Tried-and-tested measuring principle - even better

Advanced design and modern manufacturing technologies extend the scope of application and reduce the total cost of ownership.

Almost no other process measurement is as ingrained, and no other process variable is measured as often, as differential pressure. Whether flow, level, density or interface, applications with gases, liquids or steam: differential pressure measurement technology is universal and suitable for all media, and accessible to an extremely wide range in process temperature and pressure.

The worldwide growing demand for DP transmitters and increasing sales figures prove their importance in process automation. Compared to other relatively "young" measurement technologies, differential pressure measurement technology is sometimes considered old-fashioned. The technology, however, is not obsolete. Leading process industry suppliers are responding with interesting innovations to the industry's increasing demands for long-term accuracy, stability and connectivity, and especially the functional safety of differential pressure measurement technology.

Measurement cell design makes the difference

A newly designed measurement cell, which is 100% linearised and calibrated, makes the difference and offers a unique and improved view of DP measurement technology. The starting point of this development is a purely mechanical design with advanced mono silicone piezoresistive sensors for differential pressure and absolute pressure, the filling process and linearisation of external effects caused by static pressure and temperature.

The measuring cell is the core of the differential pressure transmitter. Here, the individual pressures on the high-pressure side and on the low-pressure side are transmitted via a diaphragm and filling fluid to the primary sensors in the liquid capsule, with high pressures of up to 700 bar. For stable design and accuracy at high process pressures and varying ambient temperatures, the influences of these effects must be eliminated in three steps: smart mechanical design, electronic compensation capabilities and linearisation.

The influences of high line pressures and the effects of unintentional overpressure must also be minimised by the mechanical design as the primary measurement. The membranes are matched to the basic shape of the capsule by a special manufacturing technique. Mechanical movements are limited to be within the elastic material properties. So that plastic deformation is prevented and cannot lead to a change in the measurement effect. 

The influence of ambient temperature effects must also be reduced. This is achieved, by reducing the capsule size and minimising the volume of the filling fluid. In the final mechanical production step, the capsule is filled with precisely dosed volumes of the degassed filling fluid in a controlled and automated environment. The resulting capsule, is further processed in 3-D linearisation.

3D - Linearisation

Even if the unwanted influence of ambient temperatures and pipe pressures is greatly reduced by the mechanical design, modern DP transmitters have to cope with the increasing demands of industrial processes and the reduced time available for commissioning and maintenance. Therefore, the capsule features three sensors, for differential pressure, absolute pressure and temperature, measured by the front-end electronics. Residual static pressure and ambient temperature influences are linearised in successive production steps.  For this purpose, mechanically prepared measuring capsules, are installed in batches on a mould and placed in a heating chamber.

The capsules go through a linearisation cycle, for the entire range of static pressure, temperature, and bi-directional differential pressure, resulting in a 3-D matrix of linearisation coefficients. These sensor characteristic individual linearisation coefficients are placed in a correction polynomial and uploaded to the front-end electronics. So that it delivers the actual corrected digital measurement values to the amplifier electronics. After this first raw data acquisition, a second final calibration is performed in the heating chamber, while the capsules remain installed on the mould, to ensure that all performance specifications are met. After calibration, the measurement capsules can be modified according to customer-specific requirements for HMI, communication, accessories and other functions. All production and calibration data can be traced for each individual transmitter, with the relevant parts marked with a data matrix code (DMC).

Industrial processes

To meet requirements for static pressures up to 700 bar, different capsule pressure ratings and differential pressure ranges have been designed. What are the practical benefits of such extensive design and manufacturing measures for the user? Conventionally, the typical field installation of a DP transmitter requires several key steps for proper commissioning. Once the transmitter is mechanically installed, the typical zero measurement is performed and the range is calibrated using a portable field pressure calibrator. For processes under high static pressure, the transmitter should be isolated from the process, and calibrated under these conditions, by trained and experienced personnel. Conventional transmitters are usually re-verified in the field at regular intervals. Using the KROHNE total 3-D linearised differential pressure transmitter, these time-consuming pre-commissioning calibrations can be reduced. The significant extension of re-verification intervals further reduces the total cost of ownership. Regularly changing pipe pressures now no longer cause deviations, due to the internal absolute pressure sensor and total 3D linearisation - periodic readjustments are now no longer necessary.

Standardised process connections, various mounting options and a modular HMI speed up installation and make it much easier - the newly introduced Bluetooth communication makes commissioning and on-site inspection a breeze. Quick installation routines for all flow, level, interface, density and differential pressure, delivered in 12 different languages, ensure that commissioning and maintenance staff, can quickly carry out instrument implementations.

With this new development, KROHNE is the only manufacturer to perform 100% 3D linearisation and calibration on every DP transmitter.  As a leading measurement instrumentation (MIV) supplier, KROHNE offers complete DP, flow and level solutions with application-specific sizing, engineering and documentation.

The combination of optimised mechanical design based on years of experience, together with state-of-the-art sensors for differential pressure, absolute pressure, temperature and 100% 3-D linearisation offers absolute added value for today's applications. It significantly reduces the total cost of ownership, especially in modern highly critical processes with high temperatures and pressures. High accuracy and availability are guaranteed under modern process safety guidelines.