November 2009

Calibrate well, keep good data

As of now, most calibration data sheets are still hardcopy, usually printed out from a calibration software package. Some are still totally manual with no pre-printed information, and that is OK; it works. We are seeing increased use of paperless calibrations where the data is either automatically collected by a documenting calibrator or manually entered into a handheld device as an electronic record. In any case, the following information should be included as part of a calibration data sheet:

1. Instrument tag number/identification number: This is a unique identifier or unique combination used as the main tracking number for each instrument. In most cases, the tag number is the process/piping and instrument diagrams/drawings (P&ID) tag number, if applicable. If the instrument is not associated with a P&ID, there should be some consistent tag number system at your facility that uses the ANSI/ISA-5.1-2009, Instrumentation Symbols and Identification standard.

Several facilities use an additional identification number sequentially assigned as an additional tracking number. The tag number references the instrument location within a system, and the instrument identification number stays with the instrument. This way, the history of any instrument installed in the instrument location (tag number) is traceable, and the history of any instrument is traceable. Many instruments stay in the same location for the life of the equipment, in which case, this is not as important. But failures occur, and instruments need to be replaced. In other instances, we install spares temporarily to keep a process running when an instrument is removed for calibration.

This traceability is important in a pharmaceutical facility that produces penicillin, for instance. Any instrument removed from a penicillin manufacturing area cannot be used in any other manufacturing area. The instrument identification number used to track where an instrument has been is a useful tool in ensuring that instrument is not installed where it should not be. Even if you do not have similar needs, it is good practice to use both types of identification numbers.

2. Nameplate data: The manufacturer, model number, and serial number should be listed on the calibration data sheet.

3. Calibration range and tolerance: This defines the upper and lower limit used for calibration. Ideally, this is the input and output range, if applicable. A good format for the calibration range of a temperature transmitter would be 0-100°C=4-20 mA with a calibration tolerance of ±0.16 mA. The calibration range is not always the same as the instrument range or capability of the instrument.

4. Location: Be as specific as possible about location of the instrument. You do not want your new technician wasting hours searching.

5. Calibration procedure number: This is the calibration procedure used to perform the calibration. In some facilities, the entire calibration procedure is printed on the calibration data sheet.

6. Last calibration data, due date, and interval: Include this information to ensure the calibration is being performed periodically as required.

7. As-found, as-left data: This relates the test points specified with the corresponding test standard value. If all as-found data is within tolerance with no adjustments, the as-left data would be N/A or the same as the as-found data. Make every effort to record as-found data for failed instruments prior to making any adjustments, in order to provide the most data for evaluation.

8. Test standards: Record the unique identification of any test standards used to perform the calibration and, if required by procedure, record the calibration due data of the standards. It would be best to record this prior to beginning the calibration to ensure each standard is within its calibration periodicity. But of course you checked this when you obtained the standard from the shop. The most important reason for documenting the test standard is for reverse traceability in case you find a standard is out of tolerance. If this is the case in the next calibration, it is critical any calibration performed using that standard since its last calibration is known and evaluated to determine a course of action.

9. Comments: The technician needs some place to record any comments or observations.

10. Technician signature and date of calibration

11. Supervisor or reviewer signature and data

12. P&IDs: P&IDs are drawings that provide a detailed overview of a process system. They include major components, utilities, flowpaths, supporting equipment, and instrumentation. Although P&IDs are commonly used throughout industry, there is not yet a standard P&ID. Process Industry Practices (www.pip.org) has developed a P&ID practice; however, Process Industry Practices is not a developer of standards.

From the P&ID of a particular system, a technician can determine pertinent information about the instrumentation and controls applicable to performing calibrations and understanding the system operation. The most important of these include:

  • Components of an instrument loop
  • Functional identification
  • Methods of signal transmission (pneumatic, electronic, hydraulic, software link, etc.)
  • Controller inputs and outputs
  • Control valve characteristics (fail position, direct/reverse rating)
  • Flow sensor types

P&IDs also see use to organize project documentation. Using P&IDs as the base for all information in a large project provides a single reference point for data provided on other documents. This makes sense since P&IDs define system boundaries. It is important to control and keep P&IDs maintained up to date. During startups, mark up the master P&IDs, and highlight them to reflect the state of a system at any time. If the technician discovers any discrepancy at any time, the technician must take the responsibility to ensure the controlled drawing is properly revised.