The first thing you need to know when selecting a pressure gauge is what pressure range (pounds per square inch, or “psi” is standard) you need. Since the accuracy of most pressure gauges is best in the middle third of a gauge, you should always select a gauge with a range that is about twice your normal operating pressure. For example, if you have an air compressor with a normal working pressure of 50 psi you want to select a gauge with a 100 psi range.
The rule of thumb with pressure gauges is that when the operating pressure of the system is normal, the needle should be pointing straight up or in the “twelve o’clock” position. This makes the gauge easier to read at a glance – if the pointer is straight up, things are normal. Gauges are most accurate in the middle third of their range, so you’ll also get a better measurement at your most typical pressure.
If you cannot find a gauge at exactly twice your working pressure, go to the next step up. For example, if you want a 160 psi gauge and it is not in stock, a 200-psi gauge can be substituted. If the range is too low and the gauge is over-pressurized it will break. You want some room to spare at the top of the scale so that if your pressure goes above normal, it won’t damage the gauge.
Dial size refers to the diameter of the circular face of the gauge. The easiest way to choose a size is to measure the diameter of your old gauge. Gauges range in dial sizes from 1-inch for tight spaces to 10-inches for reading from a distance. When selecting a gauge choose one that fits within the physical space available and a gauge size that is easy to read where you have it installed.
If dial size is not important to you consider selecting a 2 ½” gauge. If there is such a thing as a “standard” gauge size, this is it. The 2 ½” gauge is by far the most common gauge size and you will find hundreds of options in that size range. You can also view the manufacturers’ dimensional drawings if you need more information.
There are two basic connection types for mounting: back and lower. The back mount is what it sounds like. The connection protrudes from the back of the gauge. In a lower-mount (sometimes called a stem mount), the connection is on the bottom of the gauge. Which type you need will usually be pretty obvious. Manufacturers divide back mounts into “center back” and “lower back,” (right in the middle of the back vs. down toward six-o’clock), but unless you’re trying to fit the gauge into a tight space, or mount it in a panel, the difference usually doesn’t matter.
You also have to consider connection size. For 2-1/2″ dial sizes, a 1/4-inch NPT (National Pipe Thread) is by far the most common size. If you’re replacing a gauge, you want to be aware of the existing connection size. Other common connection sizes are 1/8-inch NPT (for 1.-1/2″ & 2″ dials) and ½-inch NPT (for 4″ and greater dial sizes).