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Calibration of Stopwatches

Wow! What a debate!

I have calibrated many stopwatches and process timers during the 
last 5 years and I may try to answer some of the questions that are araising 
from this forum.


Clarification of Metrology Terms
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Metrology is a confusing world but yet, much work is done to clarify and
uniformise
the usage of terms by national metrology organisations throughout the world.

Here are some definitions that could help clarify the debate:
(I hope I won't have any problems with the copyright!)

Calibration: ?

{The set of operations which establish, under specified conditions, the
relationship
between values indicated by a measuring instrument or measuring system, or
values
represented by a material measure, and the corresponding known values of a
measurand.

Note: 	1	The result of a calibration permits the estimation of errors of
indication 
		of the measuring instrument, measuring system or material measure, 
		or the assignment of values to marks on arbitrary scales.

	2	A calibration may also determine other metrological properties.
	3	The result of  a calibration may be recorded in a document, sometimes 
		called a calibration certificate or a calibration report.
	4	The result of a calibration is sometimes expressed as a calibration factor, 
		or as a series of calibration factors in the form of a calibration curve.}

Note from the author:
	Nowhere is it stated that a calibration is intended to adjust the error of
the measuring
	instrument.  Its primary purposes is to establish the reading error of the
measuring instrument
	(comparison with an appropriate standard) and the production of a
calibration report.

Verification: ?
Translated by the author:

{Same as calibration (comparison with an appropriate standard) to verify
that the instrument is 
performing according to predefined specifications, but without the need for
reports as long as 
you keep a track of the act. For this you can use stickers, a simple
preformat verification certificate 
(The inst... performed according to... sign here, date...) etc.}

Note from the author:
	A verification is usually performed on GMP non-critical or non-GMP
instruments as part of Good 
	Engineering Practices GEP and to save on time and expenses 
	(QC revision, documentation management).  In house we simply apply a
sticker and archive the 
	verification raw data sheet in the instrument history file making it
available for customer audit .

Adjustment: ?

{The operation intended to bring a measuring instrument into a state of
performance and freedom from 
bias suitable for its use}.

Note from the author:
	Do not confuse with "user adjustment" which you have to add the following
sentence to the above 
	definition: {...employing only the means at the disposal of the user.}?.
This means that the 
	ajustments are made on the user interface side of the instrument and not
from the inside.


A real world example for these definitions for any instrument would be:

	Calibration
	Comparing the instrument against your company standard (which is
appropriate for the procedure)
	and obtaining "as found" data to calculate the deviation between these two
instruments and 
	registering these data in a calibration raw data sheet.

	Adjustment
	After noticing that the unit under test UUT is out of the manufacturer
specs, you perform an adjusment
	on the zero and span potentiometer inside the instrument in order to bring
it back to specs.

	Calibration
	After making these adjustments, you will have to perform an other
calibration in order to get the 
	"as left" value for the UUT which will again registered in the calibration
raw data sheet.
	Apply a sticker and make the calibration report.

	Verification
	You can apply the same reasoning with verification but without the need to
make a formal report
	(to save	time and expenses).

For more information I suggest obtaining a copy of these standards.

? International Vocabulary of Basic and General Terms in Metrology
CAN/CGSB-158.1-M87
    which have a conterpart in ISO, BIMP, IEC and OIML but I dont know the
reference.

?La fonction métrologique dans l'entreprise NF X 07-010 afnor 86422


Stopwatch and Timer Calibration / Verification
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Depending on the usage of the timer, you may need the service of a specialised
calibration lab with a cesium clock (that is for very precise
calibration/verification 
requirements for instruments such as oscilloscopes, high speed data aquisition 
systems, etc.), but when we are talking about the majority of
pharmaceutical and 
biothech applications, we could rely on cheaper calibration/verification
methodes

The first method that as proven to be good enough is to use one calibrated
stopwatch
which will be used as your company's time standard, and have all the other 
compared with it.

The second method is to get connected to your national standard institute 
(like NIST, NRC, BIM) via their talking clock or a short-wave radio clock
and calibrate/verify 
your timer against it.

Both methods have systemics errors which are well documented (via your
natinonal standard 
institute or calibration report) and need to be treated in your
calibration/verification procedure 
uncertainty of measurement statement.

Even if the timing needs for a particular timer is less than an hour, it
may be wise to 
follow current industry practices by using a 24 hour
calibration/verification cycle.
This will give you a standard base for inter-timer performance comparison.
This longer cycle also diminishes the impact of the human factor, which we
estimate at 
aproximatively 0,2 sec for timer activation-deactivation manipulations.


Stopwatch-Timer Reliability
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75% 	of all timers that I have calibrated were within manufacturers spec.
22% 	of all timers that I have calibrated have small discrepensies, but still 
	out of manufacturers spec. (³ 3 sec./day), however where still usable for
	the timing purpose they where needed for (no adjustment required).
3% 	of all timers that I have calibrated have large enough discrepensies
leading 
	to either adjusting or withdrawal of the timing instrument.

Some timers are provided with an adjustment capability (potentiometer) but
I agree that it
is not a large percentage of them.  To give you an example, the company 
Sper Scientific is selling hand held multi-function chronometers with a good 
accuracy and ajustment pot (found at Cole Parmer).

Forthese 3% there is an example that I could share with you that might help
some of 
us recall that we should not always be a
I-Have-Blind-Faith-In-All-Instruments person.
Most of these instruments are reliable and stable, but there is still a too
large proportion 
that are influenced by their environment and nobody is protected from a
defect, even
if the instrument is new.

I once performed a calibration on an autoclave timer which was a routine (
and left by some
to be not necessary) yearly calibration.  Since all sterilisation cyles
where not more than 
30 minutes I used the same time frame for the calibration.  At first I
discovered an error of 
2 min./cycle, but to make sure that a manipulation error was not made, I
recalibrate it a 
second time.  After 4 attemps a range of error between 2 min. and 5
min./cycle was discovered!

Suffice to say that my client threw away the autoclave timer card and
unfortunately they where
quite busy in the following weeks looking for possibly contaminated lots.



Regulatory Point of View
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Some say that there is no need to perform calibration/verification routines on
process timers.  Nonetheless, regulatory agencies require that you have to
be able to prove
the accuracy and the traceability of all critical measuring instruments
used in a cGMP cGLP
environment.  This reasoning has to be applied to all critical instruments,
from the expensive 
TOC analyser to the cheap stopwatch even if it was obtained for free at the
gas station for a full 
tank of gas... (if it is used in the above mentioned environment of course).

It is not the price of the instrument that determines the need for
calibration/verification, it is its 
defined usage and/or the regulatory requirements.


François Leduc

Calibration Specialist
Validation / Compliance Specialist

Validapro Inc.

f.leduc@validapro.com
Tel: (450) 668-1144
Fax: (450) 6681797







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