By: Marc
R. Cartier
This paper is intended to
introduce you to the technologies and installation techniques associated with
level measurement of fluids. It is
further focused on water based, non-flammable, fluid measurement. It defines currently available technology
and points out current advances being made that may affect the future of level
sensing. The material should help
simplify the complex task of selecting the best available technology. Knowing the
best level sensor to use for an application requires careful understanding of
the process involved; the measurement principles best applied; the control
scheme to be employed; and project constraints of time and money.
Introduction:
Selecting the best level device for a given application involves
reviewing the application without missing a single detail. This is an attempt to help you remember the
littlest detail and avoid the proverbial “gotcha.” Similar to pressure and temperature, level sensing usually
demands reliable sensing technology due to the aspect of safety. Redundancy in any safety system is vital in
order to maintain confidence in the system, and critical level systems often
employ multiple sensors and alarms.
Knowing the technologies, which enable level sensing, their strengths,
weaknesses, and relative cost can help you optimize your sensor selection.
Point level sensors are designed to trip at a
single setpoint. Often multiple point
level sensors are used to control the filling of a process vessel. Some sensors enable multiple point sensing
built into a single shaft for simplified insertion. When deciding whether or not to use a single point level sensor,
or many, you will have to factor in the cost of insertion, testing, replacement
and for safety systems the Safety Integrity Level (SIL) provided by the
sensor(s) chosen.
By far the most commonly employed sensor due to its simplicity and low cost are
float level sensors, which operate on the principle of buoyancy, “If it
floats, it works.” Contained within the
float is a magnet or a conductive fluid that triggers a detection circuit
within the sensor body at the prescribed level. Switch differentials for inexpensive floats are typically from
one-eighth to one-half inch. Some
floats are hinged allowing a 180-degree reversal in installation for either
rising or falling level alarming. In
order to overcome the common fault of restricted movement in dirty liquids, the
float sensor has been modified for use with a tethered cable. Switches within the float assembly are
triggered by a conductive fluid, which shorts internal contacts within the
float at a predetermined slant. The
common float is designed for use with water, however special floats are
available with specific gravities allowing use in lighter or heavier fluids.
Tuning Fork sensors operate by sensing the change in
resonance, which occurs when the fork tines are immersed. This vibrating sensor can be used in liquids
with foam, bubbles, and non-coating solids, which would interfere with the
movement, required of float sensors.
The simple electrical circuitry required of tuning fork sensors allows
for moderate price points. The accuracy
of tuning forks is usually rated to +/- 1 mm.
Their ability to operate in viscous, non-coating fluids is a primary
advantage.
Conductivity sensors use the fluids conductance to
complete an electrical circuit.
Alternating current is used to prevent electrolytic action from
occurring. Sensors can be either isolated (Non earth ground referenced) or
non-isolated. This usually will have a
direct impact on the cost and applicability of the sensor. Electrical components such as pump motors or
mixers can have leakage currents affecting conductivity sensors. Various electrode and insulator designs
exist to accommodate various installations.
Care must be taken in the electrode design to minimize the formation of
a meniscus when fluids are retreating from the electrode since this may delay
the reset of the circuit. Oils, non-conductive or coating materials should be
prevented from contacting the electrodes.
Again, due to simple electrical circuitry, costs for this sensor type
are moderate. Multiple sensing points
require separate detector circuits for each electrode pair.
Electro-Optic sensors rely on the refractive index of
fluids to disperse light away from the sensors angled lens. An internal light sensitive transistor
detects the strength of the reflected light generated by an LED. The lenses can be made of any light
conductive material such as Polysulphone, Polypropylene, PFA, Isoplast, or
glass. Fluids, which can coat the lens,
or are highly reflective, can interfere with the proper operation of the
sensor. The costs for optic sensors are
low, just slightly higher than float sensors.
