Continuous Level Sensing

 

Level is a very important parameter that is measured in various industries and across an extremely wide range of applications. In simple terms, level measurement is the measurement of the height of particular media with respect to a reference point or the base of the containing vessel.� Since there are many applications where level measurement is performed, applications could be measuring the level of solids or liquids. Some of the typical applications for liquid level measurement would be lift stations in wastewater treatment systems, bore hole or well level measurement, level of a liquid in a tank, etc. Some of the applications for the solid level measurement would be grain storage silos, dust collectors, pneumatic conveying receivers, and more.

Broadly, there are two types of level measurement, continuous level measurement and point level measurement. In continuous level measurement, the actual level of the media is known at all times from a level transmitter or transducer. Level transmitters most often provide an output of a linear 4 to 20 mA signal proportional to the media height. The level measurement can also be transmitted via a communication protocol to a control system.� Often this continuous level output is used to control a pump or valve that maintains the level or it used to report the level status at all times.�

This article concentrates on continuous level measurement and will discuss the sensing of liquid level using various methods offered by Dwyer Instruments, Inc. to cover a wide range of liquid level applications.� Some of the various techniques for sensing level would be as follows: hydrostatic pressure, ultrasonic, capacitive, and radar.

Hydrostatic Pressure

Hydrostatic pressure monitoring with a differential pressure sensor is generally used for liquid level sensing in tanks or reservoirs. Basically a differential pressure sensor has two pressure inputs, a high pressure port and a low pressure port. In this technique the high pressure port is connected to the bottom or the reference location of the tank and the low pressure port can be left open to atmosphere in case of an open tank. The level can be read as the function of the head pressure with calibration in inches of water column or psi taking into account the specific gravity of the fluid. In the case of a closed or a pressurized tank the low pressure port must be connected to the top of the tank and the differential pressure sensor will give the difference of the total tank pressure and the head pressure of the liquid in the tank.

 

3100.jpgMercoid� Series 3100 Smart Pressure Transmitter is a high accuracy (0.075%) microprocessor-based, high performance transmitter, which has flexible pressure calibration, push button configuration, and is programmable using HART� Communication. The Series 3100 is capable of being configured for differential pressure applications with the zero and span buttons. Additionally, a field calibrator is not required for configuration. 

 

 

629wLED.jpgSeries 629 Differential Pressure Transmitter monitors differential pressure of air and compatible gases and liquids. This design employs dual pressure sensors converting pressure changes into a standard current or voltage signal. Small internal volume and minimum moving parts result in exceptional response and reliability. Terminal block, zero, and span adjustments are easily accessed under the top cover. The Series 629 Differential Pressure Transmitter is designed to meet NEMA 4X (IP66) construction. 

 

Hydrostatic pressure monitoring with a submersible pressure sensor measures the pressure exerted by a fluid at equilibrium, at a given point within the fluid, due to the force of gravity. Hydrostatic pressure increases in proportion to depth measured from the surface because of the increasing weight of fluid exerting downward force from above. In this type of measurement the hydrostatic pressure sensor is submersed into the liquid and measures the height above it.

 

It is to be noted that the hydrostatic properties of liquid are not the same and are very highly dependent on the density of the liquid. Hence the hydrostatic pressure is directly proportional to the density of the liquid. The formula to measure hydrostatic pressure is Height x Density x Gravity.

 

Dwyer Instruments offers four different types of hydrostatic level transmitters fitted with piezoresistive sensing elements. They are Series SBLT, MBLT, PBLT, and FBLT. The SBLT is for level monitoring in clean water applications, the MBLT is a slim version specifically designed for wells and borehole applications, and the PBLT and FBLT are for level monitoring in applications involving sludge and slurry type fluids.

SBLT2-1.jpg

Series SBLT2 and SBLTX Submersible Level Transmitters consist of a piezoresistive sensing element, encased in a 316 SS housing. Bullet nose design protects the diaphragm from damage. The SBLT2 incorporates lightning and surge protection utilizing dual arrestor technology, grounded to case, eliminating both power supply surges and lightning ground strike. The SBLTX is UL approved intrinsically safe for use in hazardous locations when used with proper barrier.

 

 

MBLT small.jpgSeries MBLT Miniature Submersible Level Transmitter is only 0.63" in diameter making it ideal for level monitoring in well and borehole applications. Constructed for years of trouble free service, the MBLT has a welded 316 SS body and 316 SS nose cap. Body top is also 316 SS and tapered to prevent damage or snares when pulling the unit out of the installation. Featured in the MBLT is a precision �0.10% or �0.25% of full scale accuracy output, better than BFSL or BSL rated outputs used by most competitors. Lightning and surge protection is included standard to stand up in harsh applications.

