Quote:
Originally Posted by jr_tech
Have you checked the accuracy/linearity of the basic meter movement in the device? I suspect that may be a source of errors greater than 1.25% no matter what you can achieve by selection of other components.
jr
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+1 - Basic Metrology will always tell you that without center null, the best accuracy achievable with a D'arsonval meter movement full scale versus cardinals is ~2%. It's easier to achieve max accuracy with an analog movement at
one point on the scale. Differential voltmeters using an analog meter are always null (center null) for this reason. Linearity with an analog D'arsonval movement like that shown is +/- .25% at best in panel mount (near another magnetic source) applications.
Good luck in achieving 1.25% - if you do, you'll have beat ol' man Fluke and his millions....
As to selecting resistors, make sure you match the TC, else you'll have resistors fighting each other when the temperature varies. In a VTVM, cold resistance is not operating resistance, as you'll have about a 10-15 degree rise (celcius) when that tube heats up. Enclosed? More temp coefficient variations...