colfax

Ok, so just had the tires rotated at Discount Tire, front to rear.

Also had new TPMS gadgets installed in the four driving wheel, not the spare, because why not they were now 11 years old and the batteries just to fail soon.

Before the rotation the spare was at the very top of the five psi display on my dash, my question is - is is still the top? They did not touch the spare, is there any chance
that it could have moved in the display without being touched because now the psi is all the same, reading around 36, per the TPMS dash display.

Another question, I am at 6000 feet altitude, my TPMS Dash psi reads 36 and this is a result of my using my three hand held gauges to read 40 each tire.
I read that one says relative and the other says atmospheric air pressure, talking about hand gauges versus TPMS display.
Yes I know that the TPMS display is supposed to warn me if a tire loses air fast. Some of this forum say the TPMS is THE most accurate tire psi measurement so they inflate by it.
Others say they inflate per their hand gauge. I would not be asking this if I lived close to sea level where the hand and care display would in theory be the same.
But I live where there is a four psi difference, the hand gauge is always reading four psi higher than the TPMS, which should I go by for correct tire inflation ???

Fhobbs

jmcraney

I had hoped to avoid weighing in on this subject again as some of our members have passionate views about the accuracy of the values in the TPMS pressure display. Thanks to my friend, Fhobbs, I have another opportunity to try to make sense of this again and I hope to leave most of you with an understanding that leaves you happy.

There are three common types of pressure displays: Absolute, Gauge and Differential. The displays might be labeled, PSIa, PSIg or PSId but they are frequently not labeled.

Here is a link to information about measuring air pressure that might be helpful: https://en.wikipedia.org/wiki/Pounds_per_square_inch

PSIa is a display of the absolute pressure. Absolute pressure is referenced to a vacuum. Some think of a vacuum as a negative pressure, but by definition a vacuum is zero pressure. This means that a tire with the valve core removed has a PSIa of 14.7 PSI when measured at 0' MSL.

PSIg, by definition is absolute pressure minus ambient air pressure (ambient air pressure means the air pressure where the measurement was taken) . That means that a tire with the valve core removed would have a PSIg of Zero at any altitude.

PSId displays the pressure difference between two sources. The PSId of two tires with the valve cores removed would always be Zero.

PSIgtp (my term) is another type of display that is used to display tire pressure that is measured with the gauge pressure method and is referenced to 0' MSL air pressure and defined as PSIg plus 14.7 PSI.

Here are the pressures you would measure for a tire inflated to an absolute pressure of 33.0 PSI at different altitudes with various devices if the temperature for all measurements were the same:

Altitude = 0' MSL
Ambient Atmospheric Pressure = 14.7 PSI
TPMS = 33 PSI
PSIa = 33.0 PSI
PSIg = 18.3 PSI (absolute air pressure minus ambient air pressure)
PSIgtp = 33.0 PSI (gauge pressure at 0' MSL plus atmospheric pressure at 0' MSL)

Altitude = 3000' MSL
Ambient Atmospheric Pressure = 13.2 PSI
TPMS = 33 PSI
PSIa = 33.0 PSI
PSIg = 19.8 PSI (absolute air pressure minus ambient air pressure)
PSIgtp = 34.5 PSI (gauge pressure at 3000' MSL plus atmospheric pressure at 0' MSL)

Altitude = 6000' MSL
Ambient Atmospheric Pressure = 11.8 PSI
TPMS = 33 PSI
PSIa = 33.0 PSI
PSIg = 21.2 PSI (absolute air pressure minus ambient air pressure)
PSIgtp = 35.9 PSI (gauge pressure at 6000' MSL plus atmospheric pressure at 0' MSL)

So, you can see that for us lowlanders it doesn't matter what type tire pressure gauge we use. But for you highlanders the gauge type makes a difference.

Several types of gauges are in use and the PSIa and PSIgtp seem to be the most popular. Colfax, above, the originator of this thread, seems to be using a PSIgtp type gauge.

The pressure recommendation label on the front door jamb says that a cold inflation pressure of 33 PSI is best, but nothing about altitude. I personally believe that a cold inflation pressure of 33 PSIa is correct. Seems to me a cold inflation pressure of 33 PSIgtp at 6000' MSL would leave the tires under inflated.

