The default NPLC settings for Fast, Medium and Slow on the 2638A are listed below as 0.2, 1 and 10 respectively.
Q: What is NPLC?
A: NPLC is number of power line cycles. In DC Voltage, DC Current, and Resistance measurements, accuracy is reduced by power line induced AC noise. Using NPLC of 1 or greater increases AC noise integration time, and increases measurement resolution and accuracy, however the trade-off is slower measurement rates.
You can select a custom NPLC in the “test setup” menu. “Custom” is the default and is defaulted to NPLC of 10 (Slow). With this default setting of Custom, you can then set individual NPLC setting in the channel menu under channel options. Please note that the custom settings option is not available when using the 2638A front panel DMM inputs.
However, even with the standard NPLC settings of Fast , Medium and Slow it can be challenging to determine precise scan time for a mixed channel setup, (DC, RTD , TC etc.) as shown scan rate table. When you mix NPLC settings on different channels all you can really be assured of (without empirical testing) is that the channel will yield results with the digit resolution shown in the table above.