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Decipher this Specification

Decipher this Specification

Decipher this Specification

(OP)
"The visual warning required... must be installed such that it does not require more than +/- 15 degress side-to-side head rotation as viewed..."

Do you interpret this as meaning (A) a total of 15degs of head rotation (7.5degs left & right), or do you interpret this as (B) a total of 30degs of head rotation (15degs left and right).

"But what... is it good for?"
Engineer at the Advanced Computing Systems Division of IBM, 1968, commenting on the microchip.
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

RE: Decipher this Specification

I'd go for B.

RE: Decipher this Specification

+/- X to me always means +X or -X, meaning total range of 2X.

Hg

RE: Decipher this Specification

(OP)
Thanks for the quick response.  There was some discussion here at work, and I needed some additional ammunition.

"But what... is it good for?"
Engineer at the Advanced Computing Systems Division of IBM, 1968, commenting on the microchip.
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

RE: Decipher this Specification

ditto to the above

TTFN

RE: Decipher this Specification


Hi MadMango,
I hope, for your companys sake, that the others involved in the discussion, are not responsible for any engineering decisions. Thats pretty basic "cast-in-stone" stuff to be debating.

±.002" = total .004"
+.005"/-.002" = total .007"

from (the City of) Barrie, Ontario.

What happens if you get scared half to death twice?

RE: Decipher this Specification

I agree that it will be B in writing, but when speaking to say "plus minus 15 degrees" can mean approximately 15 degrees which could be interpreted as 7 1/2 each way.

Stephen A

RE: Decipher this Specification

Of course the warning sign to prevent accidents is suitably insured against ricked necks....................

RE: Decipher this Specification

Stephen A,

I have to disagree. To me, "plus minus 15 degrees" when spoken means +15/-15 degrees = 30 degrees.  To interpret it as 7 1/2 degrees each way you would say "15 degrees plus or minus".

RE: Decipher this Specification

or within 15°

from (the City of) Barrie, Ontario.

I tried sniffing Coke once, but the ice cubes got stuck in my nose

RE: Decipher this Specification

I agree: 15 degress to the left, 15 degrees to the right. I prefer to use ~ rather than +/- to indicate approximatley.

But more confusing to me is the term "as viewed."  It would make more sense if it read:

"..must be installed such that it does not require more than +/- 15 degress side-to-side head rotation to be viewed directly..."

Or something like that.  Without turning the head at all there's almost a 90 degree field including peripheral vision. Maybe it makes more sense in context.

--Jonathan

RE: Decipher this Specification

Arghh!

I thought I clicked preview rather than post.  Obviously not, otherwise I would have caught the two mistakes in the first sentence.  Degrees. Approximately.

RE: Decipher this Specification

+/- 15 deg. doesn't mean "approximately 15 deg."  It means you can go up 15 deg. or you can go down 15 deg., no more.  Nothing approximate about it.  Tolerances can be very precise.

I suspect that what was elided after "as viewed" was something like "from the front" or "from above", as a means of defining "side-to-side", as opposed to an observation method.

Hg

RE: Decipher this Specification

(OP)
Correct, I truncated the rest of the requirment as it wasn't relevant to the discussion.

The funny thing about these wonderful Federal requirements is how poorly written they actually are.  As some must have noticed, there is no specification of head rotation in the vertical plane, only the horizontal.

"But what... is it good for?"
Engineer at the Advanced Computing Systems Division of IBM, 1968, commenting on the microchip.
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

RE: Decipher this Specification

They're written by people not that much different than those that frequent this site.

TTFN

RE: Decipher this Specification

(OP)
IRstuff, I'm not sure if that was meant as an insult to the members here or as a compliment to the Government.

"But what... is it good for?"
Engineer at the Advanced Computing Systems Division of IBM, 1968, commenting on the microchip.
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

RE: Decipher this Specification

That's an exercise for the reader

TTFN

RE: Decipher this Specification

Specs and codes are written documents produced by engineers, who are not known for their writing skills.  A lot of it is a case of "I know what I meant, what's the big deal?"

