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Scale Up

Calibration features on temperature and process controllers are often underutilised. However, if used correctly, controller calibration can achieve significant improvements in system efficiency and product quality across a range of industrial applications.

Many manufacturers of temperature and process control equipment calibrate their controllers at the factory to the stated accuracy within the product specification. This calibrates the controller as an individual instrument, but not the overall system where the controller is installed.

There are many exterior influences that have an effect on overall system accuracy – for example, sensor factors such as accuracy and positioning, along with cable type and length, can all introduce errors.

Some of those inconsistencies can be eliminated by using the input calibration feature on the controller. If the controller is not calibrated for a system, even a small error can have a serious effect on the quality of output.

Precision System

In the pharmaceutical industry, which is highly regulated due to the extreme need for safety, precision is key – and the success of a manufacturing business depends on meeting quality control requirements. In order for manufacturers to maintain both safety and product quality, it is necessary to establish a thorough system, including well-documented process instructions and complete data records for all production batches, taking in time and temperature information. Regular system accuracy tests are also needed to ensure the system is correctly calibrated within the allowable parameters.

Sometimes, the need to maintain tight temperature tolerance is driven not by standards, but by the need to maximise efficiency in certain processes. No manufacturer wants to keep running product through a process multiple times to get the right result, or cause downtime because output is of poor quality. Ensuring the system is accurate will reduce the likelihood of these situations occurring.

Measured Values

A controller displays a measured process value from a sensor that is positioned as closely as possible to the product within the process equipment. The sensor provides an analogue signal through the sensor input, which the controller converts to digital form for displaying. To calibrate the controller, the value shown on the instrument is compared to a calibrated temperature measurement source to determine the error.

There are two common ways to calibrate temperature sensors. One is single-point – or zero-shift – calibration, and the other is two-point calibration.

Single-Point Calibration
This is used in situations where there is an error value that is common to both the bottom point of the operating temperature range and the top end. It is useful when there is a constant error. Single-point or zero-shift calibration is defined as the distance to which an instrument will move from zero when the temperature level changes. Zero-shift should be set at the mid-point of the operating range – unless it is totally linear, in which case it does not matter because that relationship continues all the way through the range.

Two-Point Calibration
This is the most accurate form of calibration, used when the error at the top and bottom of the range are different. For two-point calibration, the machine should always be operated within the appropriate temperature band. For example, if the machine will always operate between 200°C and 400°C, the minimum and maximum calibration measurement should be taken at those points, respectively. The error at those points should be determined so the calculation is made within the window of temperature being used. Even if the controller is capable of calibrating between 0°C and 800°C, the reading should not be taken at those extremes because the machine will never operate at those temperatures.

Some processes require certification of calibration – and may require an approved engineer to perform independent calibration services. In the UK, this approval is provided by the United Kingdom Accreditation Service, while other territories use their own accreditation body – for instance, the US National Institute of Standards and Technology, and Deutsche Akkreditierungsstelle in Germany. Following calibration, an accreditation service will provide a certificate to show the system has been calibrated to meet the requirements of the application.

Enhanced Controllers

Correctly calibrated equipment, particularly if enhanced and optimised, can make significant improvements to efficiency and profitability in the pharma industry. In particular, enhanced controllers are coming onto the market to improve the accuracy, speed and functionality of inline blending applications, where accurate and consistent blending of multiple components is a crucial factor in product quality.

Additional flexibility and configuration options mean that these new controllers can be adapted, based on the user’s exact requirements. Companies can also take advantage of advanced display options, such as textual data pages, trend and bar graphs – allowing the engineer to expand the operational functions of the pharmaceutical plant. Ideal for batch mixing control of liquid, gas and solid ingredients, enhanced controllers offer a range of features, including control of ratio, sums, selectable master setpoint and monitoring of valve operation.

The application programs inside these controllers use a number of control loops across separate blending lines. This helps enable fluctuations in the process to be identified and rectified quickly and effectively to maintain product quality and production line efficiency.

Wider Optimisation

Enhanced equipment with improved accuracy and ease of use makes the task of calibrating equipment easier, which benefits the entire plant. The right calibration can ensure that legislative requirements are met, as well as providing further benefits, such as reducing waste and helping to facilitate higher throughput on the production process by optimising the whole control system.

These advances are in line with the growing need to expand the capacity for gathering process information, provide simplicity of use for the operator, and integrate co-working control elements; it is these factors that are driving the development of solutions offering single display, such as human machine interface, operator panels, or the routing of information back to a personal computer.

This means that the controllers of the future will increasingly become a single product, rather than individual devices – bringing to the user better application and control functionality, in addition to the monitoring of enhanced system data.

The trend is towards new levels of controller customisation, in which the operation of the device is tailored to the application. The latest display and programming technologies allow the controller to use the operator's terminology, gain easy access to data, and ensure process changes are optimised for efficiency. There is also the potential for controllers to integrate with a wider network. The use of ethernet communication is simplifying integration, with standard cabling and non-engineering-based connectivity further supporting the powerful blend of easy operation and increased control capability that continues to enhance temperature control.

Industrial temperature controllers are continuing to react to the changing demands of customers, creating powerful new solutions that improve efficiency and quality for a diverse range of applications.

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As European Head of Technical Support at West Control Solutions, Ian Parnell is responsible for advising and supporting customers in the installation, set-up and configuration of temperature control products in challenging and complex applications. Ian uses his 25 years of knowledge and expertise in temperature instrumentation to create technical documentation and in-depth product manuals. He has had a long career at West, including roles as Applications Engineer, Service Manager and Production Supervisor.
Ian Parnell
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