Developments in Instrumentation Calibration Tools
- Mike Shelton, Product Manager, GE Measurement & Control

The process sector is probably the main industrial user of plant instrumentation and this is used for the measurement of a wide range of parameters, such as pressure, temperature, flow, level and various electrical measurands. This accurate and reliable measurement is essential for continuing plant performance, efficiency, quality, safety and compliance. To ensure this accuracy and maintain reliability, it is vital that all measuring instruments are regularly and correctly calibrated.

Calibration? What, Why and When?
Calibration involves comparing, at intervals, the value measured or set by the plant instrumentation with a known reference standard, which is traceable to national or international standards, to establish the accuracy of the plant instrumentation. It is important that calibration is carried out on a regular basis, because all measuring instruments drift in accuracy over time. Having established the need to calibrate, it is then important to decide when to calibrate. Sometimes calibration strictly follows manufacturers’ recommendations. Some organisations prefer to calibrate on a regular frequency basis, such as annual shut-down. The safest bet would seem to be to calibrate more frequently, but this entails increased time and cost, both in lost production and in calibration resource. Historically, calibration often meant taking an instrument out of service and sending it to a standards laboratory, or sometimes to an on-site laboratory, equipped with the necessary quality of calibration equipment. This is still done for some specialised instruments but with the introduction of the latest generation of smart, portable calibration instruments, which can integrate with today’s sophisticated calibration and maintenance management software, it is now possible to automate the calibration process for some measurands and often even possible to increase calibration intervals while still meeting standards relating to process and product quality and safety.

How to Calibrate?
If it is decided to calibrate in situ, it is first necessary to consider the ambient conditions of the various calibrations. Working in potentially explosive atmospheres, for example, presents real challenges, particularly for technicians who need a multitude of test tools and ancillaries, including gas bottles and regulators, to cover electrical, temperature and pressure applications. The difficulties are further complicated because of the regulations governing working practices in potentially explosive atmospheres. Even with suitable equipment, technicians are required to follow strict procedures and safety guidelines documented in text documents, drawings, data sheets, spreadsheets etc. It is simply not practical to carry all this material into the field in paper form. In addition, all calibration and maintenance actions and data have to be documented, often with pen and paper. Errors result and time is wasted transcribing the data and then up-dating the maintenance systems. It is an unfortunate fact that, even when calibration has been carried out correctly, poor documentation can be the difference between a forced shut-down and a safety approval.

Selecting the Right Tools
Selecting the right calibration tool can be confusing. Many tools are dedicated to one measurand so that an instrument technician often needs to carry a number of calibrators to meet all the challenges he faces in a typical working day. Fortunately, multi-function calibrators, which combine a standard multimeter and with additional capabilities such as loop testing and pressure calibration, are now available.

The latest hand-held, multi-function/calibrator is operated by a touch and swipe screen, similar to a smart phone. It can measure and simulate the electrical parameters, temperatures and frequencies associated with the majority of process instrumentation but is significantly more accurate (eight to 16 times) than a typical multimeter. As a modular tool, its duties can be expanded by the addition of a uniquely flexible pressure system. This comprises a range of 32 high accuracy pressure measurement modules and three pressure generating stations so that pressure can be generated pneumatically from 95 percent vacuum up to 20 bar to 100 bar, and hydraulically up to 1000 bar. Its broad range solves the transportation and EHS issues of using gas bottles and regulators.

Such a calibrator also allows instrument engineers to view written procedures, safety notes permits, drawings, diagrams and datasheets and has sufficient memory to store a massive plant instrument database. Consequently, results can be recorded automatically, enabling full historical analysis in the field, without the need for a PC, so that automated procedures / work orders can be downloaded remotely and results up-loaded to anywhere in the world, enabling timely completion of audit records, service invoices and calibration certificates etc.

