SENSORS AND MEASUREMENT Fundamentals of hydrostatic level measurement

Enrico Bossart evel measurement has seen considerable change For many years, hydrostatic pressure L over the past decades – from purely mechanical level measurement through to complex electronic measurement has been the most important sensors using various measuring principles. The measuring principle in continuous level ­large number of different technologies for measuring measurement. This trade article presents level (such as hydrostatic, reed chain, magnetoresis- tivity, radar, ultrasound, optical and many more) the fundamentals of hydrostatic level ­today offers the user the possibility to choose the measurement in a practical and under- most suitable sensor technology for his individual application. standable format. In continuous level measurement, hydrostatic pressure (also known as hydrostatic level measure- ment) is the principal sensor technology and ­measuring principle, with a market share of approxi- mately 40 %1 by sales volume. Still considerably ahead of ultrasound and radar technologies, hydro- static sensors for level measurement are installed in more than every second measuring point. Thus, the Author: Enrico Bossart, Dipl.-Ing. (BA) (graduate engineer), immense importance of this technology is reflected Product Manager, Wika Alexander Wiegand SE & Co. KG, in its steady growth from US$ 650 M in the pre-crisis Company Division - Electronic Pressure Measurement year of 2008, to around US$ 770 M in 20131.

What is meant by hydrostatics?

Hydrostatic pressure sensors are used for the mea- surement of level or filling height of a liquid. Hydro- static pressure measurement is suited for level ­measurement due to the hydrostatic effect of non- flowing fluids. This physical principle describes the effect of the weight force of a stationary, meaning non-flowing, liquid on a measuring point. This weight force is usually described as “hydrostatic pressure”. The most important condition for hydrostatic level measurement is what is called the “hydrostatic para- dox.” This means that, regardless of the shape and volume of a vessel, the hydrostatic pressure at the measuring point of a tank or vessel is proportional only to the filling height (Fig. 1). Thus, despite the apparent contradiction of an over-proportional ­increase or decrease in the volume or weight of a ­liquid with change in height, the hydrostatic pressure at the measuring point is solely proportional to the absolute filling height, and not to the filling quantity. SENSORS AND MEASUREMENT

01 Illustration of the hydrostatic paradox

A static liquid generates, through its specific density acts on the medium as an additional “force” always and the acting force of gravity, a corresponding resembles the ambient pressure acting on the whole weight force or hydrostatic pressure which increases system, including the level sensor. If one therefore proportionally with the filling height. Consequently, ­uses a pressure sensor with a relative pressure the hydrostatic pressure represents a direct measure ­measuring cell, a pressure sensor that is compensated of the fill level or degree of filling of a tank or vessel. or vented (just like the tank) to the ambient pressure, Since, the hydrostatic pressure is not dependent it “automatically” compensates for the effect of this upon the volume or the shape of a vessel, it can be ambient pressure on the level measurement. This used directly for the measurement of the fill level, means that a relative pressure sensor in vented but must be further processed to “measure” the fill ­vessels and tanks completely “cancels out” the atmo- quantity. From the measured level, using a ‚tank spheric pressure overlying on the liquid from the ­linearisation table‘, the actual fill quantity present in ­level measurement. Thus, the hydrostatic pressure cor- the vessel can be calculated. A tank linearisation responds only to the filling height of the liquid Fig.( 2). ­table documents, for different pressure values, the Therefore, the filling height of an open tank or respective fill quantity present in the vessel. From ­vessel is calculated using the following equation: the values documented, a curve can then be derived h = p / (ρ * g) which presents the user a corresponding fill volume p = hydrostatic pressure [bar (relative)] for every hydrostatic pressure measured. This calcu- ρ = density of the liquid [kg/m³] lation is usually carried out in the PLC, so that the g = gravitational force or grav. acceleration [m/s²] operator has a display of the actual fill quantity h = height of the liquid column [m] ­present in the vessel, directly on-screen. In order to determine the fill quantity from the As a simple rule of thumb for water as a medium, one ­hydrostatic pressure with a high accuracy, the pres- can assume that a pressure of 1 bar (relative) corre- sure sensor should be positioned, ideally, at the sponds to a filling height of 10 m. height of the required zero measurement. Based on this measuring point, the sensor will measure the Rule of thumb (media of density ~ 1,000 kg/m³ ): ­ ­hydrostatic pressure as a direct scale of the distance h = 1 bar (relative) / (1,000 kg/m³ * ~ 10 m/s²) = 10 m from the selected measuring point to the medium‘s surface. The measured hydrostatic pressure of a The rule of thumb demonstrates that the hydrostatic ­liquid, in addition to the weight force of the medium, pressure in a vessel is proportional to the filling height also consists of the ambient pressure acting directly of the medium, as long as it maintains a constant den- on the liquid‘s surface. The overlying pressure of the sity, completely independent from shape/fill quantity. ambient air or gas pressure can be considered as a force which acts on the pressure sensor in addition to Level measurement in sealed, the liquid‘s weight force as a hydrostatic pressure. gas-tightvessels

