TPMS accuracy?
Thread Starter
Intermediate
Joined: Dec 2013
Posts: 35
Likes: 5
From: Littleton
TPMS accuracy?
Has anyone compared the pressure read on the dash to an accurate tire guage? I have experienced 2-3 pounds difference using a digital guage that is supposed to be accurate.
Safety Car
Joined: May 2000
Posts: 4,827
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From: Albuquerque NM
If you're in Littleton Colorado the difference is due to the altitude. The TPMS is a sealed unit, calibrated for sea level. The higher your actual alittude, the further off the TPMS will read. E.G. I'm at 6,000 feet. My TPMS reads 28 but an actual gauge reads 32 PSI.
Thread Starter
Intermediate
Joined: Dec 2013
Posts: 35
Likes: 5
From: Littleton
Here's an article from Tire Rack regarding pressure change due to altitude. No mention of measuring device inaccuracies due to altitude.
http://www.tirerack.com/tires/tirete...jsp?techid=167
Le Mans Master
Joined: Sep 2003
Posts: 8,847
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From: Longview Texas
To answer the original question, yes, I have consistently seen a difference of about 1 psi between what I measure with the gauge and the TPS reading.
However, I have other gauges that may read plus or minus 1 psi since they are not 100% accurate... I think my gauge is supposed to be within +/- 2% of the reading in the middle of the gauge and +/- 3% at the upper and lower limits.
Never really thought about the difference in how the TPMS measures the pressure and a tire gauge measures the pressure, but it seems that the pressure in the tire actually stays the same. With a gauge, you are measuring the pressure above atmospheric pressure, so as you go up in altitude, the pressure in the tire appears to increase. The pressure inside the tire has not actually increased, but rather the reference pressure has decreased.
I would think that since the TPS is inside the tire, it's reading would stay the same since it does not have a reference of what atmospheric pressure is (that is, unless the on board computer measures the atmospheric pressure and corrects the reading for the readout).
In reality, since higher altitudes normally are also associate with lower temperatures, the pressure would appear to drop since the drop in temperature does actually change the pressure inside the tire.
However, I have other gauges that may read plus or minus 1 psi since they are not 100% accurate... I think my gauge is supposed to be within +/- 2% of the reading in the middle of the gauge and +/- 3% at the upper and lower limits.
Never really thought about the difference in how the TPMS measures the pressure and a tire gauge measures the pressure, but it seems that the pressure in the tire actually stays the same. With a gauge, you are measuring the pressure above atmospheric pressure, so as you go up in altitude, the pressure in the tire appears to increase. The pressure inside the tire has not actually increased, but rather the reference pressure has decreased.
I would think that since the TPS is inside the tire, it's reading would stay the same since it does not have a reference of what atmospheric pressure is (that is, unless the on board computer measures the atmospheric pressure and corrects the reading for the readout).
In reality, since higher altitudes normally are also associate with lower temperatures, the pressure would appear to drop since the drop in temperature does actually change the pressure inside the tire.
Last edited by TEXHAWK0; Nov 12, 2014 at 11:51 AM.
Le Mans Master
Joined: Sep 2003
Posts: 8,847
Likes: 795
From: Longview Texas
If your atmospheric pressure is 11.7 psi, then gauge would read 33 psi, but it seems that TPS should still read 30, since the pressure inside the tire has not actually changed.
Last edited by TEXHAWK0; Nov 12, 2014 at 11:52 AM.
Safety Car
Joined: May 2000
Posts: 4,827
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From: Albuquerque NM
A hand held tire pressure gauge measures the DIFFERENCE in pressure between inside the tire and outside the tire. At altitude the pressure outside the tire is less than at sea level. The TPMS sensor does not measure the outside pressure, it compares the pressure inside the tire to a reference value, namely, sea level air pressure.
Considering that the additional air pressure inside the tire is what is holding the car up (runflats not with standing), you want to use a hand held gauge which relates tire pressure to outside pressure to set pressure values, THEN see what that gives you on the TPMS, and use that as your reference. E.G. When the tire gauge says 32 PSI I read the TPMS, it says 28, so when the TPMS reading varies from that I check and add as necessary.
Considering that the additional air pressure inside the tire is what is holding the car up (runflats not with standing), you want to use a hand held gauge which relates tire pressure to outside pressure to set pressure values, THEN see what that gives you on the TPMS, and use that as your reference. E.G. When the tire gauge says 32 PSI I read the TPMS, it says 28, so when the TPMS reading varies from that I check and add as necessary.
Le Mans Master
Joined: Sep 2003
Posts: 8,847
Likes: 795
From: Longview Texas
A hand held tire pressure gauge measures the DIFFERENCE in pressure between inside the tire and outside the tire. At altitude the pressure outside the tire is less than at sea level. The TPMS sensor does not measure the outside pressure, it compares the pressure inside the tire to a reference value, namely, sea level air pressure.
