How can you optimize processes with continuous level measurement?
How much is in there: for optimum production processes and maximum resource efficiency, precise, continuous control of level, volume or mass in storage or process tanks is essential. We have the most suitable level measurement technology for every medium, every tank type, and every application, whether hydrostatic, potentiometric, via weight measurement, radar or by differential pressure.
Many factors influence the choice of technology:
- Type, size, shape, and material of the container
- Fluid media or solid particles such as granules, powder…
- Material properties (liquid, pasty, adhesive, foaming, conductive / non-conductive…)
- Required measurement accuracy
- Media temperature and density and their dynamic variation
- Changing media or always the same medium
- Location (outside, inside) and climatic influences on the measurement technology
- Pressurized environment (atmospheric pressure or pressurized tank)
What is hydrostatic level measurement?
The hydrostatic pressure is the pressure inside a liquid and always acts vertically towards all limiting walls of the container. As the level in such a vessel rises, so does the pressure. A sensor (transmitter) at the bottom of the vessel, can measure, display and output this pressure variations to the PLC. Since the pressure acts on all sides, the sensor diaphragm can be mounted at the bottom of the vessel or laterally at the bottom edge of the vessel, depending on which installation situation is more suitable.
To transmit the measurement results to the PLC, pressure transmitters use a piezoelectric signal converter internally, which converts the mechanical process pressure from the pressure diaphragm into a proportional voltage signal. This is then converted into a 4…20 mA standard signal or other protocol according to the customer’s adjustment.
Modern measuring systems, such as the L3, already offer the possibility of converting the measured pressure values in the sensor electronics and thus directly outputting volume or mass. For this purpose, further parameters must be determined, such as the container shape, the medium, and the process temperature (for the calculation of the respective specific density). In the case of the L3, the integrated temperature compensation provides a higher accuracy over the entire process temperature range than conventional hydrostatic level transmitters. This enables the display of the sensor in gallons, pounds, PSI or other volume or pressure units with a very high measuring accuracy, even with dynamic temperature curves.
In an open system (vessel with atmospheric pressure), a pressure sensor at the bottom of the vessel is sufficient, since the external pressure conditions do not change.
A closed system (pressurized vessel), on the other hand, can be subjected to varying pressures, which affects the pressure at the bottom of the vessel. To measure the level in such a system, two sensors are required which separately determine the process pressure at the bottom and the head pressure at the top. The differential pressure can then be calculated from this in the PLC or an evaluation unit, and thus the correct fill level displayed.
How does a hydrostatic pressure transmitter work?
The pressure sensor is installed in the vessel wall with the pressure diaphragm perpendicular to the vessel contents. The process or level pressure deforms the diaphragm. This deformation is transmitted by a capillary fluid to a measuring cell with a piezoelectric signal converter, which converts the process pressure into a corresponding voltage signal. The electronics in the sensor head convert this in turn into the industry standard used, such as analog 4…20 mA or HART 7.0, according to the customer’s adjustment.
This allows the hydrostatic pressure to be output as an electrical signal to the PLC.
How do relative and absolute pressure gauges differ?
In relative pressure sensing elements, the back of the diaphragm is vented, meaning the transducer measures process pressure relative to atmospheric pressure.
In absolute pressure cells, the vacuum created during the manufacturing process remains between the diaphragm and the base body, i.e. the sensor measures the pressure relative to the vacuum.
Since the atmospheric pressure can change, e.g. due to meteorological influences, the measuring accuracy is generally higher for absolute measuring cells.
What is temperature compensation for pressure transmitters?
The L3 pressure, level and flow sensor has been specially designed for measuring
liquids in the food and beverage industry, where a high accuracy under dynamically changing temperature conditions is crucial for process control. This sensor uses a piezoelectric signal converter and in addition an integrated temperature sensor to measure the pressure and temperature of the internal capillary fluid. The mV signal of the signal converter and the resistance of the temperature sensor are converted to an adjusted pressure value by the signal electronics in the sensor nozzle.
