate the accuracy of a fixed monitor.

While many hygrometers continue to be used in this way, specifiers are increasingly integrating them into higher level process management systems.

As a result, instrument manufacturers have evolved a series of highly developed sensors specifically equipped to meet this requirement.

Perhaps the most important feature for sensors used in this way is the ability to communicate their measurement data directly to a central control system, rather than solely to a local display.

Many sensors now provide the ability to transmit information using digital protocols, which are unaffected by interference from external EMI sources and ensure that data remains accurate irrespective of external influences.

For example, careful arrangement of the internal sensor electronics means that Michell Instruments' three wire transmitter, Transmet, can transmit data using either analogue or digital communication with an RS232C, RS485, 0-20mA or 4-20mA linear output, according to the requirements of the control system.

Alternatively, operators requiring observation, both local and a re-transmission signal to external equipment, may choose an instrument such as Michell's Cermet II.

This instrument offers both a local digital display and data communication capacity as well as two 10A alarm relays, which can be set to switch should any predetermined humidity levels be reached or breached.

Using a hygrometer system to communicate data in this way provides ongoing, continuous feedback on gas humidity and, subsequently, overall process performance.

The dewpoint data provided can either be monitored manually or used as part of an automatic control system, to trigger an automatic feedback loop and adjust another process parameter, if pre-set dewpoint values are exceeded.

The reduction in operator input resulting from this move places even greater emphasis on sensor accuracy and traceability.

Changes in manufacturing techniques and the use of high technology materials have resulted in generally improved sensor performance.

For example, the accuracy of capacitive sensors, which have traditionally been seen as a lower cost technology, has improved from +/-3 or 4Cdp ten years ago, to +/-1Cdp, making them much better for most process applications.

Traceability is also important if the ongoing reliability of the sensor is to be assured.

Several major manufacturers now maintain their own traceable laboratories used to calibrate sensors before use and to confirm annually that sensors are continuing to offer traceability to national and international standards.

For example, Michell sensors are calibrated at 13 points across the measurement range with readings referenced against a NAMAS calibrated cooled mirror dewpointmeter.

Calibration data for each sensor is stored within its EEPROM to maintain the performance over time.

One side effect of the increasing integration of process equipment into an interlinked system is that there is a reduced need for daily manual control.

At the same time, process controllers will typically have a reduced level of specialist knowledge of the individual operating technologies.

In combination, these factors have led to a general trend towards instrument simplification, which, in turn, has allowed the development of a number of extremely cost effective dewpoint monitoring solutions.

For example, Transmet has been designed without a local display or control panel, but instead works solely as a transmitter of information to a central monitoring point.

In addition, up to 32 Transmet units - each monitoring a separate measurement point - can be linked using a single network converter, which further contributes to the cost effectiveness of the transmitter.

Frost and Sullivan's report into the process water analysis market noted the increasing use of highly complex "smart" electronics and computer controlled displays in humidity monitoring systems.

The trend looks set to continue as end users continue to demand simple to operate yet accurate sensors, which can be easily incorporated into their management control systems.

The more that sensors are streamlined for such applications, the greater the divergence is likely to be between extremely high accuracy, multi-functional cooled mirror dewpointmeters used for calibration applications and simple "plug and play" devices, which also offer high accuracy and traceability, but with a lower degree of operator input and control.

In fact, as manufacturers diversify to meet the demands of end users, the market is likely to see the introduction of a full range of humidity sensors designed to provide the exact level of function, flexibility and manual control required in each application.

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