 

PBLT2-1.jpgSeries PBLT2 and PBLTX Submersible Level Transmitters consist of a piezoresistive sensing element, encased in 316 SS housing. This Series is perfect for wastewater and slurry applications with features to protect the unit from these demanding applications. Large diameter 316 SS diaphragm seal is non-clogging and damage resistant to floating solids.� The PBLT2 incorporates lightning and surge protection utilizing dual arrestor technology, grounded to case, eliminating both power supply surges and lightning ground strike transients.� The PBLTX is UL approved intrinsically safe for use in hazardous locations when used with proper barrier.

The Series FBLT Submersible Level Transmitter is designed with a flush diaphragm tip that will not clog in harsh applications such as sewage lift stations. Narrow 316 SS body design allows the FBLT to fit into stilling wells and narrow installations. �The FBLT features a robust FKM fluoroelastomer diaphragm that is PTFE coated for a stick resistant surface that will hold up in aggressive fluids. The diaphragm cavity is filled with a gel that will not leak out versus oil or grease that our competitors use.� The FBLT incorporates lighting and surge protection to stand up in harsh applications.

Ultrasonic

Ultrasonic level sensors are used for non-contact level sensing applications. Ultrasound is sound with a frequency greater than the upper limit of human hearing (20KHz). The ultrasonic transducer transmits ultrasound wave pulses through air directed towards the media of which the level is desired. The ultrasound waves then reflect off of the material as an echo and travel back to the transducer.� The level of the media is determined by the transit time of the ultrasound wave pulse from the sensor to material and for the echo to come back to the sensor.

 

 

ULT-1.jpgSeries ULT Ultrasonic Level Transmitter provides reliable, accurate, non-contact measurement of liquid levels. Ultrasonic non-contact technology offers no moving parts to wear, jam, corrode, or get coated like contact technologies.� Mapping software makes effective measuring surface only a 3" diameter column.� No concerns with ladders, pipes, or other tank intrusions in the remaining sound cone. Unit is FM approved explosion-proof making it ideal for use in hazardous locations. The ULT features easy programming with 6 digit LCD display and simple menu structure.� Output ranges are adjustable with choices of inputting tank dimensions or by filling and emptying the tank while calibrating, which automatically scales to levels it senses.� Window cover allows early viewing of display. Fail-safe output options and diagnostic capabilities make the ULT a good choice for critical applications.

 

Capacitive

 

Capacitance is the property of two or more conductors to store a charge when there is a voltage difference between the conductors.� The material between the conductors affects the capacitance also.� Insulating materials do not allow free movement of electrons, however in an electric field the molecules of these materials will tend to align with the field thus storing energy.� This is called the dielectric effect and these materials are often referred to as dielectrics.� When placed between two conductors the energy storage capability of these dielectrics will allow more charge to be stored on the conductors for a given voltage difference thus increasing the capacitance between the conductors.� The ratio of capacitance change caused by these dielectrics is referred to as the dielectric constant.� Different materials have differing dielectric constants and will consequently change the capacitance between two conductors more or less depending on the value of this constant.

Capacitive level sensors determine the level of material by changes in probe capacitance resulting from the movement of dielectric materials between the probe and the reference ground electrode such as a tank wall. Since measuring very small capacitance changes (less than 1 pF) can be problematic in industrial environments, capacitance level sensing tends to be most effective for materials with a dielectric constant greater than 1.2. Since the difference in capacitance is being measured, it is also possible to detect the level of two immiscible liquids that have different dielectric constants such as oil and water.

The Series CRF2 Capacitive Level Transmitter is a level transmitter providing a two-wire 4 to 20 mA output to indicate level of liquids, powders and bulk materials.� The CRF2 comes with either a rigid or flexible probe depending on application installation need and probe length required. Featured in the CRF2 is easy push button calibration of zero and span. Custom order the CRF2 to any length probe that you need for your application. FEP covered probe is ideal for use with corrosive media. This technology also provides immunity to external RF sources like walkie-talkies and cell phones as well as minimal interference with radio communication or other electronic systems.

 

Radar

 

Guided wave radar uses low energy, high frequency electromagnetic pulses, produced by the sensor, which are transmitted along the probe immersed in the media to be measured.� When these impulses hit the surface of the media, part of the impulse energy is reflected back up the probe to the sensor, which then utilizes the time difference between the impulses sent and the impulses reflected to determine the fluid level.� This is often called Time Domain Reflectometry.

 

The Series GWL Guided Wave Radar Transmitter for Liquids is a level transmitter providing continuous level indication of liquids. The sensor can output level indication as a continuous measurement reading through its 4 to 20 mA analog output, or it can alter that information into freely adjustable (NC) switching output signals. This series is available with either a rigid or flexible probe depending on the application installation required, as well as a custom probe length. One of the GWL characteristics is virtually no installation restrictions making it ideal for small tanks, tall and narrow nozzles, and various other types of processing and storage applications. The guided wave radar transmitter features exceptional performance in liquids with low reflectivity such as oils and hydrocarbons, and factory settings can be configured via HART� Communication protocol.

 

 

References:

http://www.thefreedictionary.com/hydrostatic+pressure

http://en.wikipedia.org/wiki/Level_sensor#Ultrasonic