This temperature compensation avoids measurement errors that are caused, for example, by the temperature effect or temperature drift: with changing temperatures, the specific density of a medium also changes, among other things. If this density is calculated for the level output at 20°C, but the process temperature is 80°C, then the measured value output is incorrect.
Conventional sensors show a temperature drift of up to 0.4% per 10°C. At 110 °C it is over 2.5%! The L3, on the other hand, shows a temperature drift of 0.03% per 10°C due to the reference on the calibrated measuring range. At 110°C, the temperature effect is less than 0.4%, i.e. six times lower.
What hydrostatic sensors does Anderson-Negele offer?
With a whole range of different pressure sensors, the most diverse requirements of hygienic applications for dairies, breweries, the beverage and food industries can be covered. For the particularly high requirements of the life science industry, many models also exist in a special pharmaceutical version.
The range extends from the low-cost all-rounder P42 with IO-Link, to the high-end L3 model. The latter offers intelligent electronics with improved temperature compensation thanks to an integrated temperature measuring cell, density compensation for media and integrated tank linearization for different, integrated or customer-specific container shapes. This allows direct output in gallons, pounds, PSI or many other units with significantly higher measurement accuracy than comparable devices. For outdoor applications or in cold or humid environments, the LAR can avoid measurement errors caused by climate-induced drift due to a special, hermetically welded measuring system.
What is potentiometric level measurement?
The potentiometric measuring principle works with the change in the voltage ratio between the electrode rod of the sensor, which projects into the liquid, and the metallic wall of the filled tank. This changes proportionally to the height of the medium in the tank itself, can be recorded with high precision and output as a measured value via the electronics. This measurement technique is only applicable for liquids which feature an electrical conductivity, at Anderson-Negele from <50μS/cm.
The potentiometric measuring method is suitable for closed and open process, feed, and storage tanks as well as for pressurized tanks. For non-metallic tanks, a sensor variant with a reference rod can be used.
How does a potentiometric level sensor work?
The sensor consists of an electronic unit and a measuring rod that protrudes into the liquid in the tank. Installation is possible from above, from below, diagonally and, thanks to a version with a bent measuring rod, also in the side wall of a tank. The length of the measuring rod can be precisely matched to the tank in 10 mm increments (intermediate sizes on request) up to a maximum of 3 m.
In the medium, the sensor generates an electric flow field, formed by the electrical conductivity and the capacitive properties. This creates a voltage ratio that is exactly proportional to the immersed part of the rod length. Since only the ratio of the voltages is considered, the properties of the medium, in particular the electrical conductivity, don’t influence the measurement result.
In the NSL, the sensor determines the immersion state of the electrode rod in the medium as additional information via a second, patent-pending, measuring method. This is based on the evaluation of the electrical resonance properties and ensures that foam is detected and proportionally blanked out. Faulty measurements due to adhesion and foam are thus reliably avoided.
For which products and applications is potentiometric level measurement suitable?
Due to the insensitivity to foam and buildup, the excellent measuring accuracy, and the extremely short response time, the measuring technology is suitable for a wide range of media and applications in dairies, breweries, milk and beverage processing companies, and in the food and life science industries. The only basic requirements are the conductivity of the medium and the container height limitation to max. 3 m.
The flexible and modular NSL sensor system offers reliable and precise application, even with difficult media and in demanding applications. That covers:
- all kinds of fluids such as product, CIP media, water…
- applications such as milk heaters, bottling plants, ice cream plants, separators, brewing tanks, lauter tanks, horizontal tanks…
- strongly or differently foaming media such as beer, juices, dairy products…
- pasty media or media adhering to the sensor rod, such as creams, ice cream, ketchup, sauces…
- pressurized tanks
- fast level changes, e.g., during filling
- non-metallic tanks
- different media in one tank (automatic adaptation to changing media)
- tank shapes with limited installation possibilities
- production environments with high mechanical or
- chemical stress (cleaning agents)
Which signals and parameters can be acquired and output?
