ing is one of the most common functions th st co fu tio

for process control, but it also uses a wide range of different technologies to achieve the sam goal therefore i portant to sel ct the correct design for each appl ati n to ensure a rel abl and accurate system GregW

hous , technical sales and

sensors expert at Bür er UK offers a n explanati on some of the various designs available

for process con rol, but it also uses a w de ran e of different technologies to achieve the same goal; it is therefore important to select the correct design for each application to ensure a reliable and accurate system. Greg Wainhouse, technical sales and sensors expert at Bürkert UK, offers an explanation on som of the various designs avai bl


he top 10 level sensing technologies break down like this. The first

distinction to make is the purpose of the level signal – is it required to provide a point level such as an alarm or a

continuous signal for level monitoring? 1. Float – One of themost common

themovement of the floa can either be internal and solution. The actual switc simple in design the float forms of point level detec

hingmechanism offers a reliable tion. Although

t as the level activated by

rises, or external where amagnet

switches a reed contact as the position of the float is moved by the liquid level. 2. Tuning fork – This sensor is set to vibrate at its mechanical resonance frequency and when it becomes

submerged in either liquid or solid th e frequency changes. This change is detected and converted into a switching command by the on-board electronics. The discrete sensors discussed so far are often used in conjunction with continuous level detection systems, acting as alarm points for over-filling (high-high) and leak or dry pumping (low-low).


3. Ultrasonic – The transducers in these devices emit ultrasonic pulses that are reflected off the surface of the liqui d - the time taken is proportional to the distance between the sensor and the liquid. This time is also affected by the temperature of the atmosphere above the media, the soundwave has to travel through and a compensation factor is used by the on-board electronics to derive the precise level within the vessel. In applications involving changing atmospheric gases, foaming liquids or obj

bjects in the path of the ultrasonic pulse reduced reliability an d repeatability may be experienced.


4. Radar – In principle radar operates in a similar way to ultrasonic, but the pulses travel at the speed of light and again; the reliability and repeatability can be affected - but this time by the

dielectric constant of the fluid. However, 28 28 NOVEMBER 201 MBER 2016 | AU AUTOMA MAT ATION

SelectSelecting the mosting the most equires a number of

considered. Fortunately,tunat help is at hand from the experienced enginee

variables to be considered. F

help is at hand from experienced engineersrs at Bürkerat Bürkert

the ely,

approprappropriate level sensoriate level sensor rrequires a number of variables to be

radar can provide very accurate level data and also compensate for fixed structures within the vessel. The downside can be that the initial cost of the sensor is

relatively high, but several manufacturers are making this technology more accessible to the wider market.

5. Guidedmicrowave – Also known as guided radar, overcomesmany of the initial calibration issues associated with conventional radar sensing by using a cable or rod to guide the signal to the fluid surface and back to the transmitter. This designmakes it a lotmore reliable than through-air radar or ultrasonic, because the guide provides amore focussed signal path. However, the sensors are limited to the length of cable or rod but with

solutions up to 75mthere is a wide array of probes to suitmany applic ations. 6. Laser – This technology is a flexible, easy to set up and cost effective solution for non-contact continuous level

detection of bulk solids and opaque fluids. However, being based on light reflection, it is less well suited to clear liquids, dust and vapour contaminated atmospheres and applications affected by foam.

7. Hydrostatics – Hydrostatic pressure

solutions on the market for l i is one the most popular level

quids. Used sensing

in vessels open to atmosphere, they are unaffected by changes in media

dielectric, atmospheric dust and vapours. However, as they are mounted on the base or underside of the vessel and measure head-pressure via a ceramic or thin-film stainless steel element, the reliability of measurement technique for level will be greatly affected if the media density changes.

8. Differential pressure – Conventional hydrostatic sensors are confined to applications where the storage vessel is open to atmospheric pressure. For situations where a pressurised tank is in use, it is also necessary tomeasure the atmospheric pressure above themedia using a second sensor. The two pressure signals are then evaluated to give the

actual hydrostatic pressure of the liquid and the corresponding level. However, this arrangement is subj

bject to errors due to

changes in density and temperature, but these can only bemitigated bycompensating factors, such as additional sensors.

9. Capacitance – This relatively common approach can be used with solids, liquids and mixed materials which act as a capacitor. The sensor needs to be calibrated to the specific material to account for varying dielectric constants and differences in vessel design. More recent developments have produced a contact-free measurement systemthat has widened its application. However, the accuracy of a capacitance sensor depends on a homogeneous dielectric constant throughout the stor age vessel Th e reliability of these sensors can also be heavily influenced by media sticking to the sensitive probe.


10. Load cell – The use of strain gauges provides a non-contact measurement systemthat is built into the support structure of the storage vessel. However, this means that the design and

construction of the vessel must account for the use of load cells in order to deliver accurate level meas urements. In addition , changes to product density will also greatly affect the reliability of the measurement in the Selecting the most sensor requires a nu

mber of variables to appropriate level vessel.

be considered, especially increasingly stringent regulations requiring more precise and reliable level measurement. Ultimately, the number of different technologies availab engineers canmake .


the selection process le to process

quite daunting Fort unately help is a t hand fromthe experienced engineers at Bürkert which have considerable expertise in designing and installing process control systems for a wide variety of applications.

Bürkert Fluid Contr T: 01285 648720

T: 01285 648720 ol Systems www urkert. AT /AUTOMATION /AUTOMATION

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