Process transmitters to strike the balance between continuous stresses and accuracy
The mechanical connection of the transmitter to the process is made via a pressure port or with a flush diaphragm, depending on the model. In production processes with aggressive media or high temperatures, one must always bear in mind that the sensor and the electrical circuitry will always be close to the process and the measuring instrument may be more susceptible than in other processes. Even where the wetted parts of the UPT are from comparatively hard-wearing materials, it is recommended that in such cases, for reasons of process safety, a diaphragm seal is fitted between the process and transmitter. Its diaphragm, which can be made of special, highly resistant materials, protects the transmitter from the critical process conditions and transmits the pressure to the measuring instrument through a transmission fluid. There is also the possibility to connect the diaphragm seal to the transmitter over a capillary, so that the temperature between process and measurement can be significantly reduced.
A production plant only operates profitably if disturbances and outages can be cut to an absolute minimum or if they never occur in the first place. As a result, the demands from the plant builders and plant operators on the individual components are correspondingly high. The need for technical equipment with significant breadth in both performance and scope of application is growing − this also comes from the need to alleviate cost pressure. In the case of measuring instruments, this means that they must continue to deliver high accuracy while still withstanding extreme conditions. To achieve such a balance, WIKA has developed a new generation of process transmitters for pressure measurement.
The challenges to the technical equipment within a manufacturing process can be illustrated by the example of the food industry: There, during the production phase, the ambient air is often warm and humid. This changes abruptly when, in the cleaning phase, the plant is hosed down with high-pressure water. Such changes in the process and environmental conditions put all elements of the plant under stress, especially measuring instruments such as process transmitters.
Stress on the one side and the highest accuracy on the other is, in itself, a contradiction. Ultimately, the measuring instrument must not only simply endure the conditions, but must also function within the specification that has been promised to the operator.
The operator expects that the sensor will still deliver the required accuracy, even after many cycles like those described at the start or after comparable processes, even though the external influences could sometimes be at the limits. If the measurement deviates from the actual value, this will be referred to as the measuring deviation or inaccuracy of the sensor. Should this inaccuracy be too large, it is possible that the measuring result might no longer be usable for plant control. The parameter settings can no longer be implemented with sufficient reliability. As a result, production defects, destruction of the manufactured item or irreparable damage to the plant can occur.
Reliability and accuracy are not the only criteria for the selection of a measuring instrument. But WIKA has started from this point in the conception of the new universal process transmitter (UPT-2x). An accuracy of up to 0.1% of the measuring span is possible across a wide-ranging field of application – as the instrument's name suggests. The transmitter features pressure ranges from 0 ... 400 mbar to 0 ... 1000 bar in nine stages. The intermediate values can be set through the scaling of the measured value (with a recommended maximum turndown of 20:1).
The market is also asking for versatility on economic grounds, in order that different processes, varying environmental conditions and a range of mounting situations can be covered with a single instrument model. For the transmitter used, this means that the engineering and design must follow the breadth of operation of the sensor.
For example, the housing of the UPT is made from a plastic that is as light as it is robust. Thus, the transmitter can be used on measuring points that, due to their design, cannot allow any great weight. Should instruments with comparatively heavy cast metal housings be used there instead, over time, as a result of vibration for example, loadings which are too high can occur on the connection points and even on the instrument itself. In turn, this can lead to leakages and failures, and so to process uncertainty. The result could be contamination of the product and plant downtime.
However, generally, plastic housings have a downside. They are non-conductive, and thus can lead to electrostatic discharge. It is even possible to cause a spark simply by brushing it with overalls or a pullover. Instruments with such housings must carry warning labels and are only suitable for limited applications in hazardous areas. However, if we are following the principle of versatility, the new UPT must also be suitable for Ex areas, backed up by an ATEX approval. This is achieved by a special plastic with metal particles mixed in. The microscopically small shavings combine in the casing to form a metal mesh and thus make the body conductive. Further unsafe or uncertainty factors can be eliminated through specific accessories for the UPT. An adapter on the cable entry, for example, protects against overvoltages caused by faulty power supplies or from the effects of a lightning strike. An instrument bracket can help decouple it from any possible vibration.
The geometry, design and functionality of the instrument also show the response to increasingly more compact installation environments. With the ability to set the display to four different orientations, each 90° apart, and to rotate the process connection through 330°, in all cases, the display can be set to a favourable position for the viewer. The 58 mm wide display is easy to read, even from as far away as five metres. In addition, the instrument is easy to use overall. The electrical installation is possible with hardly any tools, and the on-site operation is self-explanatory and easy.
The transmitter electronics are also oriented towards flexibility. Over the system connection with HART v.7, the UPT can easily be integrated into all common modern control systems. Communication via the generic HART protocol is supported, in addition, by WIKA’s own DTM (Device Type Manager) and EDD (Electronic Device Description): Users can therefore work with an easily understandable and time-saving platform. All functions can be parameterised via a laptop or remotely from the central control system. For the monitoring of liquids in tanks, with any shape of vessel, there are specific algorithms that output the level in the tank or even the exact volume as data and/or transmit this to the control system.
For communication via a hand-held unit, the corresponding hardware is also available. Thus, the UPT can be used with the Emerson FC475 as conveniently as via a computer’s USB interface (via a HART communication modem). With a load of at least 250 ohms, the HART signal can be taken from the current loop. A software tool, such as Pactware, helps in reading measured values and parameters. The settings can be adjusted on the screen and transferred to the UPT individually or as a complete packet.