Digital Humidity Measurement

Measurement Advantage
The measurement of humidity and moisture throughout industry and commerce increases year on year. Most materials and processes are influenced by the presence of water to some extent, but many of today's fastest moving high technology markets such as the semiconductor, pharmaceutical, power generation and communications sectors have a significant dependence on its control.

Factors which influence the need for installing more and more humidity instrumentation include product quality, product testing, energy efficiency and the comfort of the working environment.

Calibration Advantage
The past ten to fifteen years have seen significant advances in the precision and reliability of measurement technology. Perhaps more significantly, the general understanding of humidity as a science and development of traceability of measurement values through the calibration hierarchy has also improved. As little as ten years ago humidity was often referred to a 'black art'.

Indeed, many users today still have this impression, but the network of National Standards laboratories throughout the world and their supporting accredited laboratories, mean that the user can now verify their measurements against reliable standards, and hence, trust the values they obtain more than ever before.

A wide range of techniques are employed for the measurement of humidity. These range from simple mechanical indicators, to highly complex and expensive analytical instruments. No single measurement technique can be reliably applied to all application scenarios, so often instrument selection is as important as the quality and reputation of the manufacturer.

One of the most widely used instrument types is based on electronic impedance or capacitive sensors which respond to varying levels of partial water vapour pressure. Their output is predominantly in the form of percentage relative humidity (%rh) which can be easily converted into other measurement variables such as dewpoint (°C dp), mixing ratio (g/kg) or wet-bulb temperature. They provide measurements over wide humidity and temperature ranges and are supplied in user friendly configurations such a hand-held instruments, analogue or digital transmitters, and data-loggers.

As with any electronic sensor, a limitation is their instability with respect to time (drift), caused in particular by wide variations in temperature & humidity or the presence of chemical pollutants.

So regular checks of calibration must therefore form an integral part of their effective use and maintenance. Recent developments in the technology used in today's electronic hygrometers has led to gains in precision, reliability, maintenance and user convenience, and has gone a long way to counteract the effect of sensor drift.

Digital Advantage
Sensor modules based on digital technology offer three key advantages.

Sensor characterisation:
Humidity sensors all demonstrate different response curves when compared to actual values. Traditional analogue circuitry has evolved over many years to provide excellent linearisation capabilities across the sensors full working range. Depending on the product type; two, three or four adjustment points are used to 'trim' the instrument before use to provide a linear output signal or accurate displayed value. In the digital domain, there is increased scope for the sensors response characteristics to be precisely mapped, and through the use of software, up to ten adjustment points can be used, and these may be selected to optimise measurement performance in the users specific application conditions.

As calibration 'data' is electronically recorded within the sensor modules electronic 'brain', and the output signal is in a digital format, interchanging sensor modules with no loss of measurement performance is now a precise and practical option. This means that the user can maintain a number of instruments using a single spare sensor module which can be calibrated under ideal laboratory or workshop conditions. Disruption to measurement signals is literally seconds whilst the newly calibrated module is connected, and on-site engineering time is almost eliminated.

An additional benefit is to allow a single instrument to be applied a variety of applications by using a range of alternative probe configurations.

Having measurement data in a digital format means that additional processing and communication of values is simplified. Calculation of alternative humidity variables can be performed in software, either within the local instrument, or when connected to a PC - through software.

Data acquisition equipment can receive values digitally, negating the need for analogue to digital conversion, therefore reducing the risk of signal loss. Finally, instruments can be networked together to communicate digitally.

HygroClip range:
Rotronic have recently introduced a range of humidity measurement products which embrace the many advantages of digital technology:


HygroFlex is an industrial transmitter well suited to demanding industrial process and monitoring applications using a wide range of probe configurations. They can be networked together to form a dedicated monitoring system using a single PC.

HygroPalm is an advanced hand-held instrument suitable for spot checking applications, again with a wide range of probe configurations including high temperatures up to 200°C. One of many unique features of the HygroPalm is its suitability for display and adjustment on-site of HygroFlex transmitters using a digital interface cable.

HygroLab is a benchtop unit with similar features to both the HygroFlex and HygroPalm. Using the full probe input complement and the networking facility, up to 128 measurement points can be monitored using a single PC.

This article supplied by Rotronic Instruments Ltd.

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