Yes it is still 1.5 bar BUT the amount of stored energy will be 3.33 times more in the 20ft pipe compared to the 6ft pipe. But if the Ducati system is pressurised to 15psi (approximately 1 bar), and I am not saying it is, that is a relative pressure so the absolute pressure will be about 30psi (or 2 bar) which is more than the 1.7 bar you mention.
forget about ambient pressure, 1bar inside 1bar out side =0 most caps rated to about 0.7- 1bar the water will start to boil around 110-115c ref auto data.
The whole point of a pressurised system is to raise the temperature to make the radiator more efficient; a hotter radiator dumps more heat energy. And I am sorry fin but if a radiator cap says 1bar then that means 1bar over ambient making the absolute pressure 2 bar. Otherwise what is the point of having a cap with 0.7 to 1 bar on it, you might as well take it off and have the system open to 1 bar of atmospheric pressure.
The gauge is measuring the differential between the tyre pressure and ambient pressure. So the tyres 15 psi is different at sea level and altitude, plus temperature has a baring as well.
Pressure measurement - Wikipedia, the free encyclopedia See absolute, gauge and differential pressure measurements, it is close to the top.
i assume you mean the difference between outside and inside? what ever the gauge says it it is +- tolerance? i think most things in life are only as complicated as some like to make it. accept it. the higher the pressure the higher the boiling point which leads to the higher the expansion the higher the pressure the higher the temp the coolant will boil. as you all know there are coolants out there that wont or don't expand at the same rate. the lower the out side pressure the higher the differential.
Ok, on the premise that Google is our friend, I have spent a couple of hours googling. I am certainly not an instant expert now but I do have a much better understanding of what is going on. The key component here is the humble radiator cap, it has 3 seals on it. The outer seal keeps everything in at all times, the lower seal is spring loaded and is the pressure relief valve. In the spring loaded plate whose outer edge forms the main pressure seal is another low pressure valve which is the return valve for coolant which has flowed into the overflow tank, to allow it to flow back once pressure in the system drops as the engine cools. The pressure in the system is produced by heat from the engine. With a cold engine there is little or no pressure. However, as the coolant temperature rises, it expands and pressurises. The pressure is controlled by the main valve in the filler cap and filler caps have different pressure ratings according to manufacturers specs. The rating of the filler cap in my 999 is 1.2 bar - it is stamped on it. Water has a nominal boiling point of 100c at sea level. However, our bikes should have as standard a ratio of 50/50 water and 'antifreeze'. Allegedly, this modifies the boiling point to 227F or 108.33C, a useful increase. However, as I hope we all accept, pressure modifies the boiling point as well, so our humble radiator cap, if it has a rating of 7 pounds will (allegedly!) increase that to 248F or120 degreesC - even better! A 14 pound rated cap will give 263F, which is 128.33C. 1.1 bar is 15.94 pounds, so more than that and the 999 has a cap rated at at 1.2 bar, which is 17.4 pounds per square inch! (According to the online calculator I used). Therefore the boiling point of the properly specified coolant in a Ducati 999 with a healthy cooling system is around 130C! Of course, having too high a cooling system pressure will cause the system to leak, so there are limits but it is apparently possible to buy aftermarket caps with a higher rating and if your Ducati has a 1.1 cap, as I believe some do, it may be beneficial to swap to a 1.2 but the extra pressure might cause problems. It does occur to me however, that over time the main spring in the cap may weaken, so buying a new cap may be beneficial. Checking the seals will definitely be beneficial...
Exactly. Our tyre pressure gauge is gauge measurement (i.e. relative to ambient) so if our tyre has 15psi in it when we measure it in our atmosphere and you take that same tyre and gauge and do the measurement in a vacuum it would read 30psi, the absolute pressure hasn't changed but the relative pressure (to ambient) has gone up. The absolute pressure has always been 30psi. A radiator pressure relive valve operates relative to atmospheric so when it says 1 bar it is one bar over atmospheric that it vents, therefore in absolute terms the fluid is at up to a 2 bar pressure and it is this overpressure, the magnitude of which is dependent upon the temperature of the closed system, that allows the system to run at over 100C.
As a kid, I remember driving in the Swiss Alps in my Dads MG. It didn't have a pressurised system which caused real problems. Going up a pass made it boil like crazy as the hard work the engine was doing combined with the altitude, which reduced the boiling point of the coolant. He put a spring-loaded wooden clothes peg on the rubber tube of the overflow from the radiator. This effectively created a very crude pressurised system and pretty much cured it.
Maybe it is slightly misleading to call the system pressurised. Perhaps it would be more accurate to think of it as a contained system. With a cold engine it will only be at or around atmospheric pressure. It is only as the coolant warms and tries to expand that the containment creates pressure. That pressure is limited to the value stamped on the cap, which will start to relieve the pressure into the expansion tank if extra pressure occurs. Once the coolant cools, the coolant that has flowed through the relief valve into the expansion tank will be drawn back via the return valve. Non pressurised systems used to have a rad with an air space for expansion above the matrix and a filler cap with a simple overflow pipe at the top of that. As I understand it, that is no longer the case and the system should be sealed and bled with no air in it, other than in the expansion tank.
And you wouldn't have been wrong, it all depends whether it is an absolute, gauge (relative to ambient) or differential measurement.
good job or thirty plus years of training would of been right down the swanny never seen an absolute gauge,
Gosh, it's making my head hurt trying to work out if the pressure necessary to move the valve would vary according to altitude. I'm guessing it would because if the pressure the other side of the valve were equal or greater, it would never move. So equally, if the pressure is less than 'normal' it should move more easily. Is that right??
