Be it pressure, flow, level, density or interface applications with gases, liquids or steam, differential pressure (DP) measurement is a technology suitable for many processes. DP sensors are capable of measuring an extreme range of process temperatures and pressures. Modern DP transmitters must be able to cope with ever-increasing requirements in terms of safety, reliability and long-term stability in industrial processes. Changes in process conditions can present a challenge for DP measurement as these changes can influence the actual measurement uncertainty.

Be it pressure, flow, level, density or interface applications with gases, liquids or steam, differential pressure (DP) measurement is a technology suitable for many processes. DP sensors are capable of measuring an extreme range of process temperatures and pressures. Modern DP transmitters must be able to cope with ever-increasing requirements in terms of safety, reliability and long-term stability in industrial processes. Changes in process conditions can present a challenge for DP measurement as these changes can influence the actual measurement uncertainty.

Total 3D linearisation means producing a three-dimensional (‘3D’) matrix of individual linearisation coefficients for each measuring cell that includes the variables of differential pressure, ambient temperature and static line pressure.

To ensure the best performance under all process conditions, KROHNE performs total 3D linearisation of each DP transmitter to compensate for possible influencing factors in combination. This provides for a robust and accurate differential pressure measurement, even under changing process conditions.

There are three influencing factors on the measurement uncertainty of DP transmitters:

1. Linearity of the differential pressure (DP).

3. Line/static pressure effects (SP).

These influences can be caused by changing process conditions and may result in inaccurate readings delivered to process control by the DP transmitter.

To overcome this, KROHNE has a new innovative approach on the DP measuring cell design. The DP cell itself has extremely small dimensions and a low mass. The reduction of unnecessary material assures fast reaction on ambient temperature changes. The small dimensions reduce the amount of fill fluid volume to assure the lowest possible ambient temperature effect. The small diaphragm footprint provides a small area force at high static pressure ranges of 400 bar/5800 psi or 700 bar/10 000 psi.

Active compensation with built-in sensors

The innovative design does not stop on the outside, the internals of the measuring cell were also re-engineered: Additional to the silicon-based piezoresistive differential pressure sensor, an absolute pressure sensor on the low pressure side and a temperature sensor are built into the measuring cell. These additional sensors are required for the total 3D linearisation that is performed on every OPTIBAR DP device produced.

These sensors can provide additional valuable information about the process, such as the static line pressure or the state of DP-cell temperature to maintain safe conditions for the operator. All sensor values can be submitted via HART, fieldbus communication or on a secondary current output to the control system.

The KROHNE total 3D linearisation starts by running a full raw data acquisition to measure the uncertainty against a known reference on a minimum of 400 discrete measurement points.

Every DP cell passes through its entire specified operational range – for example:

• Differential pressure range P: -500 … +500 mbar*.

• Ambient temperature range: -40…+85 °C / -40…185 °F.

• Static line pressure: 0…160 bar / 0…2320 psi.

The result is a three-dimensional (‘3D’) matrix of individual linearisation coefficients for each measuring cell. They are passed through a correction polynomial and are uploaded to the measuring cell front-end electronics. After this raw data acquisition run, a final calibration run is performed to confirm that each individual DP cell complies with KROHNE’s stringent 3-sigma performance specifications.

After passing the total 3D linearisation, the measuring cells are connected to an OPTIBAR DP transmitter. All relevant parts are marked with a bar code so that all production and calibration data can be traced for each transmitter.

OPTIBAR DP 7060 high performance differential pressure transmitter with orifice plate and with averaging pitot tube.

Added benefits: no periodic re-adjustments

The typical installation of a DP transmitter in the field requires several important steps for a proper commissioning. Once the transmitter is physically installed, the zero-point adjustment is performed and the span is calibrated using a portable field pressure calibrator. It should be noted that with processes with high static pressures, the transmitter must be isolated from the process and calibration must be performed by trained and experienced personnel. Conventional transmitters are usually re-verified in the field at regular intervals.

With KROHNE’s total 3D linearised differential pressure transmitters, time consuming commissioning calibrations can be reduced. The significant extension of re-verification intervals further reduces the total cost of ownership. Frequently-changing line pressures do not cause any drifts due to the internal absolute pressure sensor and the total 3D linearisation.

KROHNE offers two DP transmitters with total 3D linearisation. The OPTIBAR DP 3050 is a differential pressure transmitter with integrated absolute pressure measurement. It is the ideal choice for general flow, level and differential pressure applications. KROHNE claims the device is currently the most compact pressure transmitter on the market. This makes it particularly suitable for space-saving installation in applications with limited space, e.g. in mechanical and technical plant engineering. This compact DP transmitter features the proven-in-use DP transmitter technology of the OPTIBAR series, but it comes without the high-end options of the more advanced OPTIBAR DP 7060. KROHNE points out that the OPTIBAR DP 3050 is one of the most cost-effective DP pressure transmitters on the market. For DP flow measurements it can be combined with KROHNE primary flow elements. Equipped with diaphragm seals it is a good choice for hydrostatic level and general-purpose DP measurements.

The OPTIBAR DP 7060 is a high performance differential pressure transmitter with integrated absolute pressure measurement. It offers a high degree of modularity for various flow, hydrostatic level or process pressure applications. For DP flow measurements it can be combined with the KROHNE primary elements (e.g. orifice plates or pitot tubes). Equipped with diaphragm seals, it is the right choice for hydrostatic level, interface, density or demanding DP process applications. Featuring a response time of just 125 ms, the DP transmitter offers high accuracy and measurement stability under all process conditions.

For more information contact KROHNE SA, +27 11 314 1391, d.rampathi@krohne.com, www.za.krohne.com

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