Then there are the documents produced not only by engineers, but by higher-level engineers who aren't the people who have to deal with and interpret or enforce the document every day.

Hg

RE: Decipher this Specification

Just from the context of this thread, it's clear that interpretation works both ways.  The writer has his interpretation and must figure out all the possible ways that the supplier might interpret it.

Additionally, there is the desire to KISS, but you wind up with a less specific and contained requirement.  The example requirement here could take a full page to completely specify, which means that you'll need to verify each and every condition and sub-requirement, which is quite monumental, particularly if it's a small portion of an overall system.

Overall, it's a complicated game of "catch me if you can."  When you write a requirement, there is a tendency to have a mental image of what you're specifying.  When you simplify the requirement, it's often based on the that mental phantom, but the supplier is under no obligation to design that particular embodiment, and thereby provide a product that's operationally unacceptable.

TTFN

RE: Decipher this Specification

But at some level you have to be able to rely on standard conventions.  I can't write my specs around someone who doesn't know what +/- tolerances mean.  There's a difference between varying interpretations of an ambiguous statement, and just plain ignorance.

Hg

RE: Decipher this Specification

Up to a point, but even assuming that you accept the meaning +/-, what does the 15º mean, is that 3 sigma, or an absolute, and if so, what measurement tolerance is acceptable and how would you instrument and calibrate someone's head and eyeballs to verify that requirement?  It this case, it may be relatively clear, but in many other specifications, it's never quite that clear.

If there's plenty of margin, then it's usually not a big deal, but often, the specification pushes the limit of the design and then, you get into a BIG, prolonged hashing of tolerances and how they are accumulated, e.g., RSS or added and what did we really sign up to in the first place.

TTFN

RE: Decipher this Specification

There are also significant figure conventions that take care of a lot of that.

I take tolerances as an absolute.  If they just barely missed it, they still missed it; the edge of the tolerance isn't what they should be aiming for anyway.  The tolerance is how much they can just barely miss some other target by.

The stuff that gets to me is language like "should be nearly parallel".  Thanks, I can really enforce that.

Hg

RE: Decipher this Specification

One problem is that while it's written like a tolerance, it's actually not and often, it's tied to some other requirement such as brightness of the display or levels of contrast supported.  The trade between viewing angle and other display characteristics consumes many calories.

TTFN

RE: Decipher this Specification

HgTX, I liked your example "should be nearly parallel".

My personal favourite was a project for which the client specified a "super-flat" concrete floor slab, because the equipment that was to be installed required extremely precise alignment. When we asked for allowable tolerances on construction and subsequent in-service movements, and explained that the term "super-flat" doesn't have any universally accepted definition of allowable deviation from nominal alignments, the client came back with +/- 0 mm, and was absolutely immovable on that point. (+/- 0 flexibility or common sense!)

We never did get the client to relax the specification, so we declined to bid for the design and construction management. Presumably, he found someone who had EXCEPTIONAL concrete construction capability. (Or more likely, they never checked the construction tolerances, or else they did, and ended up in a long and bitter legal battle!)

RE: Decipher this Specification


Wow, MadMango,
You really stared something here ... 24 posts (±1) in 24 hours ... not bad

from (the City of) Barrie, Ontario.

I tried sniffing Coke once, but the ice cubes got stuck in my nose

RE: Decipher this Specification

JulianHardy,

+/- 0 mm over what area?

I could lay a "super flat" floor of +/- 0 mm over any given area of 0 m^2.

Just think of all the money you could have saved on shims had you bid the job!

RE: Decipher this Specification

Did you explain that the actual technical term is "super-duper-flat"?

Hg

RE: Decipher this Specification

Actually I believe that colloquial acronym would be

RFF

Really F***ing Flat

RE: Decipher this Specification

Hey, watch it, if I laugh too much they'll think this forum isn't work-related.

Hg

RE: Decipher this Specification

It is scary how many people actually got it WRONG.

Putting Human Factor Back in Engineering

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