Over time, the dial gauge on a pressure canner can become inaccurate by showing either too high or too low.
If the dial shows lower than the actual pressure, then that means you will be running the pressure too high and over-processing your food, affecting food quality.
If the dial shows higher than the actual pressure, then that means there are safety issues: you aren’t achieving the pressure / temperature that you think you are, and which is required for safe processing.
Dial gauge errors: 1 pound error in a 20-minute process causes over 10% decrease in sterilizing value. 2 pound error a 30% decrease.”1
Consequently, all experts recommend a yearly testing of your pressure canner gauge before the start of canning season.
In the States, most University extension services used to offer it, and for free to boot. Not all offer the testing service anymore. Of those that still do, a few do it for free still, others charge a nominal fee now for the testing.
If you have a Presto, Presto will test your gauge for free — you just have to pay for the shipping to get it to them. Call their customer service to arrange it.
Or, if you have a Presto, you can just buy your own testing kit. See the entry on Presto Pressure Canner Gauge Testing for info on both.
If you have a weighted-gauge canner, you don’t need to worry about dial gauges, obviously. Weighted gauges never need testing, and in fact, there is no testing mechanism for them, even.
If you have a dual-gauge (dial and weight) pressure canner (such as All-American, and Presto with the 3-piece weight added), then you can just ignore the dial.
Note: according to the justplainmarie blog, car radiator repair shops may test and fix your gauges for you. Other bloggers, though, have said they have tried asking at places near them and had no luck.
Andress, Elizabeth L. Pressure Canning and Canning Low Acid Foods at Home. Cooperative Extension: University of Georgia. Powerpoint presentation, slide 37. Accessed March 2015 at nchfp.uga.edu/multimedia/slide_shows/CanLowAcid_web08.ppt. ↩