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.
Interchangeability:
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.
Communication:
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.
Email: instruments@rotronic.co.uk
Website: www.rotronic.co.uk
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