kosmo886

So I have two floor pumps (Topeak Elite and Lezyne Road Drive)...both solid quality. And two pressure gauges...one a Topeak digital and I forget the brand of the other. In any event, my inconsistent readings follow an interesting pattern. Both of the floor pumps register very close to each other, while the two seperate pressure gauges are also very close to each other, yet the pressure gauges always ready about 10PSI lower than the floor pumps. ALWAYS. I've triple checked this many times by attaching the pump, gauge then pump again to confirm the pressure didn't drop somehow etc. Has anyone noticed a similar issue? Seems fairly odd to me. A few PSI is no big deal, but now I don't know which gauges to trust.

upthywazzoo 

Floor pumps measure pressure at the gauge, which is affected by how much pressure there is in the pump itself. That's at least part of the discrepancy. Assuming the tire gauges you have are calibrated--I would trust those.

WhyFi

I'm not being dismissive, but does it really matter? Usually you're going to target a certain pressure and then tweak from there. As long as you're getting enough precision to do that with your pumps, I wouldn't be worried about the accuracy too much.

genejockey 

Do they have better tire gauges nowadays? The one I bought 20 years ago seemed more like an expensive tool for letting air out of tires. It was Heisenberg's Pressure Gauge - It only let you determine what the pressure in your tires HAD BEEN before air blew out while taking the gauge back off the valve.

genejockey 

You make an excellent point - as long as you use the same gauge, once you've found the right pressure, just keep pumping them up to that pressure as registered on that gauge. It doesn't matter if that number isn't accurate, as long as your gauge always registers the same at that pressure.

Mind you, not knowing what that actual number is might bother you. It would bother me.

EDIT: The OP's post reminds me of an old saying - "The man with one watch always knows the time. The man with two watches is never sure."

Leinster

This.

What matters more is that your pumps are consistent with each other, and that they repeatably and accurately pump your tires to your desired specification.

The pumps may be empirically wrong, but if they're consistently wrong, then they're right.

79pmooney

My gauge doesn't suffer lost air. Thumb and index finger. I'd just develop a similar gauge and calibrate it with either of your gauges and what feels right out there on the road and go from there. (And best part - it works mid ride and takes up no jersey pocket space.)

Steve B.

If the digital tells you the floor pumps are 110psi while the pump says 100 and you want 100, pump to 90. If the digital tells you 90 and the pump says 100, pump to 110. "ish.

kosmo886

i don’t disagree with anything said. More a curiosity if there is something fundamentally different in gauge readings on floor pumps vs. standalone pressure gauges. I find it very odd that the pumps are all off by 10psi relative to the gauges. Oh well.

Sy Reene

If this is true, I kinda feel the pump makers would know of this effect, and would just rotate the PSI scale that's under the needle the correct amount to still be accurate.

upthywazzoo 

Someone who is a mechanical engineer/pump expert feel free to correct me.

There are a couple problems that stand in the way of compensating for this easily. Firstly, the gauges cannot really measure pressure in a linear way. There is a range at which they're most accurate. A pump designer at Lezyne must choose (for example) for their pump to be accurate at 90psi. So at 90psi, the gauge is right on, and maybe between 75 and 105, it's pretty good. Beyond that it's going to be less and less dependable.Even if the designer accounts for the offset between internal pump pressure at 90psi (in the tire) and the inside of the pump (105psi, for example), the offset will not be reliable for the rest of the gauge range, unless there's some sophisticated mechanical or electronic elements used to create a differential offset. You can absolutely buy pressure gauges that are accurate to within 2% or better across their whole range of measurement--but these are certified components, and it's not likely that you'd find them on $40 pump.

This is why it's useful to find the pressure that works for your tires according to the pump gauge and just stick to that.

sfrider 

How do did you test? If you pump up to a pressure with a pump and then check with a pressure gauge you can lose a little air when switching. If you connect a pump you will lose more air due to its greater volume, since obviously air has to come out of the tire to pressurize the pump.

noisebeam

If you are OCD and need 0.05% accuracy get this plus some adapters. You can also interface with your PC and track results.
https://www.testequipmentdepot.com/d...3531dlm110.htm

MattTheHat 

My Blackburn digital gauge will take about three readings and lose about .5 PSI. How accurate it is, who knows? My guess is fairly accurate based on comparisons to the digital gauge on my compressor’s air chuck.

upthywazzoo 

Well with a presta valve, the pressure gauge on the pump will only be measuring pressure inside the pump/hose, not the tire. The pump has to generate enough internal pressure to overcome the pressure inside the tire holding the presta valve closed. A tire with a presta valve could not inflate the pump, since it's one way, by design and a presta chuck does not open the valve like a schrader chuck does. Only a standalone pressure gauge would work here, or performing at least one pump stroke.

blakcloud

Thank you for this explanation. Now it makes sense to me.

Prowler 

We keep seeing these sort of questions. If you're really concerned about accuracy, test everything on a truck tire or tractor tire. The change in pressure as you switch gauges will be less than they can measure. Bike tires are too small. Then pick your fav gauge and stick with that for your bike.

Or just wack your tires with a heavy lug wrench like truckers do and get a feel for it.

Sy Reene

No, I don't think so.. the presta valve is open after you start pumping, as long as the pump head remains affixed. The gauge is reading the overall equalized pressure of the whole system (which is fine; psi is psi). Releasing the pump head allows the presta valve to immediately close and the escaping hiss/air is that air that was in the pump's hose.

Some pump heads (like mine), have a little button on the pump head to release air from the tire (in case of having overshot your target); it's not like that button is pushing in the presta valve, it's just opening the seal a bit to let air escape.

upthywazzoo 

This blew up my brain, but it makes sense. My description would work better for a schrader valve since there must be a pressure differential there for it to open

Drew Eckhardt 

They're all accurate. The tire gauges just let some air out as a side effect.

woodcraft

You had it right the first time. When the pump generates more pressure than what's in the tire, the valve is forced open.

The valve is only held open by the pump head when it's fit on wrong, normally it is closes at the end of each pump stroke.

The bleed button on the pump only releases pressure from the hose when using presta- you can't lower the tire pressure with it.

Sy Reene

No.. at least not with my pump (Bontrager -- one of these if it matters: Bontrager SuperCharger ). As soon as I twist the head on to the valve, it gives me a PSI reading. No pumping required to get initial reading. Once attached, I can hit the release valve and deflate the tire completely if I so wished.

spelger

yes. exactly.

woodcraft

Yes, I see you can do it that way (& how Bontrager describes), but I never do. I have a pump with that kind of head, but any imperfection of the seal & the tire loses pressure.
The pump (head) I mostly use has presta on one side & schrader on the other with a lever between. If you push it on far enough to open the presta valve, the tire deflates before the lever can be closed. Basically the same for the many pumps I've used back to the Silca frame ones.

upthywazzoo 

Hm, this would imply that your chuck opens the presta valve manually, because the air has to come from the tire to give a PSI rating if there hasn't been any pumping. This also explains why your release valve can work without reactivating the presta valve seal.