The multimeter measures the forward resistance as 0 and the reverse resistance as non - zero.
A mechanical meter generally refers to an analog meter. It makes the current flowing through the meter head of the pointer reach the rated value of the meter head through resistance shunting. At this time, it is exactly at the full scale.
High-voltage diodes can be composed of multiple sub-high-voltage diodes connected in series, or they can be connected in series and then encapsulated in one package. For the GTR (transistor) type, if it is a single diode, it is made of materials and processes with very high voltage resistance, and it cannot be measured by an ordinary multimeter.
The Rx10k range of the analog meter usually uses an additional 15V battery of type 10F20 to measure resistances above 100k.
For multiple high-voltage diodes connected in series, if a single tube has a voltage resistance of 1000V, 20 tubes are needed to achieve a voltage resistance of 20000V when connected in series. The voltage drop of each tube is 0.7V, so for 20 tubes, it is 14V. Therefore, most meters can only measure diodes with a voltage resistance within 10000V. To measure diodes with higher voltage resistance, the measuring instrument itself needs to have a higher voltage.
If the forward measurement is zero, it indicates that the diode has been broken down. The normal value is 7k when the voltage is 10000V. (It should be zeroed first, otherwise the meter may go off the scale.)
The reverse measurement is 17.2K, which accounts for one-third of the dial. Normally, it should be infinite. If it is damaged, it should also show zero resistance. Since the actual object has not been seen, it can only be speculated that the tube has been broken down inside, and a large amount of carbon powder appears. When measuring in the reverse direction, the measured value is the value of some diodes that are partially broken down or not broken down. When the diode is tapped lightly with a tool, this value should fluctuate. In the 10K range, if a diode is not broken down or is only partially broken down, the value itself is 700Ω, and the measurement instrument will show zero at the bottom.
Another possibility is that the tube is normal and has a low voltage resistance. So, the forward resistance is approximately zero. When measuring in the reverse direction, do not touch the device with your hand, otherwise the resistance of the human body will be measured. If there is no diode and you directly measure your hand, you will also get this value.
There is also a less likely but existing possibility. This phenomenon may occur in germanium material diodes. Such tubes have a relatively large reverse leakage current. When the forward conduction voltage is 10000V, it is less than 3K. There is a resistance value in the reverse direction, but it is rather random, which can be large or small, yet the tube is in good condition.
