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tashirosgt
2011-Oct-05, 04:06 AM
I've been trying out a leakdown tester (http://en.wikipedia.org/wiki/Leak-down_tester). The instructions for the initial setup of the device say to adjust the pressure till the downstream gauge shows zero percent leakage. (The downstream gauge is marked in percent instead of PSI, unlike the illustration in the wikipedia.) This adjustment is done with the downstream hose completely closed. I'm curious what physics explains why any adjustment is needed? With the output closed, do the pressures on either side of the orfice equalize regardless of the input pressure? Is the adjustment merely trying to find the constant pressure that puts the downstream gauge at zero? (That would imply that the device is only designed for use with one particular input pressure.)

The output leads to a short length of rubber hose with a "quick connect" fitting on the end. If there is any leakage from this fitting, it isn't enough to hear.

profloater
2011-Oct-05, 06:56 AM
I've been trying out a leakdown tester (http://en.wikipedia.org/wiki/Leak-down_tester). The instructions for the initial setup of the device say to adjust the pressure till the downstream gauge shows zero percent leakage. (The downstream gauge is marked in percent instead of PSI, unlike the illustration in the wikipedia.) This adjustment is done with the downstream hose completely closed. I'm curious what physics explains why any adjustment is needed? With the output closed, do the pressures on either side of the orfice equalize regardless of the input pressure? Is the adjustment merely trying to find the constant pressure that puts the downstream gauge at zero? (That would imply that the device is only designed for use with one particular input pressure.)

The output leads to a short length of rubber hose with a "quick connect" fitting on the end. If there is any leakage from this fitting, it isn't enough to hear.

If there is no leak the second pressure gauge must register the same pressure as the first. As a leak increases the second pressure will drop. The orifice will then go sonic which limits the flow to a constant for a big leak. A good engine will have a very small leak past the pistons etc. Zero percentage leakage presumably means no leakage. From memory the orifice goes sonic at about 7 psi or half a bar.

Squink
2011-Oct-05, 04:43 PM
The instructions for the initial setup of the device say to adjust the pressure till the downstream gauge shows zero percent leakage.That second gauge isn't a flow rate meter, it measures pressure just like the upstream gauge.
Thus '100%' corresponds to a specific pressure.
If the system is closed, the pressure on both sides of the orifice will be equal, after kinetics gets done happening, but that pressure will not correspond to 100% on gauge 2, unless you adjust the pressure to make it so.

tashirosgt
2011-Oct-06, 12:55 AM
I bought another leakdown tester ("OTC" brand) which has both gauges marked in PSI. I find it simpler and more versatile that one with the second gauge marked in percentages ("General" brand from Harbor Freight).

It would be nice to be able to measure a rate of flow instead of a drop in pressure. It would also be nice if the adapters that screw into the sparkplug holes were long enough to stick out above the sparkplug wells.

pantaz
2011-Oct-06, 04:02 AM
Leakage testers can be great tools. Here are some articles on how to use them effectively.
http://www.mobiloil.com/USA-English/MotorOil/Car_Care/DIY/Engine_Leakdown_Test.aspx
http://www.motorcycleproject.com/motorcycle/text/leakdown.html
http://www.650motorcycles.com/LeakDownTester.html

profloater
2011-Oct-06, 07:09 AM
I bought another leakdown tester ("OTC" brand) which has both gauges marked in PSI. I find it simpler and more versatile that one with the second gauge marked in percentages ("General" brand from Harbor Freight).

It would be nice to be able to measure a rate of flow instead of a drop in pressure. It would also be nice if the adapters that screw into the sparkplug holes were long enough to stick out above the sparkplug wells.
For a fixed orifice and upstream pressure (above the sonic limit) you can calibrate the downstream pressure to give a flow rate within a range of leak rates.