Russ wrote:
What is considered the break-over point of precision with low uncertainty
versus cost to a group like this? Is there a rule-of-thumb for the cost of
each additional digit of precision after N digits?
One person's opinion:
To a group like this, I'd be inclined to say that interest begins at
a room-temperature (say, 20C +/- 3C) accuracy of 3ppm (i.e.,
guaranteed to remain within 3ppm from 18-22C for at least one year
after purchase). 3 ppm is 0.0003%. There is at least one 10v
reference with specifications in this ballpark available at an asking
price under $130 (I'm told the seller has accepted offers
significantly lower than this).
If I sell someone a reference
that I've ascertained is 2.50163v @70.3 F with a calculated uncertainty, is
it valuable as a 0.1% reference even though the error may be much less,
like +/- 0.08%?
I, for one, do not consider 0.08% to be "much less" than 0.1%. One
sneeze and it's out of spec. Indeed, I would consider a claim of
0.1% accuracy to be bordering on fraudulent based on a calibrated
measurement at 0.08%, unless the spec was qualified as "within 0.1%
at [temperature within 0.1C] as is, where is -- no claim as to
accuracy after it has been shipped to the buyer."
Speaking as someone with substantial commercial design experience, I
would never offer a voltage reference for sale as a claimed "0.1%
standard" that I did not have excellent justification for believing
would stay below 0.05% for a year over a several-degree range of
temperature and multiple trips across the country via commercial
carriers. I wouldn't expect to be able to charge more than $10-15
for the product just described, and then only if the nominal output
voltage were 10v (I think you will find that there is a very strong
preference for 10v references over 5v, 2.5v, or other voltages).
Best regards,
Charles
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