GMP 11

**Good Measurement Practice**

**for**

**Assignment and Adjustment of Calibration Intervals for Laboratory
Standards**

1.  Introduction

    1.  Purpose

> Measurement processes are dynamic systems and often deteriorate with
> time or use. The design of a calibration system would be incomplete
> without some established means of determining how often to calibrate
> instruments and standards. A calibration performed only once
> establishes a one-time reference of uncertainty. Recalibration detects
> uncertainty growth and serves to reset values while keeping a bound on
> the limits of errors. A properly selected interval assures that an
> item will receive recalibration at the proper time. **Proper
> calibration intervals allow specified confidence intervals to be
> selected and they support measurement traceability.** The following
> practice establishes calibration intervals for standards and
> instrumentation used in measurement processes.

2.  Prerequisites

    1.  Calibration history for laboratory standards

    2.  Expected tolerance limits if applicable

3.  Safety

    1.  No outstanding safety concerns

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2.  Methodology

    1.  Summary

Recommended calibration intervals are based on various examples of
traceability as described in GMP 13. As data is collected and evaluated,
the laboratory technical manager may adjust the calibration intervals to
ensure that measurement results are not invalidated by the intervals
selected.

2.  Apparatus

None.

3.  Procedure

    1.  Identification of Parameters

> *Critical Parameters*
>
> Components that contribute more than 25 % of a measurement's
> uncertainty are identified as critical parameters. To ensure an
> accurate evaluation of performance, calibration intervals are
> determined to meet a 99 % reliability target. Critical parameters are
> checked and defined below:
>
> *Mass Critical Parameters* (The example provided below corresponds to
> Option A in Mass Traceability in GMP 13.)
>
> Balance Performance
>
> Balance performance in Echelon 1 weighing processes is evaluated in
> every measurement series. An F‑test ratio evaluates the observed
> standard deviation of the process against the accepted standard
> deviation of the process. The balance performance component is also
> tested with a check standard in each weighing series. The check
> standard value is evaluated with a t‑test by compared the observed
> value to the accepted reference value. All other weighing processes
> have incorporated measurement control procedures and control charts
> that are evaluated as data is collected.
>
> Mass Standards
>
> Mass standards are dynamic with use. Wear, contamination and other
> factors can cause drift from accepted values. Thus, the following
> intervals have been set:

Table 1. Calibration intervals for mass standards

  ------------------------ ------------------------------- --------------
  Item                     Initial Cal Interval (months)   Source

  P1. kg + P1.. kg         48                              NIST

  C1. kg + C1.. kg         (alternating 2 years) 48        NIST

  P30 kg - P2 kg           12                              Lab

  P500 g - P1 mg           6                               Lab

  C500 g - C1 mg           6                               Lab

  W25 kg - W1 mg           12                              Lab

  P Pound Standards        24                              Lab

  W Pound Standards        12                              Lab

  P = primary; C =                                         
  check/control; W =                                       
  working standards                                        
  ------------------------ ------------------------------- --------------

### *Length Critical Parameters*

> Length Comparison Performance
>
> The measurement performance of each length calibration is evaluated
> with a check standard (when performing tape-to-tape comparison).
>
> Initial Length Intervals
>
> Length standards are dynamic with use. Wear, contamination and other
> factors can cause drift from accepted values. The following intervals
> have been set due to these factors:

Table 2. Calibration intervals for length standards

  ----------------------- ------------------------------ -----------------
  Item                    Initial Cal Interval (months)  Source

  100 ft Tape #1          60                             NIST

  100 ft Tape #2          60                             NIST

  25 ft or 7 m Tape       60                             NIST

  18 in Steel Rule        120                            NIST

  Length Bench            24 (if used or moved)          Lab
  ----------------------- ------------------------------ -----------------

> *Volume Critical Parameters* (Example shown corresponds to Option A
> for volume calibration in GMP 13.)
>
> Volume Comparison Performance

The measurement performance of a volume transfer calibration is
evaluated in each use with a repeatability check. Use of check standards
is the preferred method for evaluating the measurement process over
time. Traceability for volume standards may be established through
gravimetric calibrations using traceable mass standards.

