There are basically two types of pressure canner; one is the weighted gauge, and the other is the dial gauge.
The appearance of a weighted gauge resembles a disk that is attached to a vent port, which controls the internal pressure. This disk would rattle during the processing, which releases some amount of air and vapor. As they are able to control the pressure during the processing, we do not need to constantly keep an eye on its precision and accuracy when it is processing. The sounds of the weighted gauge rattling are the indication that the canner is sustaining the correct pressure. One disadvantage of weighted gauge canners is that at higher altitudes where the air pressure of the atmosphere is different, the accuracy of the weighted gauge will be affected.
The other type is the dial gauge. Unlike the weighted gauge, you will need to keep an eye on the dial gauge. The dial gauge is stuck to the cover, where you can refer to its needle that point at the pressure available in the canner. When you get a new dial gauge, do have it tested to ensure its accuracy. Experts recommend that these should be checked yearly.
It is important that both types of pressure canner are accurate. If the readings are lower than the real pressure, the food inside will become over-processed. However, when it comes to safety concerns, this is not a concern. In contrast, readings that are higher than the real pressure can cause under-processing, which may cause the food to be unsafe to be eaten. This is because you need enough pressure to get the food cooked, and being under-processed means that there is not enough pressure to get the food cooked. Thus, it is important that your gauges are accurate to avoid any mishaps.