Friday, 27 July 2018

Chuck's Tech Opinion: Bicycle pumps, pressure gauges: Can you trust their measurement?

Can you believe in the pressure measurement of your pump or tyre pressure gauge? Here are a couple of handy gauges: Schwalbe digital gauge on the left and an analogue AccuGage on the right:

Digital Schwalbe: 35g
Analogue AccuGage: 86g
Many pumps, both big and small, have gauges these days. L to R in the photo below: Lezyne Floor Drive (steel), Topeak Joe Blow Sport and Topeak Turbo Morph G: 

So, with these pumps and gauges, I decided to gather some data to see what I could find out. Using a large volume presta valve MTB tyre (Maxxis Ardent), a Challenge Grifo cyclocross tyre, and a Continental Ultra Sport road tyre, I set various pressures using the Joe Blow Sport. I chose that one simply because it was new and the gauge nice and clear: 

The protocol was to set a pressure with this pump, and then with the other pumps and gauges to check what reading they gave. So the Joe Blow acted as the reference value. Of course, we don't know which of these gauges gives the best absolute measurement. 

Absolute means closest to the actual correct standard unit measurement. Contrast with precision, which is about how repeatable a measurement is. 

Here is a close up of the gauge on the small Morph: 

I didn't expect the small dial to allow precision in measurement. And finally, the Lezyne dial, which has a crack in it: 
The clear issue here is what looks like a zero error. With no tyre attached to the pump, it reads about 20psi. (I could not find a way to zero its gauge). I did my best to minimise air escape while switching between gauges. 

OK, with that description, here are the results in a graph (units on each axis are psi):

The x=y line is the Joe Blow "reference" value. You can see that the AccuGage ('+') and the Schwalbe digital ('solid dots') followed that line very well up to about 50psi. For higher pressures, the Schwalbe still followed that line well, but the AccuGage fell a bit below it (~5psi at 80psi). Remember, we've just randomly selected the Joe Blow to act as the reference. The Morph ('open circles') seems to be reading generally 5psi higher than Joe Blow reference and the two stand alone gauges. Most obviously, the Lezyne ('x') reads consistently ~20psi higher than the Joe Blow, AccuGage and Scwhalbe. 

The other thing that came out during this test is that ease of use is a big deal. The Lezyne and the Joe Blow are the easiest pumps to use. The Morph is rather awkward in comparison, and reading the dial is not easy. Both of the stand alone gauges require a bit of practice to use efficiently. With the knurled nut of the Presta valve slightly open, you push the gauge down over the valve and the instrument measures and holds the reading. Out of the two, the AccuGage was much nicer to use because it was easier to slide over the knurled nut of the valve. This seems to be down to its external shape and the size of the opening. The needle stays where it is until you press the very convenient air release button. No batteries, no switching on or off. The Schwalbe has a smaller hole, so tends to contact the knurled nut on a Presta valve more and allow air to escape. Also, the oval shape doesn't give the hands as much purchase on the tool. Perfectly functional, but compared to the AccuGage it was a little irritating during use. On the other hand, the digital gauge is lighter and fits better in a jersey pocket.

It seems reasonable to conclude that because three gauges were pretty consistent with each other (Joe Blow, AccuGage analogue and Schwalbe digital) that these are the closest to measuring the absolute pressure value. That conclusion is consistent with the Lezyne simply suffering a +20psi zero error.

I will continue to use the Lezyne, despite what appears to be its lack of absolute accuracy. All I need to remember is that it reads 20psi high consistently across the usual tyre pressure ranges. This is purely a systematic error. The Morph gauge reads about 5psi high, but its dial has poor resolution anyway. Its utility is in its portability and the fact that for its small size, it can pump up pretty hard. For off road and remote use (e.g. with a pocket pump), the stand alone gauges would get the nod. I'd use the Schwalbe when I need to be able to differentiate between 1 or 2 psi (cyclocross) or if I need to carry a gauge in my pocket. That said, the ease of use of the AccuGage is a big attractor. 

Yes, you can trust your pressure gauge measurement, BUT only if you really understand what it's telling you!

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