"How Accurate is Your Diagnostic Tool?"
Gauges vary. It doesn't matter whether you're checking your timing or your blood pressure, you need to have confidence that your measuring tool is accurate to within a point or two.
It's an assumption one makes every time he hooks up a timing light, a compression or leak down gauge, a high buck tire gauge or, again, a blood pressure gauge.
The Timing Light might be the most important, so in the case of the timing Light, especially if it resides in the top of the tool box, it should be sent off and calibrated at least once a year . .
The rest of the gauges are probably relative and should be treated that way. One day at the races, I had five of my competitors check one of the race car's tire with their tire gauges, all but one of them the Standard Moroso that most of us use. They varied by up to a pound and a half in both directions. While it isn't anything to get excited about, simply because all of us were setting the pressure to what worked for us and the actual number was irrelevant, it did show all of us that when one asked another, "What Tire pressure are you running?" the answer wouldn't reveal much because neither of you would know how close one another's gauge might be . .