Do you trust your TPMS or hand gauge?

I use Accutire Digital gauges. I have 3 and all three are accurate to +/- 0.5 psi when I test against ISO Lab-certified master gauge at 80.0 psi.
They run about $15 on Amazon.
I have done gauge checks at a couple of FMCA Conventions and find that pencil gauges, as a group, are the worst because they get dirty and the "slip stick" in many have nicks in them from rolling around in a toolbox. 14% "failed" and were off by more than 5 psi.
Analog (dial) gauges are sometimes difficult to read as the dials may only have marks every 2 to 3 psi or so. Their failure rate was about 5%.

Concerning TPMS accuracy

I tested 6 TireTraker external sensors and 6 TST internal sensors and found them to be within +/- 2.0 psi against the Accutire master gauges. I have published the data on my RV Tire Safety blog.

At my RV tire seminars, I have suggested the following system for checking and managing gauges.
1. Get a couple of Digital gauges and compare them to each other. If more than 2 psi difference at 80.0 psi then one or both are questionable. When you have two digitals that read identical psi (+/- 1.0 psi or less difference at 80.0)
2. If you have dual rear tires and need an angle head or "dual foot" like this    

to read the outer dual you can still use that stick or dial gauge on your duals and then confirm the reading of a front tire against your "Master" digital gauge to confirm the "stick" gauge is reading correctly
3. Keep one digital as your personal master but do not use it for your daily or monthly check against your TPMS.
4. If your daily "dual foot" stick gauge or dial gauge gives strange readings compare it against your personal "Master" digital gauge that you keep packed away in a padded box. It is very unlikely that both your "master" and your daily gauge will go "off" the same amount and in the same direction (higher or lower) at the same time.

Using the above system I have been able to confirm my digitals are ALWAYS accurate to +/- 1.0 psi or less difference over the last 12 years except when the battery "died" in one digital. After replacing the battery that gauge was confirmed to match the master again.

 

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What is a good tire gauge?

You need a system, as any gauge can be damaged or fail.

Pressure gauge system
Over the past six years I have offered pressure gauge calibration check services. With well over 70 gauges of almost every description checked we have learned that many are not much better than door stops or for use as a tent peg.

There are some broad generalities we have learned.

Digital gauges

are usually the most repeatable and accurate with dial gauges.

Next are "stick"

or "pencil" gauges lagging far behind.

While it is also true that I have found some gauges of each type to be accurate, those tend to be those that are brand new. The stick gauges with the sliding bar tend to get worse with age. This is probably due to dirt and nicks in the slide stick causing inconsistent and inaccurate readings.

Before I get into my suggested "system" some are probably wondering how I can be confident that my master gauge is itself accurate.

Having worked as an ISO 9001 certification auditor, it was part of my job to understand how to establish systems and procedures that when followed could be relied upon to deliver accurate information. Even though I am retired, I still can gain access to tire laboratory test equipment and occasionally compare my gauges to current ISO certified laboratory gauges.

System:  You need at least two gauges. One is your own personal "Master Gauge". This gauge is not used to check the pressure in your tires. It is only used as a reference for your regular use hand gauge. The Master gauge should never be stored in your tool box but should be carefully packed away where it will not get dirty, wet, dropped or knocked around.

Calibration check procedure: Using one of your front tires as a source of high pressure air you would take a reading with your Master gauge and a reading with your hand gauge. Hopefully when you first set up your system they will both read within 1.0 psi of each other or better. If not you may want to check your gauges against other digital gauges to try and confirm if it is your master or hand gauge that is most accurate. Majority wins if you have two that match to within 0.5 psi. For your two gauges you should record the two pressure readings and the date. You can then take readings of all your tires using your hand gauge, recording the pressures as you go. You could also record the readings of pressure from your TPMS (tire pressure monitoring system). Now I would expect the TPMS readings to be within +/- 3 psi of your hand gauge.

Remember, TPMS are primarily a tool to warn when there is a change not to be used as an absolute accurate gauge. Again record the measured pressures. Once you have finished this initial check you can set your tires to their appropriate Cold Inflation Pressure (CIP) level using your hand gauge.

Now when you look at your morning TPMS CIP readings, you should see similar movement up or down of the pressure of all tires as daily morning ambient temperature changes. Remember the tire pressure will change by about 2% for each change of 10F ambient. I covered this change is THESE posts.

