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    Today's 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. In Portugal  Aeroquip occupies small part of the market due to high price, but stays in demand due to the above average quality. The hydraulic hose market was (and still is) an endless battle between a thousand and one brands fighting for a "place under the sun", and most Portuguese clients would go for the price, however some applications, demanding certain amount of hose and fitting quality, turned many heads into our direction, so we were receiving quite 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 oil dripping sample. That particular client brought over a small 1/4 inch R2 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 R2 fittings, and was about to start the hose assembly when it struck me that so far I hadn't seen an apparently new 1/4 inch hose present this type of failure. I asked the client, who was waiting at the counter, what the usual working pressure of the hose was.

    Oh, it is very low - he said - the recommended value is around 100 bars, 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 did they measure the above mentioned 100 bars. "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... 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 to see 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 bars, and that their normal working pressure was exceeding 1000 bars. 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 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, and is a very low pressure. One Bar is not a SI unit, but, strangely, 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 0.98 bars. (I skipped a bunch of decimals and am aware of it)

   Then, to make it more complicated, the scientists invented atmosphere (atm), which equals exactly 101325 Pa, the atmosphere pressure at sea level - so 1atm equals 1.01 bars.

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

   Oh yeah, there's also 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 am from Europe, I have to make an effort to deal with psi and prefer good old bars.

    As I was writing about the psi, I remembered a friend of mine, who once received an order for 10000 bar scale 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!

    The main point here is it's always important to pay attention not only to what people say, but also to what people gesture, and, of course, it never hurts to learn basic theory before messing around with oil-hydraulics.