Decreasing the digits of resolution on my DMM causes incorrect readings when measuring coil resistance. Hardware: Modular Instruments>>Digital Multimeters (DMM)>>PXI-4070
Problem: I am measuring a coil resistance. When I set my DMM to measure at 6.5 digits of resolution my readings are all good. When I read at 5.5 Digits my readings are off by +/-5 Ohms. When I measure at 4.5 Digits my readings are off by about +/-10 Ohms. When I am measuring resistance in the 50 to 100 Ohm range, why are my readings off when I decrease my resolution? Solution: The reason your values decrease in accuracy as you decrease your digits of resolution is because of "aperture time". When measuring a coil resistance, a back EMF is generated as soon as the switch is closed this back EMF will dissipate after a very short amount of time. When you measure at 4.5 digits the default aperture time is 20 µs (NI 4070/4072). When you measure at 5.5 digits the default aperture time is 500 µs (NI 4070/4072). When you measure at 6.5 digits the default aperture time is 100 ms. So, to ensure you are getting correct readings when measuring a coil resistance, measure at 6.5 digits or programmatically increase your aperture time to 100ms. Related Links: Attachments:
Report Date: 10/13/2006 Last Updated: 10/24/2006 Document ID: 42C4SU96 |
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