Recommended practices for calibration laboratories

General

Calibration equipment and reference standards will be affected by changes in a number of influencing parameters such as temperature, pressure, and relative humidity. The reference environmental conditions listed below are gradually being adopted internationally.

Although it is possible for standards laboratories to work under other environmental conditions and to correct their measurement results so as to represent the calibration information at the reference conditions, considerable extra work and meticulous data processing is necessary to ensure that correct results are reported. The overall operation of a standards laboratory or of a test equipment calibration laboratory is simplified when the same reference environmental conditions are maintained in calibration laboratories and in external client laboratories. This decreases the probability of errors when writing or implementing reports.

In-house laboratories

For those calibration laboratories that do not offer their services to external clients, it may be more economical to establish a set of reference environmental conditions suitable to the local scene. This requires that appropriate attention be paid to calibration data from other sources with other environmental conditions.

Environmental stability

Although the set points of variables, such as temperature, are important, a more important aspect is the stability of the environment during the time taken to make a calibration measurement. Limits on the rates of change of the environmental conditions are important to all laboratories, and should be established and maintained in all calibration laboratories.

The requirement for temperature stability applies only to the space occupied by the standards and the comparison or measuring equipment, although the larger the volume in which the conditions are stable the greater would be the confidence in the measurement results. It sometimes happens that a minor relocation of work space will improve conditions of measurement appreciably.

Micro-climates

As far as temperature conditions are concerned, there are a number of artifact standards such as standard cells and standard resistors in the electrical/electronic field whose expected high precision performance can only be achieved when the devices are maintained in much more closely controlled temperature enclosures, such as oil baths or air baths, where stability of temperature of ± 10 mK or better can be maintained. Many precision measurements can only be made when these micro-climates are present to overcome the instabilities of temperature normally found in even the best-controlled laboratories. If the laboratory work is restricted to the use of such micro-climates the ambient temperature may change without affecting the compatibility of the measurement results.

Effects of relative humidity

It is known that relative humidity greater than approximately 65% can lead to corrosion effects in some instruments or standards, or to degradation of electrical insulation in other cases. Below approximately 20%, electrostatic effects begin to manifest themselves in many ways, one of which may be electrostatic discharge problems that can easily damage some sensitive electrical circuits.

Air cleanliness

For many calibration laboratories, air cleanliness is an important factor. One of the ways to improve air cleanliness is to maintain a positive air-pressure differential of at least 12 Pascal (0.05 inches of water) between the inside of the laboratory and the adjacent area outside the laboratory, to reduce the influx of dust-laden air. Whether or not the recommended air cleanliness limits can be met without a positive pressure differential is very largely associated with the general cleanliness of the space surrounding the laboratory. If the air cleanliness limits can be met without a positive air pressure differential, there is no need to complicate the installation. Experience has indicated that 50,000-class and 100,000-class laboratories need the addition of positive air pressure.

A 50,000-class laboratory is one in which, under normal working conditions and with normal laboratory staff complement, the number of particles of 0.5 micrometer and larger does not exceed 1.8 million particles per cubic metre of air. This is equivalent to 50,000 particles per cubic foot of air.

Activities within the laboratory not directly related to the calibration activities and which tend to contribute to the generation of airborne particles should not be allowed. Such activities include soldering, smoking, eating, and drinking.

Lighting

A lighting level appropriate to the task being performed, and preferred by the people doing the specialized work, should be maintained in the work space. A lighting level of 500 to 1000 lux will be suitable for many laboratory work areas.

Vibration

The natural or induced vibrations present in the laboratory should not compromise the validity of the measurement results or affect the life of standards and associated equipment.

Acoustic noise

In order to provide a more pleasant work environment, the ambient acoustic noise should be kept below 65 dB, "A" weighting.

AC power

A reliable source of electrical energy is necessary. This source should be of suitable frequency and voltage, and with minimal distortion so as not to compromise the validity of the measurement results or affect the life of standards and associated equipment. It may be necessary to provide conditioned and regulated supplies for certain types of measurements.

