Hello everyone,
This is my first post on this forum. I hope everyone is doing well. I just transferred to the metrology field last year and still learning the basics. Here's one thing that I don't really get and would appreciate some help.
Here's the scenario. I'm taking a current measurement of nominal 0A from a power supply and want to calculate the uncertainty. The shunt standard used is 1Ω and has tolerance of 0.01%. A DMM standard is used to measure the voltage drop across this shunt. When the nominal is not zero, I can do it because I just find the uncertainty of each standards in ppm, then use Root Sum Square to find the combined uncertainty in ppm. Then I can multiply that uncertainty with the nominal to get it in absolute unit.
When the nominal is 0, 0 A in this case, I don't know what to do. The voltage reading across the shunt would be zero. If the accuracy of the DMM is (5ppm of reading +3ppm of 100mV range), that gives me ±0.0003mV. BUT the theoretical voltage reading is 0V, I cannot divide 0.0003mV/0V to get the fractional uncertainty.
Then I run across the same issue when I try to get a combined absolute uncertainty of the current measurement of 0A because it is zero. If I somehow got a combined fractional uncertainty and try to multiply it by the nominal zero, it comes out to zero; doesn't work.
What should be my approach in this case?