Solid-state devices are always powered by direct currents and voltages . Vacuum tubes require DC voltage sources as well. Thus many measuremtents are made in the DC mode. Many systems today have built-in diagnostic meters so that power supply parmeters can be monitored and logged.
Accuracies of a typical DC bench-type digital multimeter can be as good as 0.002 percent (1 count of reading for a 6 1/2 digit instrument). This accuracy is often available on the lower voltage ranges usually below 300 V；for higher voltages accuracies may be slightly worse.
The point here is to read the instrument’s specifications carefully, and if things are not clearly spelled out, contact the instrument manufacturers’ application. engineering department.
High-voltage DC measurements usually involve a high-resistance voltage divider, which usually forms a high resistance to the circuit being measured and thus draws infinitesimal current. One common need for measuring DC voltages in the range of 2 to 30 kilovolts(kV)is in measuring cathode-ray tube(CRT) final anode voltages. Many CRTs used in colour television sets and computer monitors have voltages in the range of 15 to 30 kV. The accuracy of such measurements usually is not critical and should be in the range of 3 to 5 percent of the reading. A highly insulated hand-held probe containing the voltage divider is often used and comes as an accessory from some manufacturers.