Considering that the additional air pressure inside the tire is what is holding the car up (runflats not with standing), you want to use a hand held gauge which relates tire pressure to outside pressure to set pressure values, THEN see what that gives you on the TPMS, and use that as your reference. E.G. When the tire gauge says 32 PSI I read the TPMS, it says 28, so when the TPMS reading varies from that I check and add as necessary.
Considering that the additional air pressure inside the tire is what is holding the car up (runflats not with standing), you want to use a hand held gauge which relates tire pressure to outside pressure to set pressure values, THEN see what that gives you on the TPMS, and use that as your reference. E.G. When the tire gauge says 32 PSI I read the TPMS, it says 28, so when the TPMS reading varies from that I check and add as necessary.
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Verdad Gallardo
Tech Contributor
Joined: Oct 1999
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From: Charlotte, NC (formerly Endicott, NY)
The big problem is you have no idea how accurate your very accurate digital or analog gauge is. If your gauge hasn't been regularly calibrated to a known reference standard you have no idea whether it is accurate or not. The manufacturer can make all the claims they want and they can charge a higher price to convince you it is better but it is just another gauge until it is calibrated.
As for temperature or altitude correction the Government Rules established in 2005 state the following:
Temperature and Altitude Compensation
Table of Contents
ETV requested that the agency reconsider its tentative decision in the NPRM to not include a requirement for temperature compensation as part of the TPMS standard. ETV argued that the standard must provide temperature compensation when the TPMS calculates tire pressure in order to determine the need for activation of the low pressure warning. According to ETV, temperature compensation is needed to account for the rise in pressure (4 psi) from the cold-start, ambient temperature to the normal running temperature.
ETV also stated that the TPMS should be required to account for changes in atmospheric pressure that accompany changing altitudes. ETV commented that such atmospheric pressure changes could change tire pressure by as much as 10 psi.
ETV argued that the TPMS should make the necessary adjustments to account for temperature, altitude, and load prior to vehicle motion in order to prevent nuisance warnings that may result from daily and seasonal variations in those factors and which eventually might cause the driver to ignore TPMS warnings. Alternatively, ETV argued that those factors could cause the TPMS low pressure telltale to fail to illuminate, thereby resulting in a false sense of security on the part of the driver.
We have decided not to adopt requirements for temperature and altitude compensation because we believe that such requirements would introduce unnecessary complexity to the standard. Regarding temperature correction, the test procedures for low tire pressure detection in the final rule have been amended to compensate for tire pressure fluctuation. Tires will be deflated to testing pressure within five minutes after a 20-minute period of driving, which will ensure that the tire pressure will not rise above the telltale activation pressure during the remainder of the test.
Regarding altitude correction, we do not believe that altitude will be a significant factor in tire pressure fluctuation. We expect that the effect of atmospheric pressure on tire pressure will not result in more than a 5-percent change in tire pressure over the atmospheric pressure extremes encountered during normal driving.
We note further that ETV did not provide any data to demonstrate the need for either temperature or atmospheric compensation.
In other words; "Don't worry about it!"
Bill
As for temperature or altitude correction the Government Rules established in 2005 state the following:
Temperature and Altitude Compensation
Table of Contents
ETV requested that the agency reconsider its tentative decision in the NPRM to not include a requirement for temperature compensation as part of the TPMS standard. ETV argued that the standard must provide temperature compensation when the TPMS calculates tire pressure in order to determine the need for activation of the low pressure warning. According to ETV, temperature compensation is needed to account for the rise in pressure (4 psi) from the cold-start, ambient temperature to the normal running temperature.
ETV also stated that the TPMS should be required to account for changes in atmospheric pressure that accompany changing altitudes. ETV commented that such atmospheric pressure changes could change tire pressure by as much as 10 psi.
ETV argued that the TPMS should make the necessary adjustments to account for temperature, altitude, and load prior to vehicle motion in order to prevent nuisance warnings that may result from daily and seasonal variations in those factors and which eventually might cause the driver to ignore TPMS warnings. Alternatively, ETV argued that those factors could cause the TPMS low pressure telltale to fail to illuminate, thereby resulting in a false sense of security on the part of the driver.
We have decided not to adopt requirements for temperature and altitude compensation because we believe that such requirements would introduce unnecessary complexity to the standard. Regarding temperature correction, the test procedures for low tire pressure detection in the final rule have been amended to compensate for tire pressure fluctuation. Tires will be deflated to testing pressure within five minutes after a 20-minute period of driving, which will ensure that the tire pressure will not rise above the telltale activation pressure during the remainder of the test.
Regarding altitude correction, we do not believe that altitude will be a significant factor in tire pressure fluctuation. We expect that the effect of atmospheric pressure on tire pressure will not result in more than a 5-percent change in tire pressure over the atmospheric pressure extremes encountered during normal driving.