Pressure sensors have been used for many years to
sense the level in a vessel. The fluid
“head” can be detected either through absolute (referenced to zero pressure),
gauge (referenced to barometric pressure) or differential which can determine
the difference between two gauges. The
most used is gauge pressure with differential sensors being used nearly as
often. Gauge pressure switches are
readily available and at prices that are very economical. Pressure switches are generally capable of
detecting levels greater than a couple inches to an accuracy of a few tenths of
an inch. Many have small orifices,
which couple the fluid pressure to the internal strain gauge. Fluids, which can clog small orifices,
should be avoided. Freezing of the
solution can create immense pressures capable of destroying the sensors or
piping system components so in some locations winterizing of the electronics
and impulse lines are required.
Ultrasonic sensors operate by allowing the solution
to transmit sound from one crystal to another across a small gap when
immersed. This technology allows use in
fluids of various viscosities, and is also capable of withstanding higher
pressures and temperatures. Typical
accuracy is stated at +/- 1mm. Ultrasonic sensors offer the greatest general
use as they are only affected by medium or heavy coatings, which interfere with
the transmission of sound between the crystal elements.
Continuous level sensors allow for sampling of the entire
range of fluid motion. They can be
either extremely simple sight gauges or clear tubes with graduations to very
complex radar devices capable of sensing through vessel walls and discerning
the difference between fluid, foam or vapor interfaces. The advantage of continuous level sensors is
the greater degree of control and recording capability provided over the entire
process range.
The simplest continuous level sensors, sight gauges,
have been in use for many years throughout various industries. Certain gauges can be fitted with magnetic
floats allowing for remote signal transmission and control. This adaptation can add considerable complexity
to an otherwise simple visual indicator.
Sight gauges can be very inexpensive for small volume vessels. Larger vessels often use gauges fitted with
colored flags, which flip when passed by a floating magnet. This provides a quick visual reference and
eliminates parallax error. Accuracy of
these indicators is usually just under ½ inch.
Tape measure sensors are usually utilized for drum level sensing. A tape measure fitted to a float rises and
falls with the level. A distance of at
least the drum height must be clear above the sensor to prevent restriction of
travel. Parallax error is avoided in
this technique allowing for visual indication to 1/16 inch or 1 mm.
Continuous float level sensors are often equipped with either reed switch
arrays, resistor networks, or magnetostrictive wire which allows measure of
level to an accuracy of up to +/- 1mm. Costs will vary based on the sensor
material, resolution, and lengths required.
According to various market studies, differential
pressure sensors account for much of the revenue generated in industrial
level sensors. They are a traditional
sensing technology for larger vessels.
Simpler to use and install, gauge pressure sensors are suitable for a
broad range of tank level applications at a substantially reduced cost.
Within the arena of available sensing technologies, ultrasonic
level sensors are the fastest growing sensor type. They are mounted above the process vessel
and detect the surface using ultrasonic pulses. Since they are separated by at least several inches from the
process, they are compatible with almost any process. The accuracy can be affected by dust, vapors or metal
(reflective) objects within the sensors “beam angle” and range so care must be
taken in installation to eliminate “false echoes.”
In some instances, the sensors are installed atop a tube to
prevent interference and minimize surface commotion; particularly if the vessel
in question is filled from the top.
Among the main advantages are the ability to detect level regardless of
the fluid composition and the simple (in most cases) installation.
Capacitance sensors rely on the different dielectric
properties of air and the media to detect level. With conductive liquids the tank wall and an insulated active
electrode form a capacitor. As the
level changes, the dielectric property of this “capacitor” will change
linearly. Non-conductive liquids can
utilize a bare metal rod or cable as the electrode. Various length sensors can be produced up to several hundred feet
with little overall affect on the cost of the sensor. Bare cable electrodes should never come in contact with
conductive fluids or the tank wall or a short circuit of the sensor will
occur. Advanced measuring techniques
using insulated concentric sensors allow for interface level measurement of
liquids which contain differing water, oil, or foam levels. Tank grounding for capacitance sensors must
be done properly for the system to operate.
