craftsman or buy the Sears brand with no warrantee.
Most places sell the "good stuff" with a warrantee and the "cheap
crap" without.. Some of the "cheap crap" isn't even all that bad, but
you are taking the chance - if it breaks you buy another one.
On Saturday, September 10, 2016 at 11:44:04 PM UTC-4, firstname.lastname@example.org wrote:
Jumping in late here as I hadn't been following this thread.
Please explain your math. You spent $18 to replace the module that is
now on the shelf. You spend $22 on the module that is in the laptop.
If the one of the shelf effectively cost you $40, then wasn't the one
in the laptop effectively free?
It seems like it was going to cost you *something* to get your laptop
up and running (either $18 or $22), so why do feel that the total
amount that you spent is sitting on the shelf?
On Tuesday, September 20, 2016 at 4:06:35 PM UTC-4, DerbyDad03 wrote:
Inquiring minds have to wonder how it could cost $18 to ship a memory
module for a laptop? I just shipped a box that's 26x9x8 that weighs
6 lbs from NJ to CA via Fed Ex economy ground for $13. A memory
module should cost less than $5 to ship.
> If I had a "better brand" lifetime warranted module it would not have
> cost me anything to get it replaced - just take it back to the supplier
> and walk out with a new one.
Best warranties are often found on least reliable products. With
numerous fine print exemptions. There is no replacement for spec numbers
that actually claim it does what it is suppose to do.
On Tuesday, September 20, 2016 at 9:34:45 PM UTC-4, email@example.com wrote:
But you didn't have a ""better brand" lifetime warranted module". You had
what you had. Why bring hypotheticals into this?
The module on the shelf cost you $18, not $40. The one in the laptop cost
Regardless of how *you* want to do the math, you either wasted $22 by being
impatient or you wasted $18 "just on principal". Either way, the one on the
shelf did *not* cost you $40.
On Saturday, September 10, 2016 at 10:17:50 PM UTC-4, T wrote:
This is just nuts. You can't get any meaningful MTBF by the method
you just described. What you showed was taking 1000 bulbs and testing
them for just an hour. That isn't predictive of MTBF over the life
of the bulbs. Following that method with say an ordinary incandescent,
you'd come away with the impression that they hardly every fail at all,
because incandescents rarely fail in the first hour and it has little
to do with how long they last in service. (In reality what you'd be
measuring is the infant mortality by your 1 hour method)
You find MTBF by testing the devices continuously over many hours,
finding out how many fail at 100 hours, 1000 hours, 10,000 hours
and then determining on average how many hours you get before failure.
Ed has it right, MTBF, properly calculated, is the average number of
hours that you get from an LED before it fails. In your bulb example,
you only tested one bulb to the failure point, in essence you have a
sample size of one.
"MBTF does not tell you anything about the second and so forth hours."
Of course it does. If we know that a bulb or an engine has an MTBF
of 20,000 hours, then we know that on average, that's how many hours
they go between failures. The device is very unlikely to fail at
two hours, or two hundred hours, but has a high failure rate at 20,000
hours. Are you telling us that MTBF only tells you about the first
That's scary. And if that's the case, why is it that every time you've
used the term here, you keep posting "MBTF", when it's actually MTBF?
I'm glad somebody noticed. The 'mean' time of a single failure is
exactly the time of that failure and is basically useless as a measure.
I would think that they could test 100 items until maybe 25 of them
failed and get the mean time from that selection of failures.
On Sunday, September 11, 2016 at 2:14:50 PM UTC-4, FromTheRafters wrote:
That would be getting closer to the real number, but even that gives you
an MTBF number that is too low. That's because you're only counting
the 25 that failed and not counting the 75 that were still working OK.
You know the MTBF based on averaging the number of hours it took for
those 25 to fail is a lower number than what you would calculate based on
averaging what you'd get from all 100. Knowing something about
the physics of whatever the system is, eg that it would typically follow
a bathtub shape curve, you could use some statistical methods to generate
a better estimate of the real MTBF based on the results of just 25 failing
out of 100 and stopping the test at that point.
On 09/11/2016 11:14 AM, FromTheRafters wrote:
Yup. The already do that. It is called the "bath tub curve".
More at Wikipedia:
We studied it in college.
There is a common misconception that MTBF means how long
it will last. It is pretty useless for determining
longevity. Folks often mix up "Mean Time to Failure
(MTTF)" with MTBF.
You would never believe how much sticking your finger in the
wind is involved in MTBF predictions too.
If you want to know what the manufacturer really thinks,
look at his warranty. On the above bathtub
curve, we were taught in college to set the warranty
at 90% of the useful lifespan.
In my opinion, Shop-Vac fudges the specs a bit.
They claim 6 peak horsepower yet their specs also state 120 Volts and 11.8 Amps.
So how does a 1416 watt motor yield 6HP?
Last I knew, a 100% efficient motor would yield 1 HP at 746 watts so this Shop-Vac is actually less than 2 HP.
On Wednesday, September 14, 2016 at 6:00:50 PM UTC-4, T wrote:
In the case of an LED light, which is what we are discussing,
MTBF and MTTF are the same, because there is no repair. In a
repairable system, where it's not scrapped at first failure,
you can repair it, then continue to measure the time to the
next failure. With an LED bulb the first failure is essentially
On 09/16/2016 07:26 PM, firstname.lastname@example.org wrote:
If I remember correctly when I use to do this for the military,
the numbers we used were from tables where they cooked the
parts to simulate aging. Then they did a formula on it
to come up with an MTBF. Everything was based on one hour.
MTBF is really only useful if comparing one part's
MTBF to another part's MTBF.
The warranty is where you get any real idea of how long
the manufacturer thinks things will last.
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