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#1 SloopJohnB

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Posted 23 October 2018 - 02:52 PM

Part One from Engineering 360

“Watch Your Units!” Part 1 – Even Columbus Had Trouble With Units

Posted October 12, 2018 5:00 PM by RSBenner

“Watch your units!” If you polled engineering students, many of them would tell you that this is the phrase most often repeated by professors throughout their college career. And with good reason! It is so easy to calculate an incorrect result to a problem when you fail to notice that some of the starting values are given in different systems of units. Without that initial conversion, there is no hope that your answer will be correct, regardless of how good your engineering is. As students, we often brushed the warning aside as not being a big deal. However, it is a big deal! In fact, I found enough examples where this warning was not heeded, that I wrote multiple articles. In honor of Columbus Day and the landing of Columbus in the Bahamas on October 12, 1492, here is part 1.

 

“In fourteen hundred ninety-two; Columbus sailed the ocean blue.”

I remember learning this little rhyme when I was in elementary school. We learned all about the explorer Christopher Columbus and his famous travels as he attempted to sail west from Spain to reach Asia and the East Indies. What I did not learn was that part of the problem with Columbus’ voyages (besides the fact that North America was in the way) was that he had a unit conversion problem.

Although not always accepted, it has long been known that the Earth is shaped like a sphere. In fact, the first accurate calculation of the Earth’s circumference was performed way back in the third century BC by the Greek scholar Eratosthenes. An astronomer from Baghdad during the 9th century, Alfraganus, also calculated a slightly less accurate circumference. While preparing for his journeys, Columbus studied the work of both these men, ultimately deciding to use the latter values for use in his own calculations.

According to the work of Alfraganus, one degree (at the equator) is equal to 56.67 miles. In addition to using this less accurate number in his calculations, Columbus made another error. He wrongly assumed that Alfraganus was using the 4,856-feet Roman mile when he actually meant the 7,091-feet Arabic mile. This resulted in a 25% reduction in Columbus’ calculated circumference size. In addition, Columbus thought Japan was located at 85 degrees west longitude rather than 140 degrees east. These miscalculations resulted in a 58% margin of error in his estimate of the distance it would take to reach the East Indies.

Throughout Columbus’ life, he never accepted that he did not reach Asia during his voyages. Perhaps if he had used the findings of Eratosthenes, and performed the correct unit conversion, he would have realized he had actually reached the “New World.”

Stay tuned for Part 2!

Part Two

“Watch Your Units!” Part 2 – The Space Program

Posted October 22, 2018 5:00 PM by RSBenner

“Watch Your Units!” Part 2 – The Space Program

“Watch your units!” There is that phrase again! I can hear it in my sleep.

The most common place modern engineering students will run into unit conversion problems is when dealing with the International System of Units (SI) versus US customary units (USC). For Part 2 of this series, we will look at these types of unit conversion challenges from the US space program.

The Mars Climate Orbiter

Launched on December 11, 1998, the mission of the Mars Climate Orbiter was to maintain an orbit around Mars and study the Martian atmosphere and climate. This was not to be. Instead we have the most famous, and most expensive, example of a unit conversion error.

On September 23, 1999, as the Mars Climate Orbiter attempted to insert itself into its first orbit around Mars, communication was lost and was never regained. Subsequent investigation found that USC units were used in the ground software system while all other systems operated in SI units. This caused the trajectory figures to be off by a factor of 4.45 and resulted in a closer approach to Mars than expected. It is assumed that, since the orbiter was too close to the surface of the planet, heat and drag from the atmosphere destroyed the Mars Climate Orbiter.

“People make errors. The problem here was not the error. It was the failure of us to look at it end-to-end and find it. It’s unfair to rely on any one person.”

-Tom Gavin, NASA Jet Propulsion Laboratory

They failed to ‘watch their units.’ It was a $125 million mistake.

Reference:

https://sma.nasa.gov...fvrsn=eaa1ef8_4

Constellation Program

Who would think that unit conversions could be so expensive? In the case of the Constellation Program, it contributed to the cancellation of the program.

Begun in 2005, the Constellation Program was developed as a replacement of the aging space shuttle program. The focus of the program would be on manned flights with plans to return to the moon and an ultimate goal of a trip to Mars.

One of the underlying objectives of this program was the implementation of SI units across the entire program. In addition to new designs developed for the program, the ground and mission infrastructure (launch pads, test stands, etc.) that was largely developed in the 1960s for the Apollo Program would have to be updated to achieve this goal. However, it was estimated that the unit conversion costs would be approximately $370 million! In a futile attempt to support the program’s budget, it was ultimately decided to drop this plan and retain the USC units. Unfortunately, this did not save the program.

Although overall schedule and financial problems ultimately led to its cancellation in 2010, addressing units of measure is listed as one of the “Lessons Learned” in a NASA publication about the cancellation of the Constellation Program.