The high-precision measurement process and intelligent electronics in the various NSL versions offer many different signals:
- Level height for 4…20 mA or digital IO-Link signal with <1% measuring accuracy
- Dry running warning signal
- Warning signal failure
- Signal limitation underflow / overflow
- Error limitation underflow / overflow
- Signal simulation (3.95…20.05 mA)
It enables the setting of the following parameters:
- Fill level zero point / offset
- Level Slope / Gain
- Attenuation / Filter
- Physical unit
What potentiometric sensors does Anderson-Negele offer?
The NSL measuring system is the core technology from which a comprehensive range of sensor variants is derived. This means that there are virtually no limits to the variety of applications:
- NSL-F: Level sensor with integrated electronics and programming display
- NSL-FR: Remote version with separate electronics, ideal for installation in difficult-to-access locations or in the event of strong vibrations at the process
- NSL-F-01: Curved version for installation in the tank side wall
- NSL-F-02: Dual probe for installation in non-metallic vessels
- NSL-M: Mini variant for data output to the PLC, without display
- IO-Link in Flex Hybrid technology: available for all NSL-F variants
What is the level measurement by radar?
The Anderson-Negele AGW Guided Wave Radar uses the TDR (Time Domain Reflectometry) principle. The instrument sends low power nanosecundum wide pulses along an electronically conductive rod with a known propagation speed (the speed of light). When a pulse reaches the surface of the medium that has a higher dielectric than the air/vapor in which it is traveling, the pulse is reflected. The reflected pulse is detected as an electrical voltage signal and processed by the electronics. The level measurement is directly proportional to the time of flight of the pulse. The measured level is converted into 4-20 mA current and HART signals which is displayed on the LCD display. The level data measuring values can be calculated into volume.
What are the specific functions of the Guided Wave Radar?
The AGW guided wave radar level transmitter is designed specifically for the needs of the life sciences market. It offers robust construction to stand up to agitation in the vessel. The AGW also offers the ability to remove the head (electronics) without breaking the sterile boundary to perform a dry calibration verification with the optional dry verification kit.
The AGW can be ordered pre-bent from the factory to accommodate vessel geometries. By using the Anderson-Negele E-Scope software the customer can configure a 20 point linearization table giving a mA output which is proportional to the volume in the vessel. The E-Scope software also allows the ability to tune the AGW for applications with foam and other two phase level applications.
With a dielectric threshold of dK=2 the AGW can be used in virtually any liquid media used in the life sciences industry including WFI. Finally, the tuning of the AGW is set in a way that it is not impacted when filling by a sprayball or when high humidity is present in the airspace at the top of a vessel.
What is the inventory measurement by weighing systems?
In many applications weighing systems for level detection offer a more practical and precise solution than other measuring techniques. With a field-proven sensor program, Anderson-Negele also offers precise, robust, and efficient solutions in this measuring range.
In many process vessels, storage tanks and silos, precise fill quantity determination is a particular challenge. Classical measuring systems such as hydrostatic sensors, potentiometric probes or radar often reach their limits. They are not precise, fast, or flexible enough in certain application environments, not practical for technical reasons or not economical for cost reasons.
This is particularly the case with:
- Process vessels with agitator. Here, no sensors can be installed that protrude into the medium. The amount of substance is often in motion, which creates dynamic transverse forces. In addition, it must be possible to measure and control precisely dosed and dynamically added ingredients with high precision in such vessels, even during the running process. Closed stirring systems with negative or positive pressure are also possible.
- Interchangeable movable containers for ingredients, such as in yogurt mixing. For connection reasons, but also for cost reasons, these cannot be equipped individually with level sensors.
- Horizontally stored tanks. Here, a small change in fill level already causes a large change in volume, and the inherent movement (sloshing) of liquids can make measurement by level sensors difficult. Load cells allow for more reliable results here.
- Tanks for dry bulk material, where irregular mass distribution (formation of piles, funnels, ratholes, etc.) can make surface measurement incorrect. With such tanks or silos, classic continuous level probes are often also not possible due to their size / height.
The solution: Turn your container into a precision scale. For more information, see the Kistler-Morse Website with its Weighing Systems product section.