> Initial Volume Intervals
>
> Volume standards are dynamic with use. Wear, contamination and other
> factors can cause drift from accepted values. Calibration intervals
> are as follows:

## Table 3. Calibration intervals for volume standards

+----------------------------+------------------------+----------------+
| Item                       | Initial Cal Interval   | # Source       |
|                            | (months)               |                |
+----------------------------+------------------------+----------------+
| P100 gal standard          | 60                     | # NIST         |
+----------------------------+------------------------+----------------+
| P25 gal standard \*        | 60                     | Lab            |
+----------------------------+------------------------+----------------+
| P5 gal standard \*         | 12                     | Lab            |
+----------------------------+------------------------+----------------+
| Glassware-                 | 120                    | Lab            |
|                            |                        |                |
| Autopipetes 5 L to 100 mL  |                        |                |
+----------------------------+------------------------+----------------+
| \*Gravimetric calibration  |                        |                |
| for volumes 5 gallon or    |                        |                |
| smaller, and all "slicker  |                        |                |
| plate" standards.          |                        |                |
| Laboratory must be         |                        |                |
| qualified for performing   |                        |                |
| gravimetric calibrations.  |                        |                |
| Volume transfer is         |                        |                |
| acceptable above 5 gallon. |                        |                |
|                            |                        |                |
| \*May be a "slicker plate" |                        |                |
| type. None are hand-held,  |                        |                |
| "dump" style, test         |                        |                |
| measures.                  |                        |                |
+----------------------------+------------------------+----------------+

> *Temperature Critical Parameters*
>
> Temperature Comparison Performance
>
> The measurement performance of each temperature comparison calibration
> is evaluated with a check standard and can be verified periodically
> using triple point cells, melting point cells, and ice baths (using
> documented procedures).
>
> Initial Intervals
>
> Temperature standards are dynamic with use. Shock, contamination and
> other factors can cause drift from accepted values. Recalibration
> intervals are as follows:

Table 4. Calibration intervals for temperature standards

  ------------------------ ----------------------------- ----------------
  Item                     Initial Cal Interval (months) Source

  25.5 ohm SPRT            36                            NIST

  100 ohm PRT′s            12                            Lab

  Standard Thermistor      12                            Lab

  Check Standards          6                             Lab

  Liquid-in-glass          6                             Lab
  standards                                              
  ------------------------ ----------------------------- ----------------

> *Secondary Parameters*
>
> Components that contribute less than 25 % but more than 1 % of a
> measurement's uncertainty are identified as secondary parameters.
> Secondary parameters are assigned calibration intervals designed to
> meet a 95 % reliability target. Secondary parameters are defined
> below:
>
> Mass and Gravimetric Volume Secondary Parameters
>
> Environmental Measurement Equipment

Table 5. Calibration intervals for environmental equipment

  ---------------------- ----------------------------- -------------------
  Item                   Initial Calibration Interval  Source
                         (months)                      

  Barometer              12                            NIST

  Hygrometer             24                            NIST

  Thermometer            12                            Lab
  ---------------------- ----------------------------- -------------------

> Length Secondary Parameters - No secondary parameters.
>
> Volume Secondary Parameters
>
> Water Temperature Measuring Device

Table 6. Calibration intervals for volume secondary parameters

  ---------------------- ---------------------------- --------------------
  Item                   Initial Cal Interval         Source
                         (months)                     

  Thermometer            12                           Lab
  ---------------------- ---------------------------- --------------------

> Temperature Secondary Parameters - No secondary parameters.

3.  Calculations

    1.  Initial Intervals

        1.  Assignment of Initial Intervals

> Assignment of initial intervals is based on these recommendations.
> Subsequent intervals may be adjusted based on analysis of Check
> standard data that is recorded on control charts. The initial interval
> is equivalent to the amount of time that passes before a metrologist
> makes the first observation of a result lying outside the warning
> limits of the chart when the investigation yields no apparent
> correctable cause. Warning limits are established at 2 standard
> deviations of the measurement process around the accepted value of the
> check standard.

2.  Absence of Control Charts

> If no control charts are available, the laboratory's Technical Manager
> will assign the initial interval based on engineering evidence,
> manufacturers specifications, NIST recommendations, and experience.

2.  Adjustment of Intervals

    1.  Intervals will be adjusted when determined to be necessary by
        the laboratory's Technical Manager.

    2.  The intervals will be adjusted by taking the following factors
        into consideration as appropriate:

        -   calibration history;

        -   measurement assurance data;

        -   data for the population of similar equipment;

        -   NIST recommendations;

        -   statistical analysis methods; and

        -   manufacturers recommendations.

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4.  Assignment of Uncertainty

The uncertainty associated with the establishment of calibration
intervals is not included as a part of the uncertainties associated with
the respective measurement SOPs. See SOP 29 for calculating
uncertainties for standards and the instruments.