Insane Hydraulics

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Which is better - bar, at, atm, MPa, or psi?

A conversation with a client (concerning some hydraulic hose issues) made me remember a curious episode that I witnessed years ago. Back then I was employed at a hydraulic company which, among other brands, represented Aeroquip - an Eaton's name for quality hose and fittings. At that time Aeroquip occupied only a small portion of the Portugues market due to high price, but stayed in demand due to the above average quality, so we were still receiving a number of hose orders every day.

Most of the times, when a client wants a new hydraulic hose, he shows up on your doorstep with what-s-left-from-the-old-one in hand for you to make a new one in accordance to the often unrecognizable oil dripping "sample". That particular client brought over a small 1/4 inch R2AT hose. The hose and the fittings were Aeroquip, and the client was complaining about oil leaking from under the sleeves. As usual, I separated the necessary fittings, and was about to start the hose assembly when it struck me that it wasn't normal for an apparently new 1/4 inch Aeroquip hose assembly to present this type of failure. I asked the client, who was waiting at the counter, what the working pressure of the hose was.

"Oh, it is very low" - he said - "the recommended value is around 100 bar, on occasion we go as high as 110-120, but never more than that." It was not what I heard, but rather what I SAW, that caught my attention - as the man was talking about pressures, he made a gesture as if he was moving a hand pump lever. I wondered if they were using a hand operated pump. The man said "oh yes, a small hand pump, yes" and pointed towards his truck. So I asked him if he could show me the pump, and also asked how they measured the above mentioned 100 bar. "it's easy, the pump has a pressure gauge mounted on it, let me show you..." - said the man and went for the truck to get the pump.

When he laid the pump on the counter I got even more convinced that something was not adding up here... The pump was apparently of a very high quality, with a long lever and a ve-e-ery small piston. Everything about it was screaming "extremely high pressure". When I turned it around to check out the pressure gauge... I saw a gauge that had a 140 MPa scale, with a permanent marker line on the glass, marking roughly 110 MPa. "See?" - said the man - "We normally don't even go over the line! But them damned hoses just don't seem to hold it..."

No need to say that the man was genuinely surprised to know that 1 Mpa meant 10 bar, and that their normal working pressure was exceeding 1000 bar. Applying special high pressure hoses solved the leakage problem for good.

I know, of course, that YOU know it, but I am still going to provide a small literacy training session on pressure units to see if I remember it right...

Ok, so in SI (the International System of Units or metric system) the pressure is measured in pascals or Pa, one pascal is one newton over one square meter. One pascal is a very low pressure. A bar is not a SI unit, but, strangely, one bar equals exactly 100,000 pascals. It has nothing to do with beer bars or golden bars.

Then we have a one kilogram-force over 1 square centimeter, which is a technical atmosphere (at). The one pressure unit that makes sense, as everybody can picture one kilogram and one square centimeter. A kgf is also not a SI unit, and equals the force one kilogram of mass produces in standard gravity, which would be 9.8 Newtons, so one technical atmosphere is about 0.98 bars.

Then, to make it more complicated, the scientists invented atmosphere (atm), which equals exactly 101,325 Pa, the atmospheric pressure at sea level - so 1 atm equals about 1.01 bars.

From the practical hydraulics point of view there is no difference between 1, 1.01 and 0.98, so it is common to call bars kilograms or atmospheres.

Oh yeah, there's also the psi or Pound Per Square Inch, which is the product of the avoirdupois system (based on pound mass, used in the US, UK, Canada and colonies). 1 bar equals 14.5 psi. As I operate from Europe, I prefer the good old bar and have to make a mental effort to deal with psi.

A friend of mine once received an order for 10000 bar pressure gauges. Naturally, he thought the client confused bar with psi, and supplied gauges with a 700 bar scale. It turned out the gauges were to work in a high pressure water cutting tool which, indeed, worked with pressures rating several thousand bar... boy was his face red!

Anyhow - the main point here is that it's always important to pay attention not only to what people say, but also to what people gesture when they communicate a hydraulic problem, and, of course, that it never hurts to learn basic theory before messing around with oil-hydraulics.