[volt-nuts] traceable calibration

[Andreas Jahn]

Hello together,

I have some questions to the calibration experts:

On monday I calibrated my voltage references at a friend of mine.
He has two  6,5 digits Keithley 2000.

Reading of my LM399 #2 was

6,86089 V on Keithley #1  (last cal 02 / 2010)
6,86082 V on Keithley #2  (last cal 05 / 2011)

both values sometimes tripping to the 10uV higher level.

giving 
6,86090 or 6,86083 as maximum values.

room temperature was 24,6 degrees (celsius)
humidity read from the hair hygrometer was 58%

So far so good. But what can I tell to the absolute voltage
of my reference with respect to absolute volt.

normally I would use the 1 year spec of the instrument 
which is 30ppm of reading + 5ppm of Range (10V)
giving a uncertainity of +/-260uV 
so I would spec the reference being
(6,86089+6,86082) / 2 = 6.860855V + / - 260uV

Calibration reference:
===============

On the other side I had the luck that the calibration 
protocol of the Keithley #2 was available.

Keithley #2 has been calibrated mid of 05 / 2011
at the german calibration lab of Keithley.
Calibration has been done with a Fluke 5720A calibrator
at a room temperature of 22,6 degrees and humidity 42%.
The calibrator itself has been calibrated end 02 / 2011
with due date end 05 / 2011. So the calibrator was near
end of the 90 day calibration cycle.

Readings of the Keithley 2000 in the 10V range where

reading        Error     Error (% TOL)

-10.00000     0 ppm     0%
- 5.00001     1,2ppm    3%
  0.0            100%      18,4%
  4.99999    -2,0ppm    5%
  9.99999    -1,5ppm    4,29%

further the calibration protocol states that the 
TUR (Test Uncertainity Ratio) was at least 4 or greater
and the Keithley was not adjusted during calibration.

The 90 day specs of the Fluke 5520A I have found
to be 1.5ppm of reading + 3uV relative to calibration
and: 3 ppm of reading + 3uV absolute.
(whatever "absolute" is meaning in this context).

So I am tempted to take reading of Keitley #2
adding 10uV (calibration offset between 5 and 10V)
and using the specs of the 5520A as tolerance.
For the temperature difference of 2 K between
calibration of reference against instrument I would use
the 2ppm of reading + 1ppm of range per degree celsius
spec of the Keithley.

so this would give a total uncertainity of  7ppm + 23uV 
 
Calibration history:
==============

2010:
====
I have compared my LM399 #2 already one year before
The readings of September 2010 where

Keithley #1 6,86089 V (last cal unknown)  (same as in 2011)
Keithley #2 6,86081 V (last cal 05 / 2009) (10 uV less than in 2011)

Temperature was 23,8 degrees (1 degree less than in 2011)
Humidity not recorded

And: the reference LM399 #2 is running parallel with the 
two LTZ1000A references which I have built at end of last year.

2009:
====

Keithley #1 6,86087 V (last cal unknown)
Keithley #2 6,86079 V (last cal 05 / 2009)

Temperature was 23,5 degrees (1 degree less than in 2011)
Humidity not recorded

but this measurement is uncertain by 30uV because it was 
before I wrote "top" and "bottom" on the housing of the reference. 
So I cannot tell in which orientation the calibration has been done.

Before and after calibration the measured values at home 
differed by 30 uV on this reference against my ADC with 5V Reference LT1027.
In 2010 and 2011 there was no significant shift (less than noise level of 2uV)
before and after transportation.

The conclusions from the history are:

Both Keithleys read a constant difference of 70-80uV @7V.
(so none of the Keithleys drifted during transport from calibration).

and: Drift of my LM399 #2 is less than 10-20uV against the Keithleys
during the last year.

Since the Keitleys have a LM399 as reference element
the worst case would be that all 3 LM399 references drift at the
same rate.

Questions:
========

- does one know what the "absolute" spec is meaning 
  in the context of a calibrator.
  Is this spec usually reached within a calibration lab?
  Do I have to add a additional uncertainity?

- what would You spec my LM399 #2 as absolute uncertainity?

- what can be derrived from calibration history. Can the
  absolute tolerance be limited further?

- how should the temperature difference between Keithley
  calibration lab and the not climatized room of my friend
  be handled?

- which influence has the humidity on calibration?
  (since the references are all in metal housings and 
  the 10V range does not need voltage divider resistors 
  I do not expect any significant influence)
  
- Under which conditions the instruments will be adjusted
  during calibration?
  When missing the 1 year spec or
  When missing the 24 hours spec?

- What can I derrive from the TUR spec of calibration?

- How are the ERROR and ERROR (% TOL) values
  calculated? It seems to be quite non-linear.
  10uV on the negative side gives 1,2ppm and
  10uV on the positive side gives 2 ppm for 5V.

with best regards

Andreas