# Understanding the absolute uncertainty specification for a Fluke 5500A

N

#### NickF

Hi All,
I have a discussion with a colleague about using the specifications from the Fluke 5500A calibrator. The operating manual can be found online at http://assets.fluke.com/manuals/5500a___omeng1100.pdf, and it shows at page 1.11 about DC Voltage specifications. The description for "Absolute Uncertainty" is "? (% of output + ?V)". If we check the range of 0 to 32.99999 V and for the interval of 1 year, the absolute uncertainty is defined as "0.005% and 50uV".

If I use the calibrator to generate a DC voltage of 10V, what is the absolute accuracy coming from 5500A? My understanding is that it is 0.005%*10V +50uV = 0.55mV.
If I got his point right, he explains that this uncertainty is stated for 33V and therefore the whole thing has to be multiplied by 10/33, therefore the absolute uncertainty he comes up with for 10V is (0.005%*10V+50uV)/3.3 = 0.167mV. We could not discuss it any further, because he got so aggravated by mental blockage and lack of understanding, that we had to stop the conversation

Regards,
Nick

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#### Stijloor

Staff member
Super Moderator
A Quick Bump!

Can someone help?

Thank you very much!!

#### dgriffith

##### Quite Involved in Discussions
I will look at it when I get to work (2 hrs.) and respond. What does the spec say about the 0.005%? Reading, full scale? Makes a difference.

DG

#### dgriffith

##### Quite Involved in Discussions
From the manual:
[FONT=Arial,Bold][FONT=Arial,Bold]Absolute Uncertainty Definition
The 5500A specifications include stability, temperature coefficient, linearity, line and
load regulation, and the traceability of the external standards used for calibration.
You do not need to add anything to determine the total specification of the 5500A for​
the temperature range indicated.
[FONT=Arial,Bold][FONT=Arial,Bold]Specification Confidence [/FONT][/FONT][FONT=Arial,Bold][FONT=Arial,Bold]Interval[/FONT][/FONT]
[FONT=Arial,Bold][FONT=Arial,Bold]99 %[/FONT][/FONT]​

[FONT=Arial,Bold][FONT=Arial,Bold][FONT=Arial,Bold][FONT=Arial,Bold]1-16. DC Voltage Specifications[/FONT][/FONT][/FONT][/FONT]

[FONT=Arial,Bold][FONT=Arial,Bold][FONT=Arial,Bold][FONT=Arial,Bold][FONT=Arial,Bold][FONT=Arial,Bold]Absolute Uncertainty, tcal ? 5 [/FONT][/FONT]?[FONT=Arial,Bold][FONT=Arial,Bold]C? (% of output + u[/FONT][/FONT][FONT=Arial,Bold][FONT=Arial,Bold]V)[/FONT][/FONT]

Range: (0 to 32.99999) V: 90 Day: (0.004 + 50) 1 Year: (0.005 + 50) 24 Hr. Stability: (4 + 30) Resolution: (10) Burden: (10 mA)

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For 10 Vdc, on the 33 Vdc range, within ?5 ?C of the temperature at which it was calibrated, the specification is 0.005% of 10 Vdc plus 50 microvolts (the floor).
So, I have it as 0.00005*10=0.0005 V + 50 uV = 0.00055 Vdc uncertainty at 99% confidence. 99% confidence has a nominal factor of 2.58, 0.00055/2.58 = .000213 V standard uncertainty. This is the value you would use when combining with other standard uncertainties in a budget. Of course, those RSS'd uncertainties would then be multiplied by 2 to get the expanded uncertainty at approximately 95%.
(This is why you can't just start combining quantities--different confidence levels.)​

This is the minimum uncertainty--there may be other quantities you need to consider.​

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