Author: Philip Stubbs Date: To: Hampshire LUG Discussion List Subject: Re: [Hampshire] [OT] MTBF
2009/7/19 James Courtier-Dutton <james.dutton@???>: > 2009/7/19 Philip Stubbs <philip@???>:
>> 2009/7/19 James Courtier-Dutton <james.dutton@???>:
>>
>>> To get a MTBF figure:
>>> Approx MTBF = ( Number of Devices tested * Test time ) / Number of failures.
>>
>> As ever, that is not always the case. I used to work for an aerospace
>> company, and they would quote MTBF before one sample had even been
>> built, let alone tested to failure. In that case, the MTBF was
>> calculated based on some rules. I don't know the details but I do
>> remember that number of components were a key variable. Therefore the
>> pumps they made were designed to have a few parts as possible. The
>> MTBF number was therefore a result of some clever work by
>> statisticians. I would not be surprised if that is the case here. No
>
> It is my understanding that once one has the MTBF of each single component.
> Once one knows how they are fitted together, one can calculate
> availability figures for the sum of the parts.
> So, if one has a network with redundant devices, one can take the MTBF
> together with the mean time to restore (MTTR) of each device on the
> network and come up with a total availability for the entire network.
> The knowledge area is called RAMS analysis. (Reliability,
> Availability, Maintainability and Safety).
> The case is similar for components on a circuit board. One can have
> the MTBF of each resistor, etc. on the circuit board and calculate
> the the MTBF for the entire circuit board.
> As a general rule, more devices in series makes the result worse, more
> devices in parallel make the results better. This is why more
> components in the pump made it worse because I expect few of them were
> redundant components.
Sounds like you know more about this than me. (not hard) :-)
Having a figure for MTBF is a good thing. Being able to calculate a
MTBF at the design stage is also a good thing. What irritates me is
when a marketing department picks up on a measure, such as MTBF or
Watts or Hz or whatever, decide this is a good indicator for ho well a
product performs, and then starts pressuring the engineering side to
do whatever is required to increase the number. It then becomes
distorted.
An example. Vacuum cleaners used to be rated only in watts. If you
wanted a vacuum cleaner with lots of suck, you bought one that
consumed the most watts. However, the way the watts are calculated
were standardized. Run the vacuum in free air for one minute. Run the
vacuum with its inlet blocked for one minute. Average watts consumed
is then the rating. Marketing then say to the engineers, we need more
watts. Well the vacuum consumes less power when the inlet is blocked,
so the engineers introduce leaks into the design, so that when the
inlet is blocked, the pump is still shifting air and doing work,
keeping the power consumption up. Never mind that the vacuum
performance is compromised.
The end result is I no longer have much faith in the numbers on the
box. The more colours, pictures and words on the box means more input
from marketing, and the greater the pinch of salt needed. :-)
