Guidance requested.
So I have:
- A Datron 4808 (not calibrated recently AFAIK, and no way to check),
- A 3458A which is about 6 months out of calibration (sadly not by
Keysight), - Three 7081s two of which are also 6 months out of calibration against
the 3458A, - A 720A.
The readings of the output of the Datron for the 1V, and 10V, 100V and 100V
ranges on the 3458A (NPLC100) are:
1.00000177V (high limit 1.00000480V),
9.9999964V (low limit 9.9999720V,
99.999380V and climbing slowly (low limit 99.999550V) In spec some five or
so minutes later.
999.98xxxV (and falling) (low limit 99.99450V) So definitely out of spec
To be fair both the 4808 and 3458A have only been on for about 8 hours -
things may improve :).
The two "recently out of calibration" 7081s disagree with the 3458A but I've
not made a record of their readings (a job for tomorrow). The other 7081 is
not in consideration at present as its of unknown provenance.
I also have out of calibration Cropico VS10 and Fluke 732A.
I'm going to run the 720A through its self-calibration tomorrow.
I can't begin to afford to send the 4808 to Fluke or the 3458A to Keysight
for calibration. Even local cal. labs are a bit pricey, and I'm not over
sure I trust them to get this sort of equipment correctly calibrated, given
that they normally only handle PAT testers, 3.5 or 4.5 digit DMMs, and maybe
the odd 6.5 digit meter.
So please how best to proceed from here?
Thanks
Dave
If this is for hobby nut purposes, I'd go next:
-
Test 3458A to make sure it's all nice and stable. Refer to SN18 procedure as example test.
-
Test 732A to make sure it's stable on 10V. Absolute output at this point doesn't really matter, drift is what matters.
-
Get some decent 10KOhm resistance standard. You'd like something with <2ppm/K TCR and <5ppm/year stability. In worst case get few (4-10pcs) VPG Foil VHP hermetic 10K resistors (even 1% are fine, you need stability, not absolute accuracy).
-
Test resistors, make them transportable in a nice sturdy box. Prep some decent wiring sets.
-
Find happy lad who'd be ready to measure your 732A/10K resistance standard for beer/juice/whatever suitable. Log temperatures during test to guardband uncertainty a little.
-
Get your 10V and 10K standards back, let them rest for week or so, under power.
-
Calibrate your 3458A against these standards. You should get <4ppm DCV, <8ppm Ohms pretty easily now.
-
Calibrate your 4808 vs 3458A
-
Calibrate 720A.
-
Modify/calibrate 7081's and keep an eye on all gear to maintain deltas for next 2-5-10 years.
---- On Mon, 10 Jul 2017 05:01:41 +0800 David C. Partridge <david.partridge@perdrix.co.uk> wrote ----
So I have:
-
A Datron 4808 (not calibrated recently AFAIK, and no way to check),
-
A 3458A which is about 6 months out of calibration (sadly not by
Keysight),
- Three 7081s two of which are also 6 months out of calibration against
the 3458A,
- A 720A.
The readings of the output of the Datron for the 1V, and 10V, 100V and 100V
ranges on the 3458A (NPLC100) are:
1.00000177V (high limit 1.00000480V),
9.9999964V (low limit 9.9999720V,
99.999380V and climbing slowly (low limit 99.999550V) In spec some five or
so minutes later.
999.98xxxV (and falling) (low limit 99.99450V) So definitely out of spec
To be fair both the 4808 and 3458A have only been on for about 8 hours -
things may improve :).
The two "recently out of calibration" 7081s disagree with the 3458A but I've
not made a record of their readings (a job for tomorrow). The other 7081 is
not in consideration at present as its of unknown provenance.
I also have out of calibration Cropico VS10 and Fluke 732A.
I'm going to run the 720A through its self-calibration tomorrow.
I can't begin to afford to send the 4808 to Fluke or the 3458A to Keysight
for calibration. Even local cal. labs are a bit pricey, and I'm not over
sure I trust them to get this sort of equipment correctly calibrated, given
that they normally only handle PAT testers, 3.5 or 4.5 digit DMMs, and maybe
the odd 6.5 digit meter.
So please how best to proceed from here?
Thanks
Dave
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David wrote:
So please how best to proceed from here?
The Fluke 732A is by far the most accurate and most stable voltage
reference you have. There are only a few cal labs worldwide that can
calibrate it to an uncertainty that does it justice (1/10 ppm). Most
good to excellent cal labs have 10v DC uncertainties from 1-10 ppm.
SO: First, have the 732A calibrated by one of those few labs. I
strongly favor the Fluke US cal facility (Fluke Park Laboratory,
Everett, WA, USA, NVLAP Lab Code 105016-0), with an accredited DC
calibration uncertainty of 100nV at 10v. However, it may be an issue
getting it back to you still under power from there (IIRC, you are in
the UK). I maintain three 732As and calibrate each one annually on a
rotating schedule, one every 4 months. I built a shipping container
with internal batteries that will keep the 732A powered for three days,
if necessary, and an internal charger so all Fluke has to do is plug the
shipping container in to recharge them.
If the choice comes down to a lab that can calibrate the 732A with full
accuracy (1/10 ppm), but it will arrive back to you "cold," and a lab
that cannot calibrate it to that standard in the first place, I'd choose
the former. In my experience, 732As generally retrace within 0.25 ppm
if re-powered within a few days. IOW, I recommend the Fluke US cal lab
unless you have a trustworthy cal lab with 1/10 ppm uncertainty local to
you. (But although experience suggests that you can rely on the
re-powered standard to be within 0.25 ppm of the calibrated value, you
will have lost NIST traceability.)
So whatever you do, have the 732A calibrated properly before you do
anything else.
Also, put the 732A on its own dedicated 2-3 kW "online"-type UPS. If
the UPS runs nothing but the 732A, it should carry you through all but
the longest power outages. Adding external batteries is a good plan
(industrial "online" UPSs support this). I have a backup generator, but
even so my UPS system will run the 732As, my primary frequency
references, and transfer DMM for more than a week if necessary.
Note that "calibrating" the 10v output of a 732A generally does not
involve adjusting it. Rather, the cal lab will tell you its actual
voltage to 1/10 ppm, and you will calculate from that value when you use
it. I usually have a new-to-me standard adjusted the first time I have
it calibrated, unless I obtained it with its full calibration history
from an unimpeachable source. I obtained two of my 732As this way, one
I repaired. I had that one adjusted at its first two calibrations.
Finally, note that proper calibration of a 732A is not cheap. Mine are
on a calibration contract, and I still pay over $1000 (plus overnight
shipping two ways on 40+ pounds). That said, the real value of a 732A
is its calibration history. Without a proper calibration and the
calibration history, it is just another voltage reference that needs
characterization before it can be relied on.
Best regards,
Charles