Pressure sensing in the Water industry.

Pressure monitoring is widely used in the water industry to provide
level measurement in reservoirs and bore boles, leak detection in
distribution networks, as well as control data for a wide range of
process functions.

Pressure transducers can also be used for 'drop testing' when measuring
leakage from reservoirs. Drop testing is used to check the volume of water
loss from a reservoh identify maintenance requirements and to meet the
requirements of regulatory authonties. The source of leakage is usually the
floor joints of the reservoir structure. Underground drains normally collect
this leakage - which can then be identified through routine checks. However
leakage from wall joints can be harder to detect and some reservoirs may
not have a drainage system in place.

The drop test system normally comprises a pressure transducer, connected
to a data logging unit. The sensor is submerged at a sufficient depth below
the surface (around 3 metres) and the pressure reading is 'zeroed'.
The pressure is logged over a two hour period, If the pressure is constant,
the reservoir is 'tight', while a fall would suggest that a leak is present.

There are many reasons for the popularity of pressure transducers, within
the water and waste industry. Pressure transducers are the first choice for
level measurement in boreholes, due to their compact size, and offer the
most reliable solution for applications involving 'foamy' water. This gives them
an advantage over ultrasonic equipment in wastewater applications, where
the conditions can lead to false readings. As transducers are submerged,
they are also ideal for remote sites where vandalism can potentially present
a problem.

Incorrect specification, in some cases, has led to a misconception that all
transducers require regular recalibration -which can present a significant
undertaking for water authorities who may have as many as one hundred
reservoirs to attend. Yet Gems Sensors says that its thin film pressure
transducers offer excellent long-term stability with minimum drift, and require
no recalibration for an impressive 10 million cycles. This makes them suitable
for level measurement in remote reservoirs - even in unmanned locations.

Another consideration, when choosing a solution, is its susceptibility to
outside interference such as lighting strikes. Gems Sensors worked closely
with the industry to overcome this problem. One authority, in particular,
demanded a cost-effective solution that would provide protection against
lightning strikes for its 100 sites - since each reservoir required 1-2 pressure
sensors for level measurement and up to six sensors per site for suction
pressure. The greatest danger to the sensors was presented by indirect
strikes, which have the potential to create power surges from several
kilometres away. In conventional transmitters, these surges can generate
high voltage discharges between adjoining metal surfaces within the
transmitter causing spark erosion to occur.

One possible solution is to install a separate protection unit alongside
equipment such as pressure transmitters. But typically - where pressure
devices are submerged in boreholes or stilling tubes - it is impractical
to fit separate units. This is because of their greater size and complexity
demand that they be located at ground level and connected to pressure
devices by long lengths of potentially exposed cable.

Built-in protection provided a more practical answer and was one of the
reasons that Gems Sensors was chosen to become a preferred supplier
through the signing of a company purchasing agreement (CPA).

Long-term stability and reliability were essential factors, making sputtered
thin film technology the ideal choice for the authority's remote pressure
sensing. A stainless steel sensor coated with a molecularly bonded strain
gauge and connected to a stainless steel diaphragm via a force rod is
used to convert changes in pressure to an electrical signal. This is then
conditioned and amplified by an electronics package within the transmitter
prior to transmission to upstream instrumentation.

The incorporation of a transorb, within the transmitter's circuitry, provides
a multitude of paths through which energy is diverted. The transorb is
connected to the case of the transmitter thereby allowing the potential of
all the components within the Iransmitter to be raise simultaneously, while
eliminating the need to tie the device directly to earth. This is sufficient to
cope with most lightening strikes with protection against transient voltages
of up to 2kV with a rise time of 1.2 seconds and a decay time to half voltage
of 50 microseconds. An earth connection, between the transmitter and
upstream instrumentation, then prevents damage elsewhere within the
circuit loop. The pressure Iransducers also offer protection against EMC
interference, which is especially important if they are used near permanent,
or mobile radio transmitters.

Today, Gems Sensors is responding the water industry's needs with the
introduction of triple sealing. This latest design will be available on all pressure transducers in the Gems Sensors range - whether they are based on thin
film or CVD construction. Fbr enhanced protection, the cable will now be
water blocked and a hermetic header incorporated, so only the vent tube
penetrates the sensor chambet In addition, the electronics will be conformally
coated to protect against possible damage, in the unlikely event of water

As Gems Sensors' Cohn Lussenden points out: "Pressure transducers are
becoming even more reliable, more cost-effective and, with advances such
as thin film technology require the minimum of maintenance. They continue
to be the most cost-effective, versatile, and precise method of liquid level
monitoring in the water and waste industry."


For more information contact:-

GEMS Sensors
Telephone: +44(0) 1256 320244
Fax: +44(0) 1256 473680
Web Site:

March 2002

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