As time goes on you might see a slow decrease in pressure. This is due to air permeation through the tire carcass and O rings around the valve and TPMS sensor. Eventually after losing about 5 psi you should "top off" the tire pressure. At that time you would use your hand gauge to set the tire pressures. I would suggest that before setting the tire pressures you again do a gauge comparison to learn if there has been any change in the accuracy of your hand gauge. If the difference between the two gauges has changed then you need to learn why.

Reliability: It is very unlikely that both your master gauge and your hand gauge would both go out of calibration by the same amount and in the same direction at the same time.

My System:  I use an Accutire MS-4021B

Digital Tire Pressure Gauge that I bought from Amazon for $12 as my master gauge. (Note I confirmed this gauge is off by 0.5 psi.)  I also use one as my hand gauge (seen above). Since this gauge does not have a truck "dual foot"

head I also run hose extenders on my rear duals. 

I also bought a Tire Traker TPMS. 

I have not had any leaking problems as I check the valve stems, TPMS sensor and hose fittings with a spray of soapy water to ensure there are no leaks.

Some may want to save $12 and not bother with having a master they can trust. But I don't think it's too hard to imagine a gauge reading a bit higher with time as the internal springs lose strength, so you would get misleading high readings which means your pressure is actually lower. This can lead to a failure that will cost a lot more than $12.

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Gauge accuracy

If you are reading this blog you know that along with others, I constantly complain that people do not always maintain their tire pressure.

One part of the process is using a sufficiently accurate gauge. I consider +/- 2 to 3 percent or +/- 1psi when measuring passenger tires to be sufficiently accurate.

Over the past few years I have checked more than 40 different gauges and have observed a 14 to 18 percent failure rate in two groups of 20 or so gauges.

I have two digital "Master " gauges as seen to the right, which read to 0.5psi increment. Both have been checked against ISO certified laboratory gauges and one reads 0.5 psi high and the other 0.5 low. I compare these gauges to each other to confirm they have not gone out of calibration before I conduct my gauge test. I figure the chance of both going out of calibration the same amount and same direction at the same time is very unlikely.

This year while at the Gypsy Journal Rally in Celina, OH, at my Tire Basics seminar, I offered to check gauges. Here are the results.

Actual         Indicated      Type Gauge             Error
82.0                81                Dial                       -1%
70.0                69                Dial                       -1%
77.0                75                Stick                     -2.6%
76.0                81                Dial                       +6.6%   Fail
77.0                92                Short Stick            +19.5%  Fail
74.5                81                Long Stick             +8.2%    Fail
79.5                78                Long Stick             -1.8%
77.5                78                Long Stick             +0.6%
68.5                72                Long Stick             +5.1%   Fail
68.0                68                Dial                         =
68.5                74                Long Stick              +8%   Fail

As you can see we observed a 45 percent failure rate with all the "fail" gauges giving a high reading which could result in the operator unknowingly operating their tires in an under-inflated state yet thinking the tires were properly inflated.  Given the general tendency for RVs to be overloaded in the first place or to be operated right at the limit of the load capability for the specified inflation this is definitely not the better way for a gauge to be wrong.

Having your own accurate "master gauge" is not unreasonable given the low cost of good quality digital gauges. You don't even need a special dual foot master gauge.

Do your own "calibration check" by using the "master single head gauge to check a front tire. Then use your "truck" dual foot gauge (as seen on the left in the top picture above ) to get a reading on the same tire to confirm the reading. If the readings match you can be confident all your readings will be sufficiently accurate when you use your dual foot gauge on the rear tires.

I would suggest you get a digital gauge and then visit the tire store from one of the major tire companies and ask if they would check your digital against their master certified gauge. You could also ask when their master was last calibrated. A competent certification includes affixing a dated calibration sticker to a master gauge. If they cant tell you when it was calibrated just say "thank you" and go to a more competent  tire dealer.

Finally don't just throw your master gauge in the bottom of your tool box. I suggest you keep it in a box of some type. Even a Tupperwear type container can protect your master gauge.

Also don't forget to always use a cap or screw on TPM sensor to keep dirt out of the valve core area of your valves. If you don't understand why you need a cap on the valve I suggest you subscribe to this blog as I will be posting a story showing exactly why you should always protect the valve core.

Edit:   I have conducted similar evaluations at FMCA Conventions in Perry, GA  in 2016 and Madison WI in 2015. At those events I tested about 50 gauges and found between 10% and 16%  were off by more than 5 psi.