When using the 8508A in 2-wire resistance mode, finding the total uncertainty can be a complex task.
To start, we need to consider the base specification. I am using Normal Mode and I am making the assumption that I am not going outside of ±1 °C from the temperature at which the 8508A was last zeroed. I am using the 1 Year specification and using the absolutely uncertainties to include the effect of the last calibration.
Next I'll review the applicable footnotes for this table:
[1] Specifications apply for max resolution in each function, normal mode
[2] Assumes 4 hour warm-up period
[3] Input zero or offset null required whenever the temperature moves more than ±1 °C from the temperature at which the previous null/zero was performed.
[10] Tru Ohms mode available on 2 O to 20 kO ranges. Read Rate reduced in Tru Ohms Mode. Specification for Tru Ohms same as corresponding Normal or Lo Current range
[15] The maximum display value for the Analog to Digital converter is 199 990 000 counts. This sets the maximum value measurable on each range to be a one followed by four nines. For example, the maximum measured values on the 2 V range on DC Voltage are ±1.999 900 00 V. However, the 1000 V ranges are limited to a maximum 1050 V.
None of these footnotes apply to my example, so no additional uncertainties need to be added from them. Since I have already stated that my temperature has not varied more than 1 °C, footnote 3 does not require further action.
Next I will look at the secondary specifications for Normal mode.
I am not doing transfer mode or adding anything for temp in this example so I can disregard it. I do need to make note of my measurement current because that contributes to my 2-wire adder formula.
Footnotes for the secondary specs are the same as above and like above I can ignore it all together.
Thus far I have 9% of reading plus 0.7 parts per million (ppm) of range.
Now we can look at the 2-Wire Adder formula.
I can ignore the ±3 m?/°C as I've stipulated that my temperature is still within 1 °C.
My Ir is 10 mA (taken from the secondary specifications table).
Here is my formula for the adder:
±(10 pA/10 mA) * 1E6 ppm of reading ± 50 mO
We can calculate this to be:
((10E-12 A / 10E-3 A) * 1E6 ppm) * 10 O + 50 mO, or
(0.01 ppm) * 10 O + 0.05 O = 50 mO
In the case of this example, the 10 pA/10 mA * 10E6 PPM of 10 O becomes such a small number (10E-9 O) that it is several orders of magnitude smaller than the fixed contribution. So assuming a reading of 10 O, the only additional adder you need is the 50 mO.
My initial spec was:
9% of reading plus 0.7 ppm of range
My 2-wire adder is 50 mO.
So my total specification will be:
(9% * 10 O) + (0.7E-6 * 20 O) + 0.05 O = (0.09*10 O) + (0.000007 * 20 O) + 0.05 O, or
±0.950014 O.