Calibration Cycles

ISO/IEC 17025 requires that calibration programs be established for key quantities or values of the instruments where these properties significantly affect the results. A well-drafted calibration program consists of calibration intervals, calibration guidelines, sampling, and testing procedures. Although calibration intervals are usually set by the OEM (original equipment manufacturer), asset owners follow norms without examining the equipment’s condition or its own specific calibration demands. This article clarifies the need for an optimal calibration cycle and offers several tools and techniques to determine a suitable approach while determining the calibration schedule.

 

Periodic and Non-Periodic Calibration Intervals:

Industries include various equipment (indicators, instruments, components, and devices), each with its own calibration needs. Most equipment has a Periodic calibration cycle defined, while certain equipment is designated with “NPCR (No Periodic Calibration Required)” status and is identified by the following criteria:

 

  1. The instrument does not make measurements (critical or non-critical).
  2. Instruments that monitor the operational status but do not provide a number (for example, visual indicators, check lights, etc.)
  3. External factors, such as changes in loads, environmental conditions, and operator skill, do not impact operational accuracy.

 

Periodic Calibration cycles and calibration interval:

Calibration cycles cannot be standardized for every piece of equipment. It typically necessitates the theoretical and practical understanding of the instrument, its operation principle, frequency of usage, operator skill level, and environmental factors. The following parameters may be used to categorize equipment to determine calibration frequency:

 

  1. Classification based on operating or input loads: Instruments or equipment that operate with loads within the design parameters are unlikely to be exposed to significant risks. Thus calibration cycles may be prolonged. Instruments that operate in dynamic environments and face great sensitivity (due to plastic deformation, thermal expansion, etc.) have smaller calibration cycles or are calibrated before each usage, owing to the potential for substantial damage.
  2. Classification based on safety and operating environments: The calibration frequency for instruments that are only used to indicate may be prolonged, and in cases where equipment is utilized in a safety-critical environment, specific attention must be paid to choosing an interval that will minimize the risk of personnel injury or property damage.
  3. Classification based on usage frequency and operator skill: Precision instruments (verniers, gauges, and so on) are used every day therefore a pre-determined calibration cycle is required to maintain their sensitivity and repeatability. Instruments that are not often used (such as optic and laser measuring tools) can benefit from an annual calibration cycle.

 

Need for Optimal Calibration Interval:

Too short or frequent calibrations can result in equipment downtime, higher calibration costs, and overheads. Extended calibration intervals can lead to unscheduled maintenance or breakdown maintenance, lowered product quality, and risk of product recalls. Consequently, companies and organizations must establish a standard calibration interval (that is specific to each piece of equipment) to maintain machine uptime and end-product quality.

 

Determining Optimal Calibration Interval:

Following is a two-step procedure to determine calibration frequency or interval:

Step 1: The initial step in calibrating your equipment is to understand the M&TE (equipment). The following are a few factors that aid us in determining the best calibration schedule:

 

  1. Type of instrument, operation principle, and the field of application
  2. OEM Recommendations of the calibration frequency
  3. Environmental conditions, safety requirements, and severity of use
  4. Defining the measurement uncertainty and the risk of instrument exceeding Maximum permissible error?
  5. Cost of correction and recall measures when the instrument is labeled “out of calibration”
  6. Review of previous calibration reports and maintenance checks
  7. Qualification and skill level of operators

 

Step 2: After determining the equipment type (as per step-1), one or more of the following approach(s) may be used to determine the optimal calibration interval. Numerous industries have tested these techniques, shown to work in a specific situation, and are internationally accepted:

  1. Fixed Interval Method: Fixed time intervals are often recommended by the OEM (original equipment manufacturer). They’re easy to remember, put into action, and plan. However, due to not taking the equipment’s running state into account, the correct instrument can be re-calibrated, resulting in loss of availability and resources.

