Sent my fluke 289 in for calibration/certification etc as I do every few years. No point having a super expensive complex multimeter if its not absolutely accurate. I tested it against many cheapies over the years. They are pretty inaccurate in some cases. When they calibrate its not a simple process. They test it at various temperatures and at many points along a current, and voltage scale, frequenciey, resistance, capacitances etc etc. And its got a lot of measurement scales and types that most multimeteres cant do.
I know for a fact that my cheap fluke meter reads spot on on ac volts, at the high end, like 240V. As long as its a clean sine wave. And its miles out when reading lower voltages. And it reads mV considerably low. It also undereads at 10A and overeads at say 25 or 50mA. So a simple voltage reference calibration wouldnt be any use. Hence they have some rather technical equipment and it takes a good couple of hours to fully calibrate a complex meter that does pulsewidths, super low resistances, duty cycles, capacitance, frequencies, and peak/high/low values, ac/dc and dc/ac measurements for noise etc. So £££. But then I KNOW its correct and very accurate everywhere. Is this important? For most stuff no. For some hobby stuff yes. Not on a powerchair really. But some cheap meters are *really* crap and can be say a 0.2V out on a battery measurement.
ANYWAY...
They found a bad capacitor on the board that just keeps the clock accurate when battery is removed. Thats all it does. Dont know how since it still keeps the time when I do it. Its 10 + years old. They just sent me an email saying they are awaiting delivery of new units. They are replacing it with brand new 2021 one with newer firmware. You gotta love flukes warranty! How are they still in business.