Final answer:
The question does not specify the minimum rate of null measurements needed, but null measurements are a technique that avoids drawing current and altering the circuit. Digital meters with this method can be highly accurate.
Step-by-step explanation:
When using passive devices, if fewer than 50 tests are conducted each month, residential measurement service providers are to perform null measurements at the minimum rate that is not specified in the given information. However, null measurements are a technique in which no current is drawn, thus not altering the circuit at all. Digital meters that use solid-state electronics combined with null measurements can provide very high accuracies, achieving one part in 106.