The Adjustment Dilemma

"Adjustment found necessary" -- a phrase that produces mixed feelings.

If your equipment is returned after its periodic calibration with a Calibration Certificate stating that the equipment was found to be outside of its published specifications and that it was adjusted to restore specified accuracy, you will probably have mixed feelings. On the one hand, you will have justified the cost and inconvenience of having the equipment removed from your site and calibrated. But on the other hand it will provide you with a new problem:-

• Was the equipment used in its non-compliant state?
• Would this degraded performance have had a detrimental effect in any use made of it?
• Will a recall of this work be necessary?

The last question is clearly of real concern to the quality-conscious user and one which, to prevent unnecessary customer alert and rework costs, you will ultimately want to avoid unless it is necessary beyond doubt.

The Dilemma

The validity of the statement made by your calibration provider that parameters were outside specification is critical to your own recall decision.

If the calibration process adopted by the supplier allows for adjustment during the testing procedure (often the case), incorrect conclusions are drawn regarding the "on-receipt" calibration status and the specification conformity of the newly calibrated product.

Adjustments rarely affect only one test point. Not only may they influence the calibration status of subsequent testpoints, ranges or parameters, but earlier ones too. For example, a mid-test adjustment might mean that subsequent checks are incorrectly reported to fail specification. Alternatively, the adjustment may improve performance of subsequent testpoints which should have been reported to fail specification.

The Impact

The true performance of the equipment, as last used, is lost as soon as any adjustment is made, and the user is misled. You may recall product from your customers for re-test unnecessarily, or worse, could be led to believe that non-conforming equipment was within specification during previous use.

And it doesn't stop there...... This lack of integrity also impacts your equipment's accuracy AFTER calibration. Users expect equipment to be returned to service in a specification-compliant state but, since adjustments may influence functions and testpoints checked earlier, this would be a false assumption and the equipment cannot be relied upon.

The Dilemma Resolved

The solution? This situation is one which our own metrologists considered at length when defining the calibration services we offer. The product that we recommend to quality-conscious users and those with, or seeking, ISO9000 certification avoids the problems identified earlier through the following process:-

1. An item received for calibration to this specification firstly undergoes a complete and thorough performance test.
2. All test results are recorded. No adjustments are made at this stage.
3. The results, with failed parameters highlighted (if relevant), are labeled ("pass" or "fail") and will be supplied to the customer.
4. Then, if any parameters were non-compliant, corrective adjustments will be made.
5. The full performance test is then repeated, with all results recorded. If the performance test is passed, the second set of results is annotated accordingly, and the equipment is labeled, sealed for integrity and prepared for shipment.

With the newly calibrated equipment the customer receives:-

• The entire set of "on-receipt" test results, clearly identified.
• The entire set of post-adjustment results (if relevant).
• A Calibration Certificate carrying a clear statement of the calibration status, both as-received and at completion.
• Assurance that the equipment is now in a specification-compliant condition (integrity sealed for confidence).
• Confidence that the "on-receipt" status report is accurate.

We do not believe that any other calibration process stands up to scrutiny.