In addition, the latest calibrator also offers HART and Fieldbus communication capability, so that instrumentation can be configured and commissioned as well as calibrated. It features a built-in library of all 1300 registered HART and 600 Foundation Fieldbus device descriptions (DD), with virtually unlimited memory for future updates. It can view, change, clone and store device configurations, even off-line. Analogue variables can be measured and sourced without the need for secondary calibration equipment. Genii also provides its own 24V loop supply and contains an integral 250 ohm HART resistor.

Compatible Calibration Maintenance Software
A typical process plant can contain 1000s of instruments and the analysis of all the calibration data is not a simple task. It involves scheduling, resource management and cost analysis, as well as the challenge of storing all the data and documentation to meet audit requirements.

However, the latest software developments combine calibration and maintenance management and provide a comprehensive solution for streamlining maintenance workflow by improving data integrity, scheduling work activity, automating calibration and maintenance and correcting deviations. For example the latest software platform from GE interfaces seamlessly with the latest calibrator, enabling the automatic download of procedures and the direct upload of results, ensuring paperless calibration and maintenance, with total integrity of data and no chance of transcription errors. Scheduling can be automated and worksheets sent directly to the appropriate operators. The system generates alarms for overdue activity and will highlight deviations that require action and provides compliant audit-ready data.

A major advantage of the GE software package is that it is web-based and can be installed on a company’s server and accessed through a PC and web browser from anywhere in the world. Alternatively, the software can be hosted by a third party as a service on an external server, so that there is no need for software installation and IT support at the company itself.

Calibrating Temperature Sensors
Hand-held calibrators operate essentially like a multi-meter (although today, they are between 8 and 16 times more accurate than a standard multimeter). As such, they measure the electrical parameters of instruments. With temperature instruments, such as RTDs and thermocouples, they are used to simulate the electrical output from the device to see if this agrees with the value displayed. For general purposes, this is sufficient, as the mechanical components of RTDs and thermocouples are themselves inherently stable, the greatest source of drift being in the associated electronics. Consequently, to calibrate the sensor itself, it is necessary at intervals to simulate the temperatures the sensor will measure in practice. Today’s multi-function calibrators have the capability to simulate pressure, as explained earlier, but the simulation of temperature requires a separate piece of hardware and, generally, this takes the form of a calibration bath.

Temperature Calibration Baths
There are two kinds of calibration baths: liquid baths and dry blocks. Both require an electricity supply and both generate real heat to simulate the conditions under which temperature sensors operate. The temperature generated has a known accuracy, so these devices can be used with great confidence to calibrate various temperature devices. However, with some calibrators certain sources of inaccuracy must be taken into account.

A typical liquid bath can generate temperatures from -350C to 1650C and from ambient to 2550C, depending on the heating fluid. The latest baths, such as those from GE, incorporate the latest heating and electronic control technology, combined with continuous agitation of the fluid bath, providing high accuracy and stability throughout the large homogenous measuring zone. They are factory-configured as liquid baths and are provided with a cover to hold up to five temperature devices, of various geometrical form and size, while reducing heat loss from the surface of the heating fluid. A leak-proof cover also prevents spillage during transportation around a site. The latest liquid baths are also versatile and feature additional interchangeable baths to suit different temperature ranges. They can also be converted into dry block calibrators by the insertion of a dry block insert.

Dry blocks are simple to use and are very robust for field use. Essentially, they rely on the heating up of a metal block and can, typically, simulate temperatures from -350C to 6500C. Compared with liquid baths, they heat up and cool down much more quickly. To ensure homogeneity of heat around the sensor to be calibrated, Dry Blocks offer a range of precision-bored inserts with diameters to suit a wide range of sensors. Although not quite as accurate as liquid baths, dry blocks can offer significant benefits in terms of ease of use in the field.

Conclusions
Process instrument calibration is vital to the continuing operating efficiency and safety of any process plant. However, the selection of the right tools for the job can sometimes be confusing. Fortunately, developments in calibration technology are now helping to make a task, which is often still considered as a chore, increasingly productive, reliable and user-friendly.