Level measurement in open, ­ The level measurement in sealed, gas-tight vessels, vented vessels which is frequently found in the chemical industry, requires a compensation of the pressure of the In hydrostatic level measurement in open or vented ­enclosed gas phase above the liquid. The enclosed basins or vessels, a continuous pressure compensa- pressure of the gas phase acts as an additional force tion of the ambient air with the gas phase above the on the liquid and distorts any hydrostatic pressure liquid takes place. Thus the ambient pressure that measurement at the base of the vessel. Thus, this

AUTOMATION TECHNOLOGIES 1/2015 SENSORS AND MEASUREMENT

the need for special requirements on the measure- ment technology (such as scalability of the measur- ing range or integrated tank linearisation). Conven- tional sensors stand out through their excellent ­distorting influence must be compensated through price/performance ratio. They are robust, simple to an additional pressure measurement of the gas install and operate, fast, and available with a range of phase. Frequently, a second pressure sensor is used accuracies up to < 0.1 %. Process pressure transmit- for the measurement of the gas pressure. This appli- ters, however, are primarily used in applications with cation effectively represents a differential pressure special ­demands on the measurement technology measurement, where the two separate pressure (such as bus signals, scalability of the measurement ­measurements are offset against each other Fig.( 3). range, integrated­ tank linearisation, etc.) and are This offset calculation can be made either by two mainly found in applications in the chemical and ­individual sensors or via an integrated differential petrochemical industries. Theextensive ­ adjustability pressure sensor. In this application, the sensors used and high intelligence of these programmable process can either be relative (sensor with ambient pressure pressure transmitters is also reflected in their price compensation) or absolute pressure variants (sensor level in relation to conventional industrial transmit- with sealed vacuum reference). ters, easily five to ten times as expensive as a conven- Thus, the filling height of a sealed tank or vessel is tional pressure sensor. calculated using the following equation: Specifically in the water and wastewater industry, submersible pressure transmitters (‘level probes’ or h = (p2 - p1) / (ρ * g) ‘submersible probes’) are frequently used to ­measure p2 = hydrostatic pressure [bar] the level in reservoirs, wells or other open bodies of p1 = pressure of the enclosed gas in the vessel [bar] water. Submersible pressure transmitters are specifi- ρ = density of the fluid [kg/m³] cally designed to operate while continuously sub- g = gravitational force or grav. acceleration [m/s²] merged in liquids. They mainly differ from h = height of the liquid column [m] ­conventional pressure sensors in their media resis- tance, pressure tightness, cable quality and ingress Types of hydrostatic level sensors protection. The design of differential pressure transmitters is In hydrostatic level measurement, one can differen- considered as state of the art technology within the tiate three types or designs of level sensors: conven- chemical and petrochemical industry. Differential tional pressure transmitters, process pressure trans- pressure transmitters offer the ability to measure and mitters and submersible pressure transmitters, eliminate the effect of the gas phase in a gas-tight ­available in relative, absolute and differential sealed tank, and correctly display the level of the liq- ­pressure variants. For application in tanks and uid phase within the tank. The operator therefore has ­free-standing vessels, conventional pressure trans- a hydrostatic level measurement which displays the mitters or process pressure correct liquid height without the need of any addi- transmitters are particu- tional compensation or additional sensors. The so- larly suitable, either with phistication of this measurement technique is how- a conventional pres- ever reflected in the costs of both the instrument it- sure port or a flush self as well as the corresponding installation.­ diaphragm­ design.­ Conventional pres- Advantages and limitations of sure transmitters are hydrostatic level measurement the most commonly ­employed, due to their Hydrostatic pressure and level measurement enjoys wide use throughout vari- a consistently high popularity due to it’s high robust- ous industries, mostly without ness, high reliability and simple installation of this technology. The following characteristics constitute Wika UPT-20 process pressure the greatest advantages and limitations over other transmitter measuring principles:

