Tyre pressure measurement 22 September 2022 at 8:48PM edited 22 September 2022 at 8:53PM in Motoring 55 replies 1.7K views

sevenhills

Is there a UK default measurement of tyre pressures? Our works vans have the PSI pressure written on the wheelarch.
Our new gauge does not display PSI though. The manufacturer, VW, display PSI and bar.
Not a clue where bar originated, PSI is imperial but are we now metric?

flashg67

Bar is a metric measurement, so used internationally(pretty much). New stickers needed on the vans, or I guess you'll learn what they are in bar pretty quickly anyway.
 I presume you don't mean your new gauge is pictured above, as the inner scale is PSI ......

sevenhills

flashg67 said:

Bar is a metric measurement, so used internationally(pretty much). New stickers needed on the vans, or I guess you'll learn what they are in bar pretty quickly anyway.
 I presume you don't mean your new gauge is pictured above, as the inner scale is PSI ......

Yes, just looking at that 4.2 - 61

Car_54

The current flavour of the month is kiloPascals. 1 Bar = 100 kPa

Ectophile

Barometers have been calibrated in bars for years, since we stopped using the old inches of mercury (or millimetres of mercury if you've got a French barometer like mine).  Roughly speaking 1 bar = 1000 millibars = 1 atmosphere.

Though 1 bar on a pressure gauge will mean 1 bar above atmospheric pressure.

As Car_54 points out, 1 bar = 100 kPa, which is a proper metric measure.  1 Pa = 1 newton per square metre.  So 1 bar is 100000 N/m².

jimmo

PSI stands for Pounds per Square Inch.  lb stands for pounds (libra pondo). So that gauge is showing just over 30PSI. 

Goudy

Ibf/in2 on the inner dial is Pounds Force per Square Inch which is just another way of saying PSI (pounds per square inch).

It's an imperiaI unit, though I wouldn't call this an "old" measurement as it's also a current US customary unit.

Bar, Kilopascal (KPa) and Pascal (Pa) are metric or SI units (International System of Units).
They are named after Blaise Pascal.

1 bar equals 100 kilopascals which equals 100,000 Pascals.
And one Pascal equals one Newton per Square Metre so that 1 bar equals 100,000 newtons per square metre.

If you remember 14.5 to 1, that should help you convert PSI (and Ibf/in2) to Bar and vice versa if a gauge only reads one unit.

Jenni_D

And to circle back to (part of) the original question ... tyre pressures should be as per the tyre manufacturer recommendations, not necessarily what's printed in a handbook or on a wheel arch.  

Iceweasel

Are we now metric - we've been metric since 1972. No school in UK has taught in Imperial since then.

shiraz99

Iceweasel said:

Are we now metric - we've been metric since 1972. No school in UK has taught in Imperial since then.

Yet we still measure distance in miles not km.

Jenni_D

shiraz99 said:

Iceweasel said:

Are we now metric - we've been metric since 1972. No school in UK has taught in Imperial since then.

Yet we still measure distance in miles not km.

Except in athletics ... I can't remember a Mile race for a long time.