Air Pressure Guages
After reading an article on Velonews about air pressure in CX tires I decided to purchase the SKS Airchecker digital pressure guage. Today I used it for the first time. I wanted to compare the guage on my pump to the actual pressure in my tires. I pumped the tires up to 30psi on my pump guage, which I have used as my starting point all season. I then measured the pressure with the SKS guage. OMG 18.5psi? I then pumped the tire up to 35psi on the pump. SKS guage read 25.5!
My conclusion, I have been running ridiculously low pressures all season long. Probably around 20psi for most races. A little late in the year to figure this out, but next year I will know exaclty what pressure I am running.
I don't want to say all pump guages are off, but clearly they are not very accurate as they read the pressure in the pump's air line, not what is actually in the tire.
The SKS Airchecker was under $35CAN shipped to my door from an e-bay seller. It is an awesome guage I must say. If you have pressure concerns, I'd recommend it in a heartbeat.
Something to keep in mind
Something to keep in mind when comparing measuring devices is that they will rarely spit out the same number. So which one is correct? You need some calibrated device to measure them against. So you can go crazy trying to buy the most accurate guage out there but you don't need to. You want consistency over accuracy, if you can get both great. Your pump may have been wrong, but all you need to care about is that the wrongness is always the same.
You might have been running too low all season, but maybe not, how did the bike feel, too sluggish, bottomong out all the time? Use only one of the gauges as a starting point, pump your tire to X psi and then ride and see how it feels. The biggest advantage with the digital gauge is that you should be able to more consistant on having the same pressure in your tires over an analogue gauge on the pump.
Martin www.urbanimages.com
Calibration aside, It's
Calibration aside, It's bloody difficult pumping up a tire to 28 psi +- 0.5 psi when the pump gauge reads 0-180 psi. Martin is right, it's not important what the actual number is, but rather being consistent from race to race, something a pump gauge doesn't do well at such low pressures.
Norco's house brand Axiom has a digital gauge for $22....it's worked well for me all season.
It is also possible that
It is also possible that you are measuring the pressure lost while removing the pump and placing the digital gauge. 10psi is more than I'd expect to lose this way, though.
You can get an estimate of these losses by repeatedly disengaging and re-engaging one device. I.e. pump the tire up to some pressure and note the pump gauge reading. Then remove the pump from the stem and put it back on. What does it read now? Repeat and try it at various initial pressures.
Anyhow, As Martin pointed out, with two uncalibrated gauges that disagree you still don't "know" what pressure you've been running (like that joke about someone who has one watch being more certain of the time than someone who has two) and repeatability is more important than absolute values (unless you want to compare tire pressure with others over the internet).
I mostly use the dial gauge on my pump but always check that against a "can I nearly bottom out by leaning hard on the top of the wheel with my knee or open palm" test and a little pre-riding.