Measuring Air Pressure - Air Essentials

When I talk with racers or consult with a race team, I hear it over and over again: "Tires are one of the most important components on the car." I have no real argument with that statement and tend to agree with it.

I find it puzzling, though, that tires are mistreated and their information is often misread. The most common measurement of tires is air pressure, followed by stagger. It is neglect of the air pressure adjustment that I find most intriguing. For such an important component, tire pressure measurement is usually the least robust process of all the processes on the car.

The method that most race teams use to mitigate the risk associated with measuring tire pressure is to have the same person check the tires. While this is a plan, it is nonetheless a weak one. What really needs to be developed is an SOP (Standard Operating Procedure) that outlines how the process is going to be accomplished, regardless of who does it.

What is the best way to check the tires? We could argue about this for hours. What we will concentrate on is the tool we use for checking the tire pressure: the lowly tire pressure gauge.

There are multiple ways to check tire pressure, but in reality there is only one repeatable method, and that's with a tire pressure gauge. The selection then broadens to two different types of gauges, analog and digital. Most people assume that a digital gauge is the best tool for accomplishing this measurement. The argument is based on the fact that when people see a digital display, they instantly assume that the gauge will be more accurate than an analog gauge. While a digital gauge may be easier to read, that in and of itself does not portend any greater level of accuracy.

We need to exercise great care when differentiating the two types of gauges. Many times, a digital gauge is not a digital gauge at all-it's merely an analog gauge with a digital display. This method offers no more accuracy than a standard analog gauge. It is just easier to read.

We need to define exactly how an air pressure gauge works. There is a multiplicity of methods used to measure air pressure, but the ones we need to concern ourselves with are those most commonly used in racing today. Most air pressure gauges are based on a Bourdon tube to activate the needle or in some cases a digital potentiometer. The Bourdon tube's name was derived from Frenchman Eugene Bourdon, who developed the tube in the mid-19th century.

The Bourdon tube is closed on one end and open on the other. The open end is where the pressure is applied, and the pressure source, in this case, is a valve stem. The tube does not have a round cross section but is usually a very flat oval, and the tube is also formed into a semicircle. The closed end of the tube is attached to a section of a gear or a rack that is driving a pinion gear. On the other end of the pinion gear is the needle, the part we see as it moves on the gauge, and usually they are on the same shaft. When air pressure is applied, the Bourdon tube tries to become straight. This movement causes the gear or rack connected to the end of the tube to turn the pinion gear that is attached to the gauge needle. The greater the pressure applied to the tube, the more the tube tries to return to a straight position. Once you understand how the mechanism operates, it is really a very simple device. The execution can be a bit messy, but the concept is easy to understand.

A digital gauge is based on a strain gauge, which operates by measuring the deflection of a surface and turning that deflection into an electrical signal that is then sent through a conditioning circuit to an amplifier circuit. This amplified signal is then routed to a meter, and we read that signal as a measurement of pressure. The deflection can come from a variety of sources, but mainly a piezoelectric sensor or metal diaphragm acting on a strain gauge. The pressure causes a deformation and the strain gauge measures this deformation or distortion. It is then conditioned through a variety of electrical circuits that are displayed on a meter.

Remember, though, that a gauge is not inherently more accurate simply because it is digital. The argument has always been that due to the digital readout, there is less of a chance of misreading the gauge. That is completely true. The problem is the scale. It is very easy to place a meter on the gauge that reads to the third or fourth decimal place, but you may not be measuring to that level.

Using a gauge with a digital readout eliminates the parallax factor. That is, when you are reading an analog gauge, it is very easy to look at the gauge and not maintain a completely parallel relationship to the front of the gauge. Looking slightly to the left, right, above, or below the center of the gauge face can lead to misinterpretations of the actual reading. This can cause the same tire to have a different pressure when measured by multiple people measuring the pressure with the same gauge. This is a classic example of not knowing how to use the tool and inducing operator error into a measurement.

The gauge should be held in the vertical position while in use because the majority of today's gauges are designed to be used this way. Placing the gauge in any position away from vertical could place the mechanism in a load for which it was not designed. Any additional load of the gears in a direction they were not designed to function could give you an erroneous reading.