AUTOMATION TECHNOLOGIES 1/2015 SENSORS AND MEASUREMENT

Advantages: n Proven and established measuring principle with high reliability, field-tested millions of times n Robust measuring process, uninfluenced by ­disruptive factors such as dust, foam, vapour, buildup, contaminants, etc. n Reliable measurement unaffected by many ­physical characteristics such as conductivity, 02 Level measurement in open, vented tanks ­ ­dielectric coefficient or viscosity and vessels n Level measurement unaffected by vessel geometry and existing installed equipment n Simple installation and operation of submersible ­reading of the level. Therefore, hydrostatic level pressure transmitters and conventional pressure ­measurement is primarily used in applications sensors without the need for calibration or which operate within known process limits or with a adjustment­ known medium density. Should the process have a n Direct contact with the medium strongly changing or an unknown density, this would n Numerous alternative design variations and normally be compensated for through additional ­sensor technologies for almost every application sensors. Therefore, various pressure sensors with Limitations: ­additional, integrated temperature sensors, are avail- n Unsuitable for bulk material able which enable the measurement of the medium n Accurate measurement requires either media temperature for density compensation. with constant density or continuous density measurement of the medium Viscosity and solids content are ­ deciding factors Hydrostatic level measurement ­ in practice The medium and its characteristics, in particular its viscosity and solids content, will decide between The selection of a level sensor is often affected by a ­using a pressure sensor with a traditional design with great uncertainty with respect to how suitable a pressure port or one with a flush diaphragm. ­technology is for the specific application. The great A pressure sensor with a pressure port should al- popularity of hydrostatic sensors lies in their simple ways be used when the medium has a low viscosity application, a low susceptibility to problems from and is free from coarse-grained contaminants. How- ­installation through to their continuous operation, ever, if a medium has a tendency to build-up, is high- their high tolerance to disturbances and the suitabil- ly viscous or contains a lot of particulates, then one ity of the technology for almost all application condi- should select a sensor with a flush diaphragm . In tions. Nevertheless, it is also important here to avoid contrast to a flush diaphragm sensor, a sensor with a a few pitfalls in order to use this measuring method pressure port can become blocked or the media in it effectively and safely for level measurement. can harden or crystallise. Any blockage in the pres- The influence of temperature, especially its influ- sure port slows down the measurement, or, in the ence on the density of the specific medium, must most extreme case, ­completely prevents a correct ­always be incorporated into the calculation of the pressure measurement. If one pays attention to the level for a correct level measurement. An increase in characteristics of the medium to be measured when the process temperature may lead to a lower density choosing a conventional pressure transmitter, then of the medium and a correspondingly increasing hydrostatic pressure measurement can be used reli- ­level, however, not always in the same proportion as ably even ­under the harshest of conditions. Sub- the increase in the hydrostatic pressure. This leads to mersible pressure transmitters, as a specific design an inaccuracy in the calculation, e.g. to a lower variant of a pressure transmitter, are used in contam-