Electromagnetic interference

Electrical and magnetic fields must be kept to a level that will not compromise the validity of the measurement results.

Barometric pressure

The barometric pressure can influence some measurements but is difficult to control in a laboratory environment. Metrologists should be aware of this influence and make any adjustments that are necessary to avoid compromising the validity of the measurement results.

General facilities

In order to provide better operational conditions for the measurements being performed in a laboratory, it is desirable that a laboratory providing Type I or Type II calibration services should be physically divided into two distinct work areas.

Receiving and cleaning area

This area should be equipped so that equipment can be cleaned and prepared to enter the laboratory. The equipment should be cleaned and dismantled as required to ensure that dirt, dust, protective greases, or unnecessary covers or packing boxes are excluded from the calibration area.

Calibration area

Calibration activities should be conducted in an area established for and dedicated to this purpose. It should be separated from receiving, cleaning, and other such activities. Equipment for calibration should be conditioned and stabilized to the calibration environment before calibration is attempted.

Repair and adjustment

If repair and adjustment are carried out by the calibration laboratory, it should be done in the stabilized calibration area. However, the environmental quality of the calibration area should not be compromised by activities that create unwanted particulate matter.

Mobile and on-site calibration operations

It is difficult to accomplish such separation of functions in a mobile or on-site calibration operation. In a mobile vehicle, particular care should be taken with housekeeping, to decrease the accumulation of dirt, dust, and grease.

General

To be deemed capable of making adequate measurements, calibration laboratories should provide a facility with adequate environmental controls, appropriate for the level of measurements to be made. Table I summarizes recommended environmental conditions for Type I and Type II mechanical/dimensional, electric/electronic and other laboratories, and recommended environmental conditions for Type III laboratories in general.

Control of environmental conditions

Type I and Type II calibration laboratories should have continuous control of the operating environment according to the recommendations in Table 1.

The nature of the Type III operation will require special consideration of the environmental factors listed above. Bear in mind that for Type III operations, some of the auxiliary or support equipment used in the calibration ensemble may be supplied by the client.

Type III laboratories should meet the environmental recommendations outlined in Table 1. However, it is recognized that this may not be possible at all times.

NOTES:

1. If these conditions are not maintained at all times, a stabilization period of at least three hours during which the conditions are met is recommended before calibrations are carried out.
2. Without continuous control of environmental conditions, it is possible that unusual ambient conditions may invalidate some of the calibration reports on the reference standards or on the reference instruments, or even damage the instruments. As a guideline, the conditions during uncontrolled storage should not exceed limits of 5 °C to 35 °C for temperature or 5% to 70% for relative humidity without special protection.
3. When the suggested environmental conditions cannot be established, the calibration report must include an appropriate allowance for the uncertainties introduced by the difficult conditions and the associated effects on the reference standards and instruments as well as on the instruments being calibrated.

Quality control system

The laboratory should establish and maintain an effective quality control system that will include documentation of quality control procedures, work instructions, calibration procedures and records.

Traceability of measurements

Reference standards of measurement, reference materials, and all measurements made by the laboratory should be traceable to appropriate designated standards.

Uncertainty list

The uncertainty list of the measurements made in the laboratory should be related to appropriate designated standards and kept up to date for all measurement processes undertaken. The component uncertainties in the list should be combined in an appropriate way to arrive at an estimate of the total uncertainty. The method of combining these uncertainties should be recorded. See CLAS Requirements Document 5 General Guidelines for Evaluating and Expressing the Uncertainty of Measurement Results.

Measurement process control

Each measurement process should have available adequate supporting data to demonstrate a state of control, e.g. through techniques such as Statistical Process Control (SPC), using check standards, or other means.

Calibration system

The laboratory should implement and maintain a suitable calibration system. Procedures should be available for the calibration of test equipment and measurement standards, with appropriate analysis of the uncertainties introduced by the calibration procedures.

