Modern DP transmitters must be able cope with ever increasing requirements in terms of safety, reliability and long-term stability in industrial processes. Especially changes in process conditions can present a viable challenge for DP measurement as these changes can influence the actual measurement uncertainty.
As the only manufacturer world-wide, KROHNE carries out a Total 3D-linearization, which in process recognizes and effectively compensates all influencing factors
To assure the outmost best performance under all process conditions, KROHNE carries out a Total 3D-linearisation of every DP transmitter to compensating all possible influencing factors in combination. This provides for a robust and accurate differential pressure measurement, even under changing process conditions.
Innovative measuring cell
There are three influence factors on the measurement uncertainty of DP transmitters:
Linearity of the differential pressure (DP)
Ambient temperature effect (T)
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 new innovative approach on the DP measuring cell design. The DP cell itself has extremely small dimensions and thus a low weight. 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 effects . The small diaphragm footprint grants a small area force at even highest static pressure ranges of 400 bar/ 5,800 psi or 700bar/ 10,000 psi. In sum, the measuring cell design combines high performance and reliability with a small installation footprint.
Active compensation with build-in sensors
The innovative design does not stop on the outside, also the internals of the measuring cell were re-engineered: besides the silicon based piezoresistive differential pressure sensor, a piezoresistive absolute pressure sensor on the low pressure side and a temperature sensors are build in the measuring cell. These additional sensors are required for the Total 3D Linearisation that is performed on every OPTIBAR DP device produced.
Of course, these sensors can provide additionally valuable information about the process, like the static line pressure or about 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 linearization starts with 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 linearization coefficients for each measuring cell. They are put in 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 KROHNEs stringent 3-sigma performance specifications.
After passing the Total 3D linearisation, the measuring cells are assembled to an OPTIBAR DP transmitter. All relevant parts are marked by data matrix code so that all production and calibration data can be traced back for each individual transmitter.
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 mechanically installed, typically the zero point adjustment is performed and the span is calibrated using a portable field pressure calibrator. At processes under high static pressures, the transmitter must be isolated from the process and be calibrated under these conditions by trained and experienced personnel.
Conventional transmitters are usually re-verified in the field in continuous intervals. With KROHNE’s Total 3-D linearised differential pressure transmitters, time consuming calibrations can be reduced for the commissioning. 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 – periodic re-adjustments are not necessary anymore.