 

The risk-Based approach to determine Fixed calibration interval:

The significance of each instrument and the level of influence it has on the accuracy of the overall result is determined by its usage schedule. These elements are crucial when classifying assets into high-, moderate-, and low-risk categories. This risk-based method is beneficial when developing a calibration cycle and monitoring equipment on a periodic basis. The amount of time that you monitor your equipment is directly proportional to how critical it is, as defined in the table:

 

Probability that Time or Usage affects the equipment/Instrument
Low Moderate High
Measurement’s Influence  on the End or Test Result High Monitoring

(moderate risk)

Frequent monitoring (high risk) Frequent monitoring (high risk)
Moderate Infrequent monitoring (low risk) Moderate monitoring (moderate risk) Frequent monitoring (high risk)
Low Infrequent monitoring (low risk) Infrequent monitoring (low risk) Moderate monitoring (moderate risk)

 

  1. General Interval method (standard interval for all equipment): This technique, like fixed intervals, has a defined calibration interval that is common among all equipment types, regardless of their operation or functioning process. Therefore, though this technique is simple to remember and put into action, equipment may be calibrated too often to incur additional expenses.
  2. Automatic adjustment (Staircase method) or Simple Response Method: According to ILAC-G24, each time an instrument is calibrated and found to be within the maximum acceptable error limit for measuring, the following interval is prolonged or shortened depending on whether it was determined to be inside or outside of this maximum allowable error (defined during the earlier calibration). This technique is easy to implement and requires little staff involvement, but single calibration events can trigger false readings in the subsequent intervals.
  3. In-use time method: Calibration intervals are measured in running hours (of usage), rather than in calendar time. This approach has been demonstrated accurate and is commonly used for heavy equipment such as engines and turbines as an overhaul. Though precise, this technique cannot be applied to all equipment types, and tracking usage time is important.
  4. Borrowed intervals: As the name implies, this technique derives its intervals from similar organizations that use conventional (and acceptable) interval methods in terms of calibration processes, equipment usage, and equipment handling. Adopting this approach does not need much effort; however, before doing so, one must verify the risk tolerance before doing so.
  5. Control Charts: Calibration is carried out as the reading approaches and crosses the control limits. This method uses a pre-determined lower control limit (LCL) and upper control limit (UCL), which must be set before use. When the reading passes the controls, calibration is performed. Despite its flexibility and validity, this procedure necessitates significant human resources to record and plot control charts because it relies on charting time-consuming calculations manually.
  6. Other methods: Various other techniques, software, and algorithms are being developed and tested to address the unique calibration requirements of each sector. These approaches, which are becoming increasingly popular among businesses because of their Early-warning and Predictive nature and not a one-size-fits-all strategy, have gained favor as a result of their accuracy.

 

International Guidelines and Methods to determine Calibration Cycle Frequency:

Following standards provide guidelines and methods to determine the calibration frequency:

  1. ILAC G-24: Guidelines for the Determination of Calibration Intervals: Provides internationally acceptable direction to comply with the ISO/IEC 17025:2005 standard. It also explores the creation of calibration intervals and five methods to adjust them.
  2. NCSL RP1: Establishment and Adjustment of Calibration Intervals: The NCSL International RP-1 document is the most well-known, referred to, and quoted for calibration intervals. The book is the most complete and detailed, with various strategies for adjusting calibration intervals. Some are straightforward, while others are more complex.
  3. Simplified Calibration Interval Analysis: Originally presented at the NCSL International Workshop and Symposium in Nashville, Tennessee, in 2006, this method offers a straightforward technique to modify calibration intervals based on an instrument’s past calibration success. Although other techniques use historical data, this approach is easy and quick to implement.
  4. Cost-Effective Calibration Intervals: The technique described at the 1995 ASQC Annual Quality Conference in Cincinnati, Ohio, employs the Weibull probability distribution to assess failure data and calculate calibration intervals that minimize risk and cost. This approach is intended to meet an organization’s business and quality objectives.
  5. Calibration Intervals from Variables Data: This method was presented during the 2005 NSCLI Workshop and Symposium in Washington, D.C., and published in Measure Magazine in 2006. It shows how to establish calibration intervals using varied data and regression analysis.

e2b calibration offers industry-leading ISO-certified calibration services. Our labs are ISO/IEC 17025 accredited and operated by a team of qualified calibration experts. Our verifiable services are unmatched in the industry. We are registered with ANAB. We are also ANSI/NCSL Z540-1-1994 certified. We have the NIST Traceable Wide scope of ISO/IEC 17025 accreditation. Contact e2b calibration for all your equipment calibration needs.

 

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