Calibration records

A separate record should be maintained for each in-house standard and item of test equipment. These records should be retained for the life of the equipment and should include the following information:

1. Description of equipment and unique identification
2. Date received and placed in service
3. Condition when received (new, used, reconditioned)
4. Current location, if appropriate
5. Copy of the manufacturer's instructions, where available
6. Dates on which the standards and calibration equipment were calibrated and, if in-house, the identity of personnel performing the calibration(s)
7. Calibration interval
8. Calibration results and, for measurement standards, the data obtained
9. Designated limits on the ranges or functions of the equipment
10. Source of calibration
11. The environmental conditions during the calibration
12. A statement of the cumulative uncertainties in the data obtained in the calibration
13. Details of any maintenance (servicing, adjustment, repairs) or modifications, malfunctions, or damage that could have affected the calibration status
14. Any limitations in use
15. Reference to calibration procedure(s) used, when done in-house

Client reports

Although certificates of conformity to manufacturer or other organization specifications (e.g. ASTM) are frequently provided to clients, calibration certificates with actual measured values or correction factors are the most meaningful and are preferred.

Reports and original test data should be retained for five years or five calibration intervals, whichever is longer, and made available when requested for review by the client or by a calibration laboratory accrediting agency. See CLAS Requirements Document 6 Requirements for Calibration Certificates issued by CLAS Laboratories for additional requirements related to calibration reports and certificates.

Intervals of calibration

Test equipment and measurement standards should be calibrated at periodic intervals established and maintained to assure acceptable measurement uncertainty and reliability. Intervals should be shortened or may be lengthened when the results of previous calibrations indicate such action is necessary or sufficient to maintain acceptable reliability. ISO 10012-1 Annex A and NCSLI RP-1 Establishment and Adjustment of Calibration Intervals contain guidelines for the determination of calibration intervals for measuring equipment.

Reliability means the probability of equipment remaining in tolerance during the calibration interval, and for standards, the probability of the assigned value of the standard not changing by more than the list of uncertainties. In the client's laboratory, the list of uncertainties should include any uncertainty associated with drift rate. The procedure for assigning calibration intervals and adjusting them should be fully documented. The laboratory should have a fully-documented recall system for all its test equipment and standards to assure timely recalibrations. The recall system may allow temporary extension of the calibration due date for limited periods of time under specific conditions such as the completion of a test in progress.

Labelling and calibration status

Test equipment and measurement standards should be sealed for integrity using a tamper-proof seal, where possible. The test equipment and measurement standards should be labelled to indicate calibration status and function and should have on the seal, as a minimum, the name of the organization performing the calibration, the calibration date, the calibration due date, and the calibration status. When the client is from the same organization, a reference to who performed the calibration can be used to replace the name of the organization. The seal or a separate label should indicate the function of the equipment or measurement standard by using a code format or by spelling out the function. Acceptable function names include: Reference Standard, Transfer Standard, Travelling Standard, Check Standard, Test Equipment, and Measurement Equipment. Where a function code is used, it must be documented. For items that are too small or where it would be impractical to affix a label, then the label should be affixed to the container of the item, or some other suitable form of labelling should be used. The labelling procedure should be fully documented and the documentation should include instructions on disposition of items with broken seals.

Labelling of limitations of use

Items of test equipment and measurement standards not calibrated to their full capability or which have other limitations of use should be labelled or otherwise identified as to the limitations. Limitation labels available should include:

1. Limited Use - used when not all instrument parameters have been calibrated or when the instrument no longer meets all original specifications.
2. No Calibration Required - indicates that the instrument to which it is attached does not require calibration.
3. Calibrate on Demand - indicates that the instrument to which it is attached is not on a regular instrument-recall system and is calibrated only as and when required by the user.
4. Invalidated Calibration - label acts as a visual identifier to prevent use of any instrument that has failed in operation; is past its calibration due date; is suspected of being, or known to be, outside its designated limits; or shows evidence of physical damage that might affect measurement accuracy.