--- In TekScopes@..., Don Black wrote:
I remember a similar thread some time ago where one of the members who
had worked at Tektronix recalled the testing of new exotic equipment. He
mentioned the weeping and wailing (probably teeth being gnashed too)
when some mega bucks piece failed and an $8,000 CTR was reduced to
shards of glass laying in the bottom. I guess it's much more common now,
particularly in the auto industry where every model has to be crash
tested. Formula 1 cars also now have to be crash tested and the benefits
are very plain to see. Crashes into barriers at 150 MPH that once would
have been fatal usually now result in the drivers walking away. Indeed,
it's no longer permitted but divers would often sprint back to the pits
and get into the spare car to continue the race.
Don Black.
On 26-Jan-13 1:02 PM, Rob wrote:
Thanks Steve, all that makes sense to me. I also understand how the
rubber deformation method brings some meaning to the 20G over 1ms into
perspective (or the impulse). I also understand the disconnect between
my original post and the tens of thousands of G's others mentioned. I
did not try to translate the 20G over 1ms into an actual impulse. So I
do not know how it compares to others.
Anyway, do you have a feel for if or how the specifications changed
over the years? Said another way, where the same military specs
understood when the 647 specification was written (I know Tektronix
was enough on the leading edge that some specifications were written
to what there equipment was capable of vs. any real world
need/criteria.). Would a 647 and a 7000 series scope meet the same
requirements? How about modern solid state scopes?. Conversely as
people became more acquainted with both Oscilloscopes and solid state
electronics. Where they allowed to be manufactured less bullet proof
(for lack of better wording),
The biggest reason I am asking all of this is because of the trouble I
have had shipping my refurbished scopes after I have sold them. I
package them what I consider to be very well. However the post office
is capable of defeating even the best packaging it seems. Then again,
perhaps the 7000 series scopes (non-militarized especially) are more
fragile then they appear to be on the bench and my packaging is indeed
not adequate. Would one expect a 647 or other scope with the
specification they claimed to survive better than a 7000 series or 475
or??? Given the post offices apparent ability to reach well above a
20G threshold.
As always Thank you for your time and the bandwidth.
Rob
*From:*TekScopes@... [mailto:TekScopes@...]
*On Behalf Of *Steve
*Sent:* Friday, January 25, 2013 5:16 PM
*To:* TekScopes@...
*Subject:* [TekScopes] Re: Tentative 647 scan?
Almost all Tek instruments were tested for dynamics, both earlier and
later versions. This included acceleration and vibration.
Acceleration is tested as deceleration. The instrument is firmly
attached to a large steel plate in a test stand. The plate is dropped
from a standard distance. An elastomeric moderator (block of rubber)
is placed at the bottom of the test stand, where the plate will hit to
control the G force. The moderator integrates the peak deceleration
force over time. Different types and sized of rubber are used. Soft
rubber lowers the peak G force, spreading it over a longer time. As it
gets stiffer, the peak gets higher, but the duration of the
deceleration is lower (energy = Mass * velocity^2). If you took the
rubber out (never done in practical testing), the G force would be
huge (tens of thousands of Gs) with ultra-short duration. Basically,
the tiny flexing of the massive steel elements would limit the peak
force from becoming an impulse function (infinite peak with 0 time).
Changing the drop height allows fine tuning of the test forces. An
acceleratomer is attached to the plate to monitor the force and time.
Testing is either started at a low value and worked up, or a dummy
weight with the same mass as the item to be tested is first bolted to
the test stand to calibrate the force.
The dynamics specs were not listed in the later catalogs as they were
contained within the military specification that most instruments were
designed to, which was listed. Typically it was MIL-T-28800C -- class
5 for lab instruments, or class 3 for portable instruments.
(MIL-T-28800B preceded `C', and MIL-T-28800A precede B)
- Steve
The environmental and dynamic specs have not really degraded much over the years. When scopes started to include floppy disk drives, then embedded PC, then had to relax the operating temperature range, as these components are only rated for operation up to +40 degrees C, rather than the +50 that standard class 5 requires.
Most of the vendors shifted to the +40 spec, because the drives were not available with the higher operating temp. Unfortunately, the +40 spec has become the defacto standard, and is applied to other instruments which don't even have these components. People say this is not a serious reduction in performance, as 40 degrees is not often encounter in a lab. While this is true, the +40 does limit operation in some rack mount setups, as the cooling air is often mixed with the exhaust air from other instruments. Active probes can be installed between cards in a rack, and situations where instruments with a +40 degree rating will overheat.
- Steve
