Pressure gauges are classified as either analog or digital and are rated by their precision as it relates to the scale of the gauge. They typically come in two options. The first is 3-2-3, or Class B, gauges. With Class B gauges, the positive pressure range is divided into three sections. The middle half of the range is the most accurate, within 2% of the full scale. The first and last quarters of the gauge are accurate within 3% of the full scale.
The second option is 1%, or Class 1, gauges. With Class 1 gauges, there is a single pressure range and accuracy is 1% of the full scale within the positive range.
The variation between 3-2-3 and 1% accuracy may not seem significant, but the difference between 1 and 3 percent full scale is considerable. For example, if you’re working with an 800 PSI gauge, a 3-2-3 gauge can be off by plus or minus 24 PSI of the true reading, whereas a 1% gauge can be off by plus or minus 8 PSI of the true reading. For the highest accuracy, choose a Class 1 or digital gauge with 1% accuracy or better.
Source: The BUZZ by Yellow Jacket