We note further that ETV did not provide any data to demonstrate the need for either temperature or atmospheric compensation.
In other words; "Don't worry about it!"
Bill
Last edited by Bill Dearborn; Jan 10, 2015 at 01:18 PM.
Thread Starter
Intermediate
Joined: Dec 2013
Posts: 35
Likes: 5
From: Littleton
The big problem is you have no idea how accurate your very accurate digital or analog gauge is. If your gauge hasn't been regularly calibrated to a known reference standard you have no idea whether it is accurate or not. The manufacturer can make all the claims they want and they can charge a higher price to convince you it is better but it is just another gauge until it is calibrated.
I had my gauge checked to the equipment used at Discount Tire - within .5 lbs so I believe it's safe to assume it is accurate enough.
As for temperature or altitude correction the Government Rules established in 2005 state the following:
Temperature and Altitude Compensation
Table of Contents
ETV requested that the agency reconsider its tentative decision in the NPRM to not include a requirement for temperature compensation as part of the TPMS standard. ETV argued that the standard must provide temperature compensation when the TPMS calculates tire pressure in order to determine the need for activation of the low pressure warning. According to ETV, temperature compensation is needed to account for the rise in pressure (4 psi) from the cold-start, ambient temperature to the normal running temperature.
ETV also stated that the TPMS should be required to account for changes in atmospheric pressure that accompany changing altitudes. ETV commented that such atmospheric pressure changes could change tire pressure by as much as 10 psi.
ETV argued that the TPMS should make the necessary adjustments to account for temperature, altitude, and load prior to vehicle motion in order to prevent nuisance warnings that may result from daily and seasonal variations in those factors and which eventually might cause the driver to ignore TPMS warnings. Alternatively, ETV argued that those factors could cause the TPMS low pressure telltale to fail to illuminate, thereby resulting in a false sense of security on the part of the driver.
We have decided not to adopt requirements for temperature and altitude compensation because we believe that such requirements would introduce unnecessary complexity to the standard. Regarding temperature correction, the test procedures for low tire pressure detection in the final rule have been amended to compensate for tire pressure fluctuation. Tires will be deflated to testing pressure within five minutes after a 20-minute period of driving, which will ensure that the tire pressure will not rise above the telltale activation pressure during the remainder of the test.
Regarding altitude correction, we do not believe that altitude will be a significant factor in tire pressure fluctuation. We expect that the effect of atmospheric pressure on tire pressure will not result in more than a 5-percent change in tire pressure over the atmospheric pressure extremes encountered during normal driving.
We note further that ETV did not provide any data to demonstrate the need for either temperature or atmospheric compensation.
In other words; "Don't worry about it!"
Bill
I had my gauge checked to the equipment used at Discount Tire - within .5 lbs so I believe it's safe to assume it is accurate enough.
As for temperature or altitude correction the Government Rules established in 2005 state the following:
Temperature and Altitude Compensation
Table of Contents
ETV requested that the agency reconsider its tentative decision in the NPRM to not include a requirement for temperature compensation as part of the TPMS standard. ETV argued that the standard must provide temperature compensation when the TPMS calculates tire pressure in order to determine the need for activation of the low pressure warning. According to ETV, temperature compensation is needed to account for the rise in pressure (4 psi) from the cold-start, ambient temperature to the normal running temperature.
ETV also stated that the TPMS should be required to account for changes in atmospheric pressure that accompany changing altitudes. ETV commented that such atmospheric pressure changes could change tire pressure by as much as 10 psi.
ETV argued that the TPMS should make the necessary adjustments to account for temperature, altitude, and load prior to vehicle motion in order to prevent nuisance warnings that may result from daily and seasonal variations in those factors and which eventually might cause the driver to ignore TPMS warnings. Alternatively, ETV argued that those factors could cause the TPMS low pressure telltale to fail to illuminate, thereby resulting in a false sense of security on the part of the driver.
We have decided not to adopt requirements for temperature and altitude compensation because we believe that such requirements would introduce unnecessary complexity to the standard. Regarding temperature correction, the test procedures for low tire pressure detection in the final rule have been amended to compensate for tire pressure fluctuation. Tires will be deflated to testing pressure within five minutes after a 20-minute period of driving, which will ensure that the tire pressure will not rise above the telltale activation pressure during the remainder of the test.
Regarding altitude correction, we do not believe that altitude will be a significant factor in tire pressure fluctuation. We expect that the effect of atmospheric pressure on tire pressure will not result in more than a 5-percent change in tire pressure over the atmospheric pressure extremes encountered during normal driving.
We note further that ETV did not provide any data to demonstrate the need for either temperature or atmospheric compensation.
In other words; "Don't worry about it!"
Bill