References:

https://ston.jsc.nas...-6127-VOL-2.pdf

https://en.wikipedia...nd_cancellation


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#2 Chrisc

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Posted 23 October 2018 - 04:35 PM

Interesting stuff,SJB.
And even when the units are the same they can cause confusion. I recall the consternation of some students who knew right enough that 0.5ml of adrenaline was required to jab into the patient's thigh, but the syringe was graduated in cc's...
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#3 Knot Me... maybe

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Posted 23 October 2018 - 05:01 PM

We had a big name boat builder ask me to quote on a pile of 68cm strops.

I quoted the 680mm strops but that was unacceptable.

 

"The surveyor requires 68cm, please re-quote but this time use the lengths we asked for!!!"


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#4 SloopJohnB

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Posted 23 October 2018 - 06:51 PM

Lucky they don't take the quote 10 times under priced.......

 

 

Did I get the power of ten right :think:


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#5 Chrisc

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Posted 24 October 2018 - 07:42 PM

Wow,what a lot of potential issues with this units business.
Talking with Joke, Holland is of course a metric country and yet she tells me that with their weights they have grams, kilos, ounces and pounds. 100 grams = 1 ounce, and 1 kilo = 2 pounds. But with British weights 1 kilo = 2.2 pounds or thereabouts.
What a recipe for a major ballsup.
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#6 DrWatson

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Posted 26 October 2018 - 12:25 AM

We had a big name boat builder ask me to quote on a pile of 68cm strops.

I quoted the 680mm strops but that was unacceptable.

 

"The surveyor requires 68cm, please re-quote but this time use the lengths we asked for!!!"

 

Seriously, what boat builder uses cm as their unit? I'd personally prefer that the parts on my boat were measured and fitted together to the nearest mm... or µm... 

 

But it gets to be a lot more fun when you work with old offsets for hull dimensions... feet, inches, 1/8ths, then + or - a 1/16th. Yes. plus or minus. Metric is good.


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#7 SloopJohnB

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Posted 20 November 2018 - 03:21 PM

Part 3

 

Watch your units!” Unit conversion errors can be found most anywhere, even in Japan and Canada. For Part 3 of this series, we will look at Tokyo Disneyland and Air Canada.

Tokyo Disneyland's Space Mountain Derailment

On December 5, 2003, one of the rocket vehicles of the Space Mountain attraction at Tokyo Disneyland derailed just before the end of the ride, forcing it to come to a sudden stop. Luckily, no one was injured due to the derailment. However, investigation proved the problem was caused by unit conversions.

The Space Mountain attraction at Tokyo Disneyland opened with the park on April 15, 1983. Based off the original, which opened in Florida in 1975, the coaster was built and maintained using US customary units (USC). In 1995, design specifications for the axles and bearings were updated to reflect the International System of Units (SI). However, the original drawings were never purged. So, when axles were ordered in August of 2002, the incorrect (USC) drawings were used, resulting in incorrectly sized axles. This resulted in a gap between the axle and its bearing to be over 1 mm, while the design specified a gap of 0.2 mm. Excessive vibration and stress occurring during operation due to this large gap and caused the rear wheel axle to break and the vehicle to derail.

The attraction was closed for two months during which all axles were inspected and (I assume) old drawings were purged.

References:

https://forums.wdwma...termined.33269/

 

The Gimli Glider

On July 23, 1983, Air Canada flight 143 departed Montreal for a transcontinental flight to Edmonton. However, the flight never made it there due to faulty fuel gauges and an incorrect unit conversion.

About one hour after departure, while cruising at 41,000 feet, both engines stopped operating. In addition to flight power, these engines also supply electrical power to the instrumentation and supply power to the hydraulic system. With the engines off, only a few battery-powered emergency flight instruments were operational and very limited hydraulic power was available. Despite these difficulties, the crew glided the aircraft and successfully landed it at a closed air force base, Station Gimli in Manitoba, with only minor injuries to the 69 people on-board.

Investigation into the “Gimli Glider,” as it became known, determined that a major factor in this near disaster centered around a unit conversion issue. In 1983, Canada was in the process of converting to SI units, and their new fleet of Boeing 767 jets would be the first to completely use this system. Because of the inoperative fuel gauge on Flight 143 the fuel level had to be checked manually. Calculations were then performed in order to determine how much fuel needed to be added to the plane before take-off. During this calculation, the ground crew, less familiar with the SI system, used a USC conversion factor in error. This resulted in the addition of 4,917 liters of fuel rather than the required 20,088 liters. Therefore, the reason for the engine failure was determined to be lack of proper fuel supply due to a faulty unit conversion.

What can I say except, “Watch your units!”

Reference:

https://en.wikipedia...ki/Gimli_Glider

 

https://airlinegeeks...th-anniversary/


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#8 marinheiro

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Posted 21 November 2018 - 08:52 AM

you also need to be careful to use the correct abbreviationsincorrectly  for units.

Years ago I was involved in a case where the schedule of rates used "cm" for cubic metres, not m3.

The contractor had put an appropriate price in for a cubic metre, but after the contract went bad for other reasons he put a claim in that this rate was actually per centimetre, even though it was a linear rather than volume measurement.

This went all the way to arbitration where the arbitrator quite firmly stated there was no basis for claim


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