How to Calibrate a Dial Scale for PSI

Pressure gauges that measure PSI, or pounds per square inch on a dial-scale readout, are used in many applications, including tire gauges, fluid tanks and pumps and automotive manifolds and cooling systems. Analog gauges use mechanical devices such as metering pins and diaphragms to detect and relay pressure. Pressure gauges can wear out just like any other component and need to tested for accuracy. A few simple procedures can help anyone achieve a more accurate reading from his pressure gauge.

Instructions

1

Attach a length of flexible plastic tubing to the pressure port of your pressure gauge. Use a felt pen to mark off lines on the tube that represent increments of 5 meters each, for at least six increments. Change the meters to feet by imputing the data into an online meters-to-feet calculator. For instance, 5 meters equates to 16. 4 feet; 10 meters equates to 32.9 feet and 15 meters equates to 49.2 feet. Write the number in meters and feet on the plastic tube with the felt pen.

2

Check to to see that the needle indicator on the gauge reads "0" on the dial. Find an elevation where you can stretch out the entire length of the tube from a vertical height, such as a building top or a bridge. Fill a large bucket of water and transport it to the top of your measuring point. Place a funnel on the end of the tube and pour in enough water to reach the 5-meter or 16.4-foot felt mark on the tube. Have an assistant below call out when it has exactly reached that level.

3

Calculate the PSI by multiplying 5 meters times 1.4223. The answer will be 3.1115 PSI. (Or round off the number to 3 PSI.) Write down the number and record the number your gauge indicates. Fill the plastic tube until the water reaches the 10-meter mark. Multiply 10 times 1.4223. The answer is 14.2 pounds. Mark down what your gauge indicates.

4

Skip an increment and fill the tube to the 20-meter mark, if you wish. The true pressure will indicate 28.4 PSI. Mark down your gauge reading. You can make as many measurements along the tube as you want, but five or six measurements should suffice to get an accurate discrepancy between the two readings. You might have a gauge that reads a 3-, 5- or even 10-PSI difference.

5

Find the calibration screw on the back of the gauge. Use a screwdriver to turn the set screw in the direction you need (up or down) to bring the needle over the correct PSI reading. This will bring the gauge back into acceptable calibration. If the gauge reads off more than 10 or 12 pounds, replace the gauge.

6

Take a reading with your gauge on a pressure line that is under pressure. The pressure should be constant, since you will use one gauge right after another for comparison. Write down the PSI number. Connect a new, identical gauge to the pressure line. Determine the PSI difference between the two gauges. If your gauge does not have an adjustable calibration set screw, you will have to make note of how far off your gauge reads. You can mark it, for example, "Gauge reads 3 PSI low" or "Gauge reads 5 PSI high."