Hydrostatic pressure sensors offer dependable level measurement

LEVEL MEASUREMENT
SENSING A SOLUTION
Hydrostatic pressure sensors offer
dependable level measurement
By Karmjit Sidhu
The treatment of complex industrial wastewater systems
laden with items, such as ferric chloride, ferric sulfate,
hydraulic oil, animal fats and antibiotics, involves brine
treatment and removal of solids and oils, as well as biodegradable
organics. During waste processing, liquid level measurements are
needed at raw and finished water tanks to quantify liquid volume
and overflow controls to prevent pollution. The repeatability and
stability of the readings are very important in order to ensure accurate
measurements for closed-loop control systems.
The Value of Level Sensors
Level sensors are used extensively for the processing of
wastewater in industrial wastewater treatment. Due to the size
of tanks and floatation of oils and waste, hydrostatic pressure
sensors (also called submersible pressure sensors) with venting
to atmosphere are widely used to indicate the water level.
In these applications, the pressure sensor is dropped to
the bottom of the tank and used in the differential pressure
Control Panel
or Telemetry
Device
Figure 1. Hydrostatic Pressure Measurement
in a Tank Using Submersible Pressure Sensors
20 mA
(Full)
12 mA
(½ Full)
4 mA
(Empty)
(dP) mode known as hydrostatic measurement with respect
to atmosphere. Fully submerged in the tank, the dP sensor
measures wet media at the bottom of the tank and the “dry” air
from the atmosphere (outside the tank via a vent tube embedded
in the power and signal cable), providing a linear output as
a function of liquid height.
Hydrostatic sensors offer an alternative to radar and
ultrasonics in a wide range of liquids and environments such as
temperature, dust and icing. Radars and ultrasonic sensors do
not work reliably in shallow plastic chemical tanks and require
modifications to be made to the narrow top opening (typically
2 in.) of the shallow tanks, which compromises the integrity of
the tank. Hydrostatic sensors not only are more reliable, but
they do not require modifications.
All-Metallic Submersible Level Sensor
Depending on the depth of the tank, submersible level sensors
can measure from a few feet to hundreds of feet of water
level and provide a continuous linear output proportional
to the depth. Figure 1 shows a typical pressure sensor sitting
at the bottom of the tank measuring the height of the water
column. The cable, including the copper wires and plastic tube,
provides the power and venting needed for the pressure sensor
to function properly. Without venting, however, the sensor
would be subjected to barometric changes and temperature
gradients within the tank, leading to inaccuracies in depth
measurements. While this type of sensor can be used in
freshwater and diluted brine tanks, it cannot be used in ferric
chloride and chlorine-rich water because it will corrode due to
severe pitting.
The pressure-sensing element, made from solid Teflon
(polytetrafluoroethylene), moves in a linear manner to applied
pressure on the diaphragm. The diaphragm is carefully
designed and fused to the Kynar (polyvinylidene difluoride)
body via a proprietary selective bonding method. The signal
conditioning electronics, housed inside the sensor, amplify
the signal to provide voltage—such as 1-5 VDC, or a 4-20 mA
current output signal—that is linear and proportional to the
applied pressure as a function of depth. The electronics are
protected from reverse polarity, electromagnetic interference,
electrostatic discharge and fast electrical transients. To ensure
full sealing against water intrusion, the electronics are encapsulated
in corrosion-resistant epoxy resin.
Finally, the sensor is tested and calibrated to provide a
thermally stable output from 32°F to 130°F against ambient
temperature conditions. The media temperature can range
from -4°F to 185°F.
20 ■ JAN/FEB 2013 ■ INDUSTRIAL WATER & WASTES DIGEST

Wetted submersible sensor
Severe corrosion in ferric chloride tank application
Non-Metallic Submersible Level Sensor
When injected into the water tanks, some chemicals can
attack metals and cause failure due to severe corrosion. While
the overall pH level can be well balanced, the initial injection
of chemicals can cause the sensor to be exposed to an acidic
or basic shock, depending on the location of the sensor relative
to the chemical additive. To overcome this, there is a strong
need to use submersible sensors constructed from chemical
resistance plastics such as polyvinylidene difluoride and
polytetrafluoroethylene. These two polymers are well known
for their chemical resistance properties and are widely used in
the chemical industry.
A water treatment plant in Arizona found hydrostatic sensors
to be more reliable and cost-effective than the radar and
ultrasonic measurement systems originally used in an application
involving the injection of chemicals in the treatment
plant. The chemicals, stored in a chemical tote, are corrosive
in nature and attach to metals. The plant initially used a costly
non-contact radar level measurement system that provided
unstable readings. It was replaced by an ultrasonic level detection
system that required the top tank opening to be modified
to position the sensor. After modifications, the sensor only
worked for a few days before chemical fumes attached to the
sensor sealing to cause unit failure.
Ultimately, a Kynar-Teflon level sensor utilizing a hydrostatic
measurement principle was implemented and has been
operating reliably after one year of use. The plant is now
standardizing this technology for all wastewater and oil field
injection chemical tanks.
Karmjit Sidhu is vice president, business development,
of American Sensor Technologies. Sidhu can be reached
at kssidhu@astsensors.com.