Unfortunately, not all calibrations are equal, and not all instruments require the same type or level of testing to verify that they are fit-for-purpose and performing within expected accuracy. Due to this, we are strong advocates for transparency in testing methods used by all calibration laboratories, and ensuring that laboratories have a capability that is suitable for confirming the performance of instruments prior to accepting them for calibration. 

Recent changes to ISO/IEC 17025 have thankfully placed increased requirements on calibration laboratories so that they must now disclose or negotiate with end the user about testing methods used, accuracy requirements, and their ability to assess the instruments compliance to the accuracy specification.

At APL we extensively use the Metrology Society of Australasia Test Methods for Pressure (MSA Test Methods 1 & 2), which clearly outline the testing requirements, the need to determine an accuracy specification, and rules on determining compliance.  For some specialised instruments that are not within the scope of the MSA Test Methods, we use other published methods such as Euramet CG-3 for Dead Weight Testers and ISO3567 for vacuum calibrations using our Vacuum Comparison System (VCS).  For electrical instruments, it is more common for calibration regimes to be prescribed by manufacturers, which we utilise in combination with our own internally developed method which has been guided by Euramet CG-15.

Does it comply?

Determining whether an instrument has or has not performed within specification is not always as easy as comparing the size or error/correction with the accuracy specification, as there is naturally some uncertainty in the error/correction reported.  ISO/IEC 17025 requires consideration of uncertainty in determining compliance, and more detailed guidance on valid approaches is provided by ILAC. 

At APL, we adopt the following practices by default (users are welcome to specify their own criteria if they prefer):

• Mechanical pressure instruments:

  Method Used: MSA Test Method 2

  Accuracy Spec: Manufacturer's specification or based on dial size as defined in MSA Test Method 2

  Decision Rule: Neither corrections or uncertainties can be greater than the accuracy specification at any point, and reference uncertainty must be within limits as defined by MSA Test Method 2.

• Digital pressure instruments:

  Method Used: MSA Test Method 1

  Accuracy Spec: Manufacturer's specification, unless requested otherwise by the user

  Decision Rule: The sum of |correction| + uncertainty must not be greater than accuracy specification at any point

• Digital electrical instruments:

  Method Used: APL internal method in accordance with Euramet CG-15

  Accuracy Spec: Manufacturer's specification, unless requested otherwise by the user

  Decision Rule: The sum of |correction| + uncertainty must not be greater than accuracy specification at any point 

• Medium/high vacuum gauges (i.e. Pirani & CDG)

  Method User: ISO3567

  Accuracy Spec: None used (unless requested)

  Decision Rule: No compliance statement provided unless requested. These are specialised devices, often tested over limited ranges to suit specific applications. We can discuss any specific accuracy requirements on a case-by-case basis if a compliance statement is required.

• Dead Weight Testers

  Method Used: APL internal method in accordance with Euramet CG-3

  Accuracy Spec: None used 

  Decision Rule: No compliance statement provided.  These are specialised reference devices and their performance is highly dependant on local environmental factors.  We will provide a statement of uncertainty and generated pressure envelopes at some assumed or user-specified conditions.

In all cases, we are very happy to discuss specific requirements users may have for accuracy, decision rules, uncertainty, tested points or test conditions, so feel free to discuss your requirements with us prior to calibration.