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Various sizes and shapes, some new and some very old. Also we tested some digital
gauges some cheap the others expensive. Our test was conducted at our aircraft maintenance shop and against a highly accurate calibrated air pressure standard. I took each gauge and measured three air pressures consisting of 25, 35 and 45 lbs.
All the testing is done and the results in Parts will be published at various times.
Part one consisted of the most popular tire gauge called the pocket or pen type gauge which cost anywhere from $ 1.00 to $ 2.50 range and they have a white plastic dial reading. I tested 5 new ones (Victor, Monkey Grip etc) that were purchased at nationwide chain and auto stores. Each of the new ones read entirely different pressures with the worst being as much as 6 lbs. lower at 25 lbs. While an exact duplicate gauge read 3lbs. lower at 25 lbs. The new ones where mostly 3 lbs lower at 25lbs but improved at the higher pressures of 35 and 45 lbs. There averages where about 2 lbs. at the higher pressures. I tried exercising the dial a few times by pulling it in and out and taking the readings again and some improved by as much as a lb. at the lower reading. I then test older gauges of this type 3 to 10 years and the results were worse in some cases. It seams the older the gauge the lower the accuracy and they are the worst at the lower pressure. Some were off as much as 8 lbs. By the way all of the gauges we tested read low except one and that had an obvious defect. I took the older gauges and sprayed the dial with WD-40 and exercised it about 10 times and then I went back and redid the test and improved the readings to that of a new gauge. One gauge was 7 lbs lower at 35lbs and after my operation the gauge improved to about 2.5 lbs lower. One gauge that was 10 years old had better accuracy than one that was 3 years old, but generally the older the gauge, the lower the accuracy of gauges of this type. You get what you pay for.
In conclusion. If you want 30 lbs put in the air till your pen type gauge reads 28 lbs. and don't forget to exercise the dial a few times before checking the tire pressure. On the older ones spray the dial with WD-40 or some silicone lube and exercise the dial about 10 times. The average after we got them working was about 2 lbs. low Part 2 deals with more expensive gauges with some surprising results until then-by