[Roland S]

hi

i wanted to test how much pressure my machine is generating. i bought a pressure guage and some quick connects and stuck it on the outlet just before my pressure hose. it shows about 2000 psi when running through my mosmatic at full pelt. but if the water is coming out of that then it isn't going to give me an accurate reading. a dumb question but what's the best way of checking the pressure accurately?

[Ben Marriott]

Evening
The only way to really measure the "true / head pressure" is to put the gauge directly into the head of the pump. (There is usually a gauge port on most piston pumps.)
When you work with your machine with the trigger pulled this will then give you your true / head pressure.
Unless there is water coming from your mosmatic for instance there is not actually any pressure being generated because your unloader valve is in bypass.

Putting the gauge inline or after the unloader valve gives a line pressure, this is the pressure that is coming out of the machine. If you buy a 250 Bar 3600PSI machine the head / true pressure will be 250 Bar. The pressure at the end of the lance will be 20-30 bar less.

The reason for this is that each time the water coming from the pump meets a fitting or metal component it creates friction, friction reduces the pressure available all the way to the lance nozzle. Every metre of hose reduces the pressure by 0.1 Bar, so for every 10m of hose you loose 1 Bar.

Hope this helps.

Ben

[Roland S]

hi ben

thanks for the reply. but if i had the water coming out of my mosmatic wouldn't that reduce the pressure even further as the water isn't being complety restricted - before taking friction into account, or have i got that wrong? e.g. if i had the water coming out of just the hose with no lance or fsc on the end the pressure would be barely anything, so if it's coming out of the end of my fsc or lance surely it's going to be reduced?

i'm getting 2000psi from my 250bar machine, which is obviously just over half of 3600psi, has anyone else got a guage setup as above? what pressure is your's showing.?

[Ben Marriott]

Sorry I might not have explained that well enough.

With no lance / trigger attached you are pumping water.
The nozzle size of your lance is specific to your machine in your case either 04/045 the idea being that your machine when new would give 250Bar 15LPM with that jet. 

A pressure gauge works on back pressure, you will only be getting back pressure when the machine is operating with water flowing through the lance jet.

If we set-up a pressure gauge as you have on your machine we would get 3300PSI on a new 250 Bar machine.

Ben

[BDCS]

It sounds like your nozzle is either the wrong size or worn.  Best put your pressure guage in the port on your unloader and set the unloader to your max pressure - this is the absolute max when the unloader dumps to protect the pump. The pressure shown when cleaning should be reasonably close to 85% of the max when running the right oriface and as they are cheap you should try a few to get it right. You should check the flow back to tank when running ( cleaning )and see no flow. You need to know the guage reads correctly as well,  I don't keep mine on the machine unless fault finding. I find flow sensitive unloaders last longer

[Roland S]

cheers guys that's a big help, i've got 2 x 0.2 on my mosmatic, i think i changed them a fairly recently but will put a new pair on to make sure they're not worn.

if the valves needed changing on the pump (interpump ww965) would i have any other symptoms like noise if they need replacing? how often should they be changed?

to be honest i haven't noticed a significant drop in pressure but if it's been a gradual decline maybe i haven't noticed.  i suppose the guage could be wrong as well it was only cheap but looked ok http://www.ebay.co.uk/itm/281638669216?_trksid=p2057872.m2749.l2649&var=580641312488&ssPageName=STRK%3AMEBIDX%3AIT.

[BDCS]

The valves are easy to remove and check the seal by blowing through them. Check the RTT first because its easy and the unloaders wear. Do you have a trapped pressure unloader or flow sensitive ?

[Ben Marriott]

With pump valves you should be able to press the centre disk down against the spring and it bounce back easily if it needs replacing or any of the springs are broken  / plastic brittle and flaky they won't. If this is the case you need a kit 123.

If when on the correct nozzle size with the machine running on full unloader turned all the way up you are still returning a lot of water in bypass the unloader valve may be worn as above

If this is the case the cheapest replacement is a VRT3 trapped pressure unloader valve. Flow sensitive is slightly more expensive but just as good if you don't regularly use the unloader to drop pressure.

If when all this is done the pressure is still down you may have a water seal that is leaking slightly which would account for the pressure drop. It may also be worthwhile checking that the inlet is tight onto the fitting as an air leak would cause cavitation and reduced pressure as the pump draws air in with the water.

[Roland S]

The valves are easy to remove and check the seal by blowing through them. Check the RTT first because its easy and the unloaders wear. Do you have a trapped pressure unloader or flow sensitive ?

thanks i'll check the valves then. I don't generally use the RTT or unloader, i have my triggers cable tied open. i know it's not good practice but it was what i was told when i bought it and have never changed. i have a trapped unloader currently, i may start using it, but will need to buy a spare to keep in the van in case it goes while working. are the flow sensitive better?

[Roland S]

With pump valves you should be able to press the centre disk down against the spring and it bounce back easily if it needs replacing or any of the springs are broken  / plastic brittle and flaky they won't. If this is the case you need a kit 123.

If when on the correct nozzle size with the machine running on full unloader turned all the way up you are still returning a lot of water in bypass the unloader valve may be worn as above

If this is the case the cheapest replacement is a VRT3 trapped pressure unloader valve. Flow sensitive is slightly more expensive but just as good if you don't regularly use the unloader to drop pressure.

If when all this is done the pressure is still down you may have a water seal that is leaking slightly which would account for the pressure drop. It may also be worthwhile checking that the inlet is tight onto the fitting as an air leak would cause cavitation and reduced pressure as the pump draws air in with the water.

thanks yes as i mentioned above i will check the valves. i have a spare set in the garage if they do need changing.

the air on the inlet is a good idea and something i will check, do you hold a flame to it or if it is sucking in air will it be more obvious than that?

[Ben Marriott]

the air on the inlet is a good idea and something i will check, do you hold a flame to it or if it is sucking in air will it be more obvious than that?
[/quote]

Just check the inlet is tight - if it is drawing in air their may be a pulsing or vibration when working - may also be a lot of air bubbles coming though the return hose.

Ben

[Roland S]

Thanks i'll check it out