bmta.co.uk
Test Equipment
It was decided that an automated solution was required to
minimise the risk of injury and length of calibration time. We,
therefore, had to either modify the existing systems or invest
in new equipment.
After reviewing several automated solutions that were on the
market or due for launch Trescal decided to invest in 5
Stahlwille Perfect Control systems with Torkmaster software.
Image
Fig. 2. Stahlwille Automated loader bench
Software
Due to a difference in the interpretation of the standard
between German and UK accreditation bodies, the Stahlwille
Torkmaster software did not initially fulfil all UK requirements
for continued use against ISO 6789:2003. We worked closely
with the manufacturer to specify and achieve the required
modifications to the software and also developed our solution
as an interim measure.
Part 1 certification
In the UK we are not permitted to offer a certificate of
calibration against part 1 of the standard, only a declaration
of conformance can be issued. However, in France they are
accredited to part 1 and part 2 and can issue a certificate for
either.
Statistical analysis
The position is unclear. The standard states that uncertainty
contributions “can be determined statistically with a sufficient
number of specimens (at least 10).” In the UK the Trescal
interpretation was the data from 10 different units of the same
type. However, in France, the collection of 10 sample data
sets from the same tool is acceptable.
Other assessors suggest that each tool must be considered
uniquely. At present, we have no firm conclusion but intend to
prove our chosen method statistically.
Calibration failures
ISO 6789:2003 stipulated that measures were made
no lower than 20% of the tool range irrespective of the
markings on the tool. Since ISO 6789:2017 states that
calibration will be from the lowest marked value we are
now testing some older tools much lower than
previously and are finding tools that historically passed
(at 20%) are now failing or have a very high
uncertainty due to poor repeatability. This is causing
some concern and confusion with customers but does
emphasise the risk in the design or manufacture of
some tools to the old standard.
Certification
Standards: ISO 6789:2017 requires a measurement
uncertainty interval (W’) for each transducer used in
the calibration system. Historically this was not
provided on calibration certificates and therefore BS
7882:2017, “Method for calibration and classification of
torque measuring devices” [3], was issued and Trescal
UK also had to achieve accreditation to this standard
at the same time as achieving accreditation to ISO
6789:2017.
Reporting results: to state compliance of a torque tool,
the calculated measurement uncertainty interval (W’)
has to be compared to the measurement uncertainty
interval provided by the customer. As this has
previously not been required, most customers are not
familiar with this requirement and have often been
unable to provide this data.