How Do They Work?

(Part 1 of 2)

I tested 30 tire pressure gages of different types over a range of temperatures and pressures.  What did I learn?  Basically you would do better to head out to the garage right now and check your tires rather than worry about which gage to use.  I also learned that it is not all about accuracy, and a very accurate gage can give inaccurate reading if not used properly or under certain conditions or if not used properly.  If you want to learn more about why I did this, how they work and which ones work best, read on.  I even provide a simple test anyone can do to gain confidence in their gages.

I did this for two reasons:  I own a bunch of tire gages and I always wondered how good they are.  I also had equipment available to check them for accuracy over their range.  To get a better statistical sample I solicited the help of the mac-pac (Club #289) to loan me their tire gages for the sake of science and BMW riders everywhere.  In return they got a custom calibration report.  Part 1 of this article describes how tire pressure gages work and the pros and cons of each type.  Part 2 shows the test results and recommends how to test your gauges if you have several.

 

How Tire Pressure Gages work

Tire gages can be broken into three types, Pencil, Dial and Digital.  At a fundamental level they all work the same; the pressure pushes on a deformable part causing the indication to change.  The only difference is what part is deformed and how the indicator works.  The pencil gage is the easiest to understand.  Figure 1 shows the guts of a pencil gage I disassembled in the name of science.

Figure 2 shows pencil gage operation.  The pressure pushes against a little piston.  The piston pushes against the deformable part, a spring until the spring force equals piston pressure force.  The piston movement pushes an indicator out of the housing.  The pressure is read on the indicator markings.  When pressure is removed the piston snaps back but the indicator stays in position due to the slight friction of the guide assembly until it is pushed back by the user.  It is indeed a simple and elegant design.

One the pencil gage limitations is its narrow measurement range.   Most only indicate up to 50 psi.  This is fine for motorcycle tires, but is about 10 psi short for most cars temporary spare.  There are some 100 psi models, but they are only marked every 2 psi, so they are not as useful for motorcycle tires.  Some gauges indicators are easier to use to get an accurate measurement.  The gage shown on the top of Figure 3 is marked so it is very easy to see each psi marking. 

Pencil gages are compact, cheap and rugged.  They are perfect to have in every vehicle so you can check the pressure on the road.In a dial gage the deformable part is C-shaped oval tube called a Bourdon tube.  It is named for Eugene Bourdon who patented the pressure gage in 1849.   When inside of the Bourdon tube is filled with a pressurized gas, it straightens. That straightening moves a set of levers and gears rotating the pointer around a dial.  Figure 4 shows the insides of a typical Bourdon tube type gage.  Figure 5 shows the operation of the dial gage.

Bourdon tube gages are very common in industry and come in a variety of materials, accuracy levels and full-scale pressure capabilities.  The ones used in tire gages are typically made from brass and scaled for 0-60 psi or 0-100 psi.  The Bourdon tube is quite rugged.  The levers and gears are fairly fragile and rough handling can cause damage.  Most come wrapped in a rubber bumper to minimize the chance of damage.  Constant road vibration sitting in the tool box, or trunk of your bike can cause wear on the levers and gears.

Most dial type tire gages are simply an industrial pressure gage attached to some sort of hose or adapter to connect it to the tire valve.  A hose makes it much easier to connect the gauge to the tire valve.  This is especially true on a motorcycle where the exhaust, luggage or disk brakes get in the way.  Some hose assemblies have a push button valve to make adjusting tire pressure easier.  Others have an internal check valve that maintains the reading after the gage is disconnected from the tire.  The hose assembly makes the unit larger and it also makes it much easier to use.  The relatively large size and fragility make most dial gages better suited to home and shop use than being carried on the road.

The heart of a digital gage is a tiny silicon wafer transducer assembly.  The silicon wafer is the deformable part.  They were invented in the late 1960s using the technology developed for microelectronics.  The deformable part is a thin diaphragm in the silicon wafer as shown in Figure 6.  It is machined using the same processes used to form the silicon wafer.  The diaphragm deflection is sensed by small strain gages deposited on the diaphragm non pressure side.  The strain gages resistance changes depending on how much the diaphragm moves.  The strain gage signals are read by the electronics.  The signals are changed into a form that the display can use to show you the pressure.  The electronics runs off a small battery, enclosed in the gage.  To conserve battery power the digital gage spends most of its time in a sleep mode.  The first digital gages required pressing a start button to wake them up for duty.  The latest gages wake up when they sense a pressure change, automatically start displaying the pressure, automatically capture the stable reading and turn finally themselves off after use.

The transducers are made hundreds or more at a time using the same highly automated micro manufacturing techniques used to make computer chips.  Digital gages have piggybacked on Moore's Law. The advances in semiconductor manufacturing efficiency have pushed prices down and features have gone up.  I bought my first digital gage about 20 years ago for $50.  It had a tiny display, required resetting after every reading and ate batteries like they were going out of style.  My other gage was a give away at a trade show a couple of years ago.  It has auto off and on, is just as accurate and still has the original battery.   You can pick up a digital gage at most any auto store for about $10.  Being a solid-state device digital gages are extremely rugged.  It would be almost impossible to damage one so that it would "work" and give an incorrect reading.  So, no matter how beat up the gage is on the outside, if it displays you can trust the reading.  The only exception to this rule of thumb is if the rubber seal at the end of the goes bad.  When this happens you can get inaccurate readings if you don't notice it is leaking.