# Floor pump pressure reading accuracy...



## rkdvsm (Jul 15, 2006)

I read about the inaccuracy of the pressure gauges of floor pumps. My question is what floor pump is considered to be accurate in their reading of the tire pressure?

I don't want to spend too much money on a floor pump, but I am willing to spend if it means getting a pump with accurate gauges.

Thanks.


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## Pitts Pilot (Dec 5, 2011)

It would appear that cost and accuracy are not well correlated, as my $100 Lezyne, although stunning, reads 20 lbs low according to all three of my gauges.


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## FBinNY (Jan 24, 2009)

Even a precision gauge won't necessarily be accurate on a floor pump. it isn't the gauge, but the plumbing. 

The gauge is at one end of the hose, and there's a valve between that and the tire. 

With a Shrader valve and a thumblock head that keeps the valve depressed it could be fairly accurate, but you'll lose some air when you disconnect the hose, so expect the pump gauge to read higher than what you end up with.

With PV it's a bit more complicated as you pump the hose pressure climbs above the tire pressure and opens the valve, and you push air in. At the bottom of the stroke you begin to lift the piston and a bit of air escapes the hose back into the pump before the check valve closed (varies with different pumps). This drops the hose pressure below the peak and the tires PV (which is also a check valve) closes. After all it done, the hose pressure will likely be lower than the tire pressure.

However most of the gauges used on pumps have decent repeatability, so if you pump at similar speed the gauge will read consistently time after time. It may be high or low, but once you have a handle on it you'll be able to pump to optimum pressure very reliably.


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## bikerjohn64 (Feb 9, 2012)

I would highly recommend using a separate hand-held pressure gauge to take a more accurate reading. Then after doing so with the hand-held; go back to using the floor pump and then mark on the floor pump gauge where the PSI number corresponds to your hand-held PSI. My floor pump gauge has a bunch of lines I "sharpied" on the gauge that tells me the more accurate PSI at different range of the gauge. There is one around the 30 PSI for my mountain bike settings and one around the 100 PSI for my road setting. Both those marks are about 5-15 PSI off the gauge markings.


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## rkj__ (Mar 21, 2007)

I agree with those suggesting to get an external gauge I you want accurate readings. 

My $60 MSRP floor pump reads ~12 psi low.


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## looigi (Nov 24, 2010)

I have 3 floor pumps and the all read the same within a few pounds and agree with the gauge on my compressor. Of course they could all still be off by the same amount. Inflating tires with presta valves requires you read the pump gauge pressure while the pump piston is moving downward and the air is flowing into the tire keeping the valve open. Do the last strokes of the pump moderately slowly to get a good read.


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## FBinNY (Jan 24, 2009)

rkj__ said:


> I agree with those suggesting to get an external gauge I you want accurate readings.
> 
> My $60 MSRP floor pump reads ~12 psi low.


If you've confirmed that it consistently reads 12psi low, then you no longer need the hand held gauge. Simply compensate by (over) inflating by 12 psi and you'll come out exactly where you want.

That assumes that the hand held gauge was accurate (they also vary, some by as much as 10psi).


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## rkj__ (Mar 21, 2007)

FBinNY said:


> If you've confirmed that it consistently reads 12psi low, then you no longer need the hand held gauge. Simply compensate by (over) inflating by 12 psi and you'll come out exactly where you want.
> 
> That assumes that the hand held gauge was accurate (they also vary, some by as much as 10psi).


Yes, the hand held gauge only needed to be used once. The floor pump gauge is repeatable, so I don't need to use an additional gauge each time.

12 psi is an estimate. It could be, 9, 10, 11, 12, 13, 14, but I know the pump gauge reads low. If I was actually running 20 psi in my tubed MTB tires, I'd get a lot of pinch flats.


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## danl1 (Jul 23, 2005)

FBinNY said:


> Even a precision gauge won't necessarily be accurate on a floor pump. it isn't the gauge, but the plumbing.
> 
> The gauge is at one end of the hose, and there's a valve between that and the tire.
> 
> ...


Great points. 

There are some pump heads that force presta valves open when locking, too. The downside is that they also tend to be good at bending the valve cores. 

And a point further down the thread is important, too: Precision (repeatability) is more accurate than accuracy. I don't really care if that 85 lb mark on my gauge is really 85 lbs*: I care that higher than that and the ride suffers. lower than that and the efficiency suffers. 


*Or whatever number your ride testing on your bike, roads, and tires works out for you to be best.


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## vinceflynow (Jan 31, 2012)

My Nashbar Big Orange pump is amazingly accurate. I check it against an Accugage.


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## stevoo (Oct 26, 2011)

I opened up my Blackburn pump and re-calibrated the tube on the gauge. Viola. Now it reads correct.


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## velodog (Sep 26, 2007)

How do we know that the hand held gage is accurate, and how many of us have ever had a gage calibrated?


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## stevoo (Oct 26, 2011)

I used a calibrated pressure gauge from work.


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## esldude (Jan 3, 2009)

Yeah, I have three regular gauges, two match, one is usually 2-3 lbs low. The gauge on my Lenzyne floor pump matches the higher pair of gauges give or take a pound on schrader valves. Don't have a presta gauge (well it is in a closet somewhere like for 15 years). It is easy to see/hear on the downstroke of the pump when the presta valve has opened. I just look at it and when getting close slow down on the last few strokes reading it as I go. Seems to be pretty consistent, and likely accurate.


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## nayr497 (Nov 8, 2008)

*great idea!*



bikerjohn64 said:


> I would highly recommend using a separate hand-held pressure gauge to take a more accurate reading. Then after doing so with the hand-held; go back to using the floor pump and then mark on the floor pump gauge where the PSI number corresponds to your hand-held PSI. My floor pump gauge has a bunch of lines I "sharpied" on the gauge that tells me the more accurate PSI at different range of the gauge. There is one around the 30 PSI for my mountain bike settings and one around the 100 PSI for my road setting. Both those marks are about 5-15 PSI off the gauge markings.


What a fine suggestion! Very smart way to do this.


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