Knowledge of the tank materials and grounding is vital in specifying the
best sensor configuration.
Radar level sensing is at the top of readily
available technologies for level and offers some very unique installation
advantages, at a premium price however.
Radar sensors use low power microwave radiation to detect the surface of
the liquid. The radar has the advantage
of being able to pass through materials such as dust, foam, or even glass
allowing for level detection in extreme conditions or through the sight glass
of a completely sealed vessel. This
sensor has advantages suited to the needs of special applications encountered
in food, beverage, or pharmaceutical production.
With a basic understanding of the technology available the task of selecting the proper technology becomes simpler. Critical to selecting the proper level sensor will be an understanding of the mechanical and electrical configuration of the vessel in question. You must be able to answer the following questions:
Does the sensor come in the required length or range to suit
the application?
Can the sensor be simply installed, or must unique mounting hardware be
specified?
Are there any internal devices within the vessel, or
conditions such as spraying or splashing liquids which will interfere with the
sensor chosen? If yes, can they be
easily and affordably overcome and still allow a low-cost sensor solution?
How critical is the accuracy of the sensor in the
application? Can a more accurate sensor
reduce inventory to the point where payback is achieved in a few months?
How important is safety in the application?
Does the sensor need a specific agency approval for use?
Does the sensor provide the needed electrical interface
for present and future plans of operation?
Will the electrical installation require additional enclosures to house
intrinsic safety barriers, interposing control relays, etc.?
Most level sensors are designed for simple installation into
standard pipe fittings, with tapered pipe threads being the norm. Questions regarding installation are driven
mainly by the mechanical design of each sensor. Even though a float level sensor may have a three-quarter inch
process connection the float may require a two inch opening; requiring a two
inch to three-quarter inch adapter.
Long float assemblies may not have sufficient headroom above the vessel
making installation problematic or impossible.
Don’t assume a sensor will fit without a thorough review of all the
mechanical dimensions.
Ultrasonic sensors require a clear beam field to prevent
false signal returns. Ensure that the
distance from side-wall obstructions or fill points are adequate. In worse case conditions, installation
within a vertical column of piping is done to reduce surface turbulence and
shield the beam from splashing liquid.
Would your particular installation allow pipe insertion?
Capacitance sensors may require additional reference
electrode installation if the vessel is lined or made of plastic which provides
an adequate ground reference for sensing.
If this is necessary does it require problematic adaptation of the
vessel lid?
If simple float switches are used, can they be horizontally
mounted through the wall or is vertical installation with a standpipe
preferred? Does the vessel cover need a
self-aligning bulkhead fitting to keep the sensor vertical? Reviewing the sensor manufacturers installation
instructions before specifying the sensor ensures you know you’ve selected the
proper sensor.
Is the process level stable or does it vary considerably due
to filling or emptying? A level sensing
system will generally only be as accurate as the condition of the fluid being
measured. Sloshing and splashing fluids
will require rather heavy averaging which may reduce system response time. Knowledge of fill rates and process
conditions help to determine the proper technology to apply.
With the advances being made in microprocessors, and low power integrated circuits, low power sensors and the application of wireless communication devices will rapidly gain broad acceptance.
Advantages are obtained in the reduction of wire installation costs, particularly at facilities using many vessels over a wide area. Current advances in consumer product display technology and processing power will ripple through to industrial products, providing enhanced performance from sensors, controllers and display devices. Just-In-Time (JIT) inventory efficiencies seen in electronic component distribution will be more common in liquid inventory handling systems saving millions through more efficient chemical use and delivery. Finally, advances in low power, high frequency circuits and DSP (Digital Signal Processors) will allow less costly advanced technologies such as radar, laser or nuclear measurement. MEMS (Micro-Electrical Mechanical Systems) technology advances may allow for optic and acoustic sensors capable of performing tasks as yet unknown, having a profound impact on sensor technology for level, and many other process variables.