For a current outputting device I’m using a data acquisition board which can only measure voltages.
To translate the currents to voltages I use similar to the following shunt resistor: http://docs-europe.electrocomponents.com/webdocs/13ec/0900766b813ecc53.pdf
These types of ultra-high precisions are used for 4-20mA current loops. In the data sheet the tolerance is mentioned as ± 0.005%. It means I can trust that this resistor R is such that 499.975 < R < 500.025 ohm.
So far so good… To prove and document the resistor’s resistance value, I’m using a calibrated current test device which has a certificate for 4mA and 20mA nominal currents. According to the certificate of this current regulator device, for 4mA mode the calibration institute measured 4.0055mA ; and for 20mA mode measured 20.0072mA.
I apply current across the resistor with this trusted current tester at 4.0055mA and 20.0072mA and I read voltages as 0.990V and 4.995V in order with the data acquisition board. This means I obtain resistances as 247.12930 ohm and 249.67509 ohm.
The data sheet of the ultra-high resistor claims its tolerance is ± 0.005%. It means R is such that 499.975 < R < 500.025 ohm but I’m measuring 247.12930 ohm and 249.67509 ohm at different currents which are both less than the data sheet claim.
The current tester claims the uncertainty is ±0.00040mA at 4mA mode and ±0.0035mA at 20mA mode.
Either the regulator is not regulating well or the data sheet is wrong. But both the current regulator and resistor have certificates. Should I trust the resistor data sheet or calculated resistor value from my measurements?