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Discussion Starter #1
Recently, due to a slow leak in one tire, I bought and installed a cheap little device which screws onto the valve stem and graphically indicates tire pressure. An example of the type can be see here: Tire Pressure Valve Caps , but there are similar devices by other manufacturers (or resellers, I suspect). They appear to be available for 32 psi and 36 psi.

After replacing the tire, I found that I liked the convenience enough to put one on each tire, 32 psi for the front and 36 psi for the back. These pressures are just below where I normally keep the tires when cold, so the indicator allows a quick check to ensure that a recently acquired leak has not dropped the pressure to a potentially dangerous level. It is accurate enough for that purpose, (as I verified when my slow leaker was still mounted).

Before using these gizmos, I was kicking the tires and listening to the note that creates before every ride and checking pressure with a gauge every few days. The kicking/listening method has only about 10 psi resolution -- good enough to avoid hazard but not enough to avoid excessive wear. These cap indicators are much more accurate and faster than kicking, and they can be calibrated with a bit of futzing.
 

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I am to lazy to bend over, unscrew the cap, put my guage on it, then if its off by 2 psi, run out back, turn the compressor on, top off the air pressure, put everything away and cap back on the valve

sounds like a good idea, I am a tire kicker and toe prodder (checking chain tension) as well

I wonder what tolerance is on the pressure ?



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Discussion Starter #3
indicator accuracy

...
I wonder what tolerance is on the pressure ?
There is no stated tolerance on the packages. This can be a bad sign, so I bothered to put the caps on a few times as I approached the 37 psi I like for the rear tire and and 34 psi for the front. For the 2 devices I used, (each package has 4), the pressure just at full green was within 1 PSI of the labeled value, according to my pressure gauge (which also has unstated accuracy but appears to agree with a fancier one I do not carry on the bike.) I could not bother to determine accuracy better than that, given I do not use these devices to set the pressure, but merely to avoid riding on truly underinflated tires. Of course, other devices may be further off, so I plan to "calibrate" whenever I have to replace one.

On the package are the claims
"Full Green: Indicates pressure is ## PSI"
"Half Green: Indicates pressure is <= 5 PSI Low"
"Clear: Indicates pressure is <= 10 PSI Low"
(where '##' is either '32' or '36')

So, it's an analog indication. With a little skill at interpolation, it is not unreasonable to expect to perceive a 2 PSI drop if pressure is at the full/less threshold.
 

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Good idea, but keep in mind that these hold the valve open. You are relying on the gasket in the cap and the plastic dome to hold the air in your tire.
 

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Discussion Starter #5
holding pressure

Good idea, but keep in mind that these hold the valve open. You are relying on the gasket in the cap and the plastic dome to hold the air in your tire.
Yes, all true. But consider these mitigations:

The valve in the stem also relies on mating surfaces with pressure keeping them together, as does the tire bead to rim seal. Granted, the cap gasket is larger than the stem valve seal, but it can be made pretty tight using the stem threading as leverage. After a week, I'm seeing no leakage at all.

The plastic dome only sees the difference in pressure between the tire and the "full scale" indication pressure. This is because there is some spring mechanism whose force has to be overcome before the indicator cylinder starts moving, and about 10 PSI more force to overcome to drive it to full green. The pressure differential is only a few PSI, acting against the cap area, about 0.4 inches^2. So the cap only sees maybe a pound of force.
 
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