Cold inflation pressure does't mean the pressure at Discount Tire or the service station. For most of us it means the pressure when we first start our car in the morning, before we move it.

The TPMS is not designed to provide a warning of rapid pressure loss - it is designed to provide a warning when any tire sensor reports a pressure that is 25% below the benchmark pressure for that wheel.

A wide range of benchmark pressures can be entered but the recommendation for most tires is 33 PSIa.

Colfax, I hope this helps. If you still have questions, ask again.

colfax

thanks jmcraney, your information does help, a lot!

But just so i am clear, and thank you for providing an example at my own 6000 foot altitude

Your differential of some 3 psi mirrors perfectly my own observations, my TPMS always shows at least
3 psi less than my brand new digital tire gauge, which shows the same as my two regular and gauges.

Continuing on, yesterday I set my tires cold in the morning with my digital gauge showing 40 for each
tire, and sure enough after I left the garage and traveled a little the TPMS showed 36-37 car display.
This feels pretty good to me, not too soft etc. I note that this is about 3-4 TPMS psi more than your own
preference.

Question: The manager of the Discount Tire I went to a week ago to have new TPMS things put on as they
were the originals 11 years old, told me the TPMS things are "calibrated at sea level", this was his explanation for my
question as to why my digital gauge reads 3-4 higher than the TPMS. Is he right? And if so then IF the maker
of the TPMS things "calibrated" them to my 6000 foot altitude then there would be no difference between TPMS
and my digital gauge? Or is this calibration thing nonsense that he telling me? He also threw out some vague language
about "relative" air pressure and "atmosphere" to explain this 3-4 psi difference, is he making it up as he speaks?
And finally, for now if you would bear with me, which should I as the car owner use to set my tires, my digital or the TPMS?
Thanks!
Colfax

Nospinzone

jmcraney

Colfax,

This is really esoteric stuff. Most people are not interested in this, just you and me.

You're probably wanting me to say that either your air gauge or the TPMS is wrong or inaccurate. It's likely that they are both correct and accurate. The reason for the difference is because they are measuring different things. The TPMS is measuring the absolute pressure in the tire, while the handheld gauge is measuring the Gauge-Pressure of the air in the tire. At an altitude of 0' MSL they are the same but other altitudes they are different. It's like comparing apples and oranges.

I believe that the correct pressure to use, in most tires, is 33 PSI Absolute, when they are cold.

It is not practical to use the TPMS to adjust your tires because the feedback loop would be too large - you would have to put air in or let air out and run around to look at the dash between each adjustment. You have to use a handheld gauge and you should use a gauge that has 0.5 PSI resolution, or better 0.1 PSI resolution. And the gauge should have a bleeder valve. The best approach is to over-inflate the tires and bleed them to the desired pressure.

We've already determined that a pressure of 35.9 PSI at 6000' MSL would be indicated for an absolute pressure of 33 PSI.

So, some morning go out to your car and with one of your handheld gauges, adjust the pressure on all tires to 35.9 PSI. Then start your car and check the pressure in the dash - I'm betting on a indication of 33 PSI.

I think we better leave the guy at Discount Tire alone. He probably does't want to know this. And since I don't know how big he is, I don't want him to know my name.

colfax

Mr. jmcraney, thank you sir!

Caflashbob

jud149

While we're on the subject, I'm wondering how long the batteries generally last. My tire pressure warning light came on a while back, first blinking for one minute and then just staying on . Access to individual tire pressures was inop. After input from this Forum, I decided to wait until I got new tires. About a week later though, all was well again and has been for about 1 1/2 months. I'm wondering if a battery is starting to go. My LS is an '11 with about 58k on the od.

Also, has anyone else had this intermittent problem?

NotFasty

You sound like what I experienced here: https://www.clublexus.com/forums/ls-...ms-thread.html

I think we haven't had enough cold weather here for the issue to recur but I'll be watching them. I'd suspect I need new sensors for my 2012,

colfax

I have an 07 and recently had Discount Tire install new tires.

Because it was now some 11 years old I figured the original TPMS individual batteries had to be very ready to give up the ghost.

Discount Tire charged me $60 per wheel to put on new TPMS units on the four driving wheels, I did not replace the one on the spare, yet.