This young racer is using...
This young racer is using the correct gauge position to get the most accurate reading. The gauge is being held in the vertical position and the view is straight on. This reduces the parallax problem that can be encountered when viewing the gauge from less than straight on.
Depending on the quality of the gauge you are using, there may be some "slack" in the mechanism. This slack can cause hysteresis, which is a variation in the gauge reading caused by the mechanism of the gauge itself. The Bourdon tube and the associated mechanicals are being loaded due to the position of the gauge, and this load could cause some non-repeatable readings. This is due to the Bourdon tube being loaded in a direction it is not designed to operate. For example, you may be reading 14.5 psi on the gauge, when in reality you may actually have 14.0 psi. The whole reason you are measuring air pressure is to achieve a specific number. Using the gauge in a way it was not designed to operate could give you a reading that is less than accurate.
You must remember that an air pressure gauge is a precision instrument. It is not a wrench, a socket, or a screwdriver, and as such, it should not be stored and treated like it is a handtool. Exercise great care when storing and transporting gauges. Many gauges designed for racing applications come with a protective foam enclosure around the gauge or a molded rubber shock mount to prevent the unit from being damaged by shock or other handling damage. Just throwing a gauge in the bottom of a toolbox that is full of other tools is just asking for accuracy issues. It is very easy to chip a tooth of the gears that drive the needle if the gauge is handled improperly. This is true if you are using a digital or an analog gauge because these are delicate instruments and need to be treated with care.
This photo really shows how...
This photo really shows how delicate these gears are. It does not take much in the way of over-pressuring the gauge or dropping the gauge to damage these gears.
It is also very easy to damage the gauge if you apply more pressure than the gauge was designed to measure. If you connect a gauge with a range of 0 to 15 psi to a tire with 55 psi, you can "wrap" the gauge. This can also cause the gear teeth to slip or the Bourdon tube to become damaged, which requires replacement of the gauge. You need to exercise great care when you are dealing with this type of instrumentation. That said, the gauges that are commonly used in our sport for this type of measurement are very robust and still deliver reasonable levels of accuracy after some slight abuse. The rougher you treat these instruments, the greater the level of inaccuracy you may experience.
If you suspect that the gauge is delivering an inaccurate reading, the easiest thing to do is check the measurement against other gauges. If you find a significant difference, you may want to call the manufacturer or the dealer you purchased the gauge from and see if they offer a calibration service.
When getting an air pressure reading, make every attempt to look at the gauge directly parallel with the face of the gauge. This will reduce the parallax factor and go a long way to getting reliable, repeatable measurements. Make sure that the interface between the valve stem and the gauge is clean and free from any dirt or debris that could cause unwanted leakage during the measurement process. Make every effort to push the gauge firmly onto the valve stem with as little air leakage as possible. The obvious reason for doing this is to lose as little air from the tire as possible.
The copper-colored tube is...
The copper-colored tube is the Bourdon tube. On the lower left, it is clear to see that the open end of the tube is soldered to the brass inlet port for the gauge. When charged with air, the tube tries to straighten out, moving the white gear and driving the brass pinion gear attached to the needle on the face of the gauge-the part we look at to get our reading.
This is not an overly complex process. The problem is that many racers are often in a hurry when they are trying to make these measurements, and it is very easy to use the gauge incorrectly and get erroneous readings. The importance of these measurements to the performance of the car makes the ability to gather good data critical. We need to make sure that the misuse of the air pressure gauge does not contribute to poor measurement.
The range of the gauge is also a feature you need to be keenly aware of in the process. If you are measuring tire pressures in the 7-8-psi range, you should not select a gauge with a range of 0-100 psi. The sweet spot on the gauge is the middle. You should be using a 0-15-psi gauge for pressures in the 7-8-psi range. This is not as critical on a good digital gauge, but on an analog gauge it seems to be a very tried-and-true way of selecting and getting the accuracy you need.
The objective should be to train the team on how the tool is to be used, and possibly document the process with step-by-step instructions. Do not make the process of measuring air pressure a task specific to a man or team member, but rather a process-based task that can be accomplished by every member of the team with the same level of accuracy. As a team, we need to be able to wear a number of different hats on race day. Because we all want to reach Victory Lane, we should take the time to make sure we are not giving away anything due to poor measurement practices.