AUTOMATION TECHNOLOGIES 1/2015 SENSORS AND MEASUREMENT

inated media, such as wastewater, as well as in clean Trend 1: alternative materials media, such as fuel or groundwater. For this, both flush product variants as well as variants with large, In recent years one has been able to observe the ­widened pressure ports are used, in order to ensure a trend for hydrostatic level measurement technology high reliability of level measurement in the to replace alternative measurement principles, and submersed­ application. to be used more and more in many process measure- In the differential pressure measurement with ment applications. Thus, a clear trend has been ­process transmitters the mounting position is a ­recognisable towards the measurement of aggressive common source of inaccuracies in the level media (e.g. acids and bases) which go beyond the ­measurement. The measuring points of the medi- usual application conditions for the operation of um and the gas phase are typically connected to conventional pressure sensors in machine building. the differential measuring cell of the transmitter by These media, often present in process industries, are oil-filled capillaries. The height difference of these being met by pressure sensor manufacturers with a measuring points to the differential pressure trans- clear adjustment in their product design. Thus, more mitter lead to an additional hydrostatic pressure and more, one finds the use of alternative materials within the capillaries themselves. This effect gener- and coatings for conventional pressure sensors and ates an additional over- or under-pressure in the their wetted parts. Titanium, gold, ceramic, Teflon hydrostatic pressure measurement, ultimately and many other materials are already available on ­distorting the measurement. The resulting inaccu- the market and are common material options for racy of the measurement must be corrected within pressure sensors - and their importance will increase ­installation by a position correction and proper in the future. configuration of the differential pressure transmit- ter, so that a fully automatic compensation of this Trend 2: hygienic design disturbance factor can be made. It is further on strongly recommended to ­position the transmitter The keyword “hygienic design”, originally derived ­below the height of the level measurement point in from the pharmaceutical and food and beverage ­order to eliminate any negative hydrostatic ­industries, is also finding ever-increasing appeal in ­pressure or under-pressure from the level mea- the chemical industry. The specific needs of these surement. ­industries – such as a maximum possible purity of the measured production batch, an optimised What is the cleanability of all wetted parts of the process instru- future of hydrostatics mentation or high media and ambient temperatures – have been implemented in a number of specialised Through the wide spread utilization of industrial hydrostatic pressure sensors. The benefits of hygienic pressure sensors and due to the annual manufac- design originate from the growing requirements in turing of millions of hydrostatic sensors, hydrostatic the manufacture of chemical and petrochemical pressure sensors have achieved a significant price products especially regarding purity and quality. advantage over many other level measurement Smaller batch sizes, faster and more frequent batch methods. The utilization of pressure sensors for lev- changes, consequently a higher flexibility in pro- el measurement will continue to grow above all in duction – without the risk of cross-contamination general applications without any special require- through product residues from previous batches – is ments on the measurement technology. Conse- provided by the hygienic design of all wetted parts. quently, in the future, hydrostatic level measure- As a result of these benefits, hydrostatic level ment will ­continue to show a growing market share ­sensors with hygienic design are now also finding­ against many other level measurement principles, a more ­frequent use in many conventional appli- and will enable cost-effective level measurement in cations for process instrumentation in the chemical many new applications. industry.

AUTOMATION TECHNOLOGIES 1/2015 SENSORS AND MEASUREMENT

Trend 3: more intelligent pressure sensors

Hydrostatic pressure sensors have previously often been used in simple applications with a high sensi- tivity to price. Due to a continuing rise in complexity of control technology and process control, and also through the substitution of alternative measurement principles by hydrostatic level measurement, the 03 Level measurement in sealed, gas-tight tanks ­demands on hydrostatic pressure sensors are and vessels ­increasing further. Requirements such as digital communication, programmability or internal tank linearisation have already been implemented in pro- Summary cess pressure transmitters. These requirements are to be expected in the future in supposedly simpler Hydrostatic level measurement technology has, in and lower-priced industrial transmitters. Leading recent years, gained an exceptional market impor- suppliers of industrial pressure sensors have already tance as a result of its simple and easily understood responded through new, special models. It is there- usage. Through its high durability and its tolerance to fore to be expected that, in the future, users of a large number of disturbances and unsteady process­ ­hydrostatic level sensors will find a considerably conditions, this technology will, for the foreseeable ­increasing number of conventional industrial trans- future, maintain its status as the most important mitters on the market with extensive configuration ­sensor technology principle for level measurement. possibilities. New applications and the substitution of alternative measurement principles will promote a further Multiple measurands growth in the distribution of hydrostatic pressure sensors. In recent years, the manufacturers of indus- In the measurement of chemical parameters, such trial pressure sensors have, through alternative as chloride, oxygen and nitrogen content, measur- ­materials and an ever-growing complexity of pres- ing systems for the measurement of multiple sure sensors, shaped hydrostatic level measurement ­measurands are a tested, accepted technology. In technology to suit the requirements of future needs. the measurement of physical parameters, on the literature: 1VDC Research Group, Inc.: What’s driving the process other hand, previously one has focused on the level measurement & inventory tank gauging markets, 2011 ­measurement of a single measurand through a ­corresponding sensor, e. g. a temperature sensor for www.wika.com/hydrostatic-level temperature measurement. In recent years, howev- er, the demand for combined instrument solutions for physical measurands has been seen more and more. The combination of pressure and tempera- ture measurement within a single pressure sensor can already be found frequently, primarily to mini- mise the number of instrumented measuring About points. The trend towards combined sensor princi- Company name: Wika Alexander Wiegand SE & Co. KG ples will, in the following years, lead to an increas- Headquarter: Klingenberg, Germany ing number of available pressure sensors on the Turnover: € 800 Mio. (Wika Group) market, and a growing market relevance for the Employees: 8,500 (Wika Group) combination of hydrostatic pressure respectively Products: pressure, temperature and level measurement technology level and temperature measurement.

AUTOMATION TECHNOLOGIES 1/2015