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Correct way to certify hydrostatic testing when it is not 100% (and Sample Size)

#1
I work in the hydraulic hose industry and came across an issue yesterday that has me stumped because our company has been doing it this way for 30 years. We perform hydrostatic testing on our hose assemblies per the requirements of SAE J517. The standard says to test every sample, double the working pressure of the hose and hold pressure for not less than one minute. Our company had interpreted the “test every sample” as to we determine a sample size. We use an AQL of 1.0 by default when a sample size is requested. When testing is done, the paperwork gets turned in for formal test certification to be created. On the test cert we list the specification, pressure, time duration as well as all of the sample represented in the lot, so if the order is for 20 hoses and we only tested 13, we are certifying that all 20 hoses passed testing. Is this misleading? Should we only be certifying to the number of hoses tested?
 

Ronen E

Problem Solver
Staff member
Super Moderator
#2
Yes, it's misleading. Either state that only 13/20 units were tested (I wouldn't recommend it) or state that the lot was tested at an AQL=1.0. Selecting that level of verification means that you (and whoever accepts your shipment, provided that they are aware of this practice) is okay with 1.0% of the units actually failing, in the long run. Please also consider that AQL sampling is not purely statistics/mathematics-based, and it includes an economical component acting behind the scenes.
 

Randy

Super Moderator
#3
Keep it simple and honest "Testing has demonstrated that......." or "Test samples have shown......" and leave it there. It's not a lie and it's not misleading.
 

Ronen E

Problem Solver
Staff member
Super Moderator
#4
Keep it simple and honest "Testing has demonstrated that......." or "Test samples have shown......" and leave it there. It's not a lie and it's not misleading.
Whether the statement is honest or misleading would depend on what comes instead of your "...............".
Wordsmithing is exactly what makes the difference between real "simple and honest" and misleading. And yes, a statement can be both "not a lie" and misleading.
 

Randy

Super Moderator
#5
Whether the statement is honest or misleading would depend on what comes instead of your "...............".
Wordsmithing is exactly what makes the difference between real "simple and honest" and misleading. And yes, a statement can be both "not a lie" and misleading.
How can either one be misleading? Have not tests been performed? Were the tests done of samples and not the whole? You drive a car everyday made up of parts that have not been 100% tested, fly on airplanes and eat food. OK so they don't use some scientific, gee-whiz, whamo-bamo test protocol and they just grab a line every now and then, test it to whatever pressure they do and it passes... Can we prove it wrong or invalid? Nope.....And like a light bulb the stinking thing can fail at any time
 

Ronen E

Problem Solver
Staff member
Super Moderator
#6
There's nothing misleading in informing that some of the units have been tested, as long as everyone is on the same page as to the level of testing (i.e. that it's not 100%). It's only misleading if stated in a way that can create an impression factually different from the truth.
 

Mike S.

An Early 'Cover'
Trusted
#7
This may be a bit off topic, but I'm curious:

You have a lot of 20 hoses. Do you test 13 of those actual hoses, and, if they pass, ship the tested 13 with the untested 7?

Does the test in any way degrade the hose, even if it passes?
 
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