Reynolds Number

[rpayne]

I am doing calculating the Reynolds number in English units and I am getting units with it.

I can't figure out why either. I would appreciate some help.

See Picture below

[AlanStevens · Accepted solution]

Use lbf rather than lb in viscosity.

Alan

[AlvaroDíaz]

Hi Alan. How are you? I love ask in class this question (hope well translated to english): "how much it weighs one kilogram of sugar?" Can you help me cheking if that is well formulated in english, and showme how to ask it with pounds without explicit saying avoirdupois? That, in case that the question was entirely correct, like with kg, but I'm not sure if that's the case for pounds.

Best regards.

Alvaro.

[AlanStevens]

AlvaroDíaz wrote:

Hi Alan. How are you? I love ask in class this question (hope well translated to english): "how much it weighs one kilogram of sugar?" Can you help me cheking if that is well formulated in english, and showme how to ask it with pounds without explicit saying avoirdupois? That, in case that the question was entirely correct, like with kg, but I'm not sure if that's the case for pounds.

Best regards.

Alvaro.

HI Alvaro, 

In English:  "How much does one kilogram of sugar weigh?"

SImilarly: "How much does one pound of sugar weigh?"

‌However, why anyone still wants to use these outdated English units is beyond me!!

Alan

[Fred_Kohlhepp]

AlanStevens wrote:

However, why anyone still wants to use these outdated English units is beyond me!!

Alan

Speaking from the United States:

It goes to familiarity.

When I was in school (long ago), we were trained in the MKS system of units, our professors were sure that US conversion was just around the corner.  My first job, however, required me to "retrain" into those "outdated English units," so I could communicate with all of the older engineers.

In that same time period I had a conversation with my uncle, a "roughing carpenter," nearing retirement.  When I told him that conversion to metric in the US was inevitable, he said, "Well, then I'll be finished."  He knew, from long association and use, how to frame a building--the slope of a roof (or porch floor), the rise over run was in inches per foot.  While we could use centimeters per meter the carpenter, standing at the building site could recite the measurements he needed, he had learned it by long use.  If they were converted to metric an entire new set of numbers would need to be learned.

Now, long into my career, I'm in the same position as my uncle.  Acceleration due to gravity is no longer 9.8, it's 32.2 or 386.  (I really like Mathcad, because it's just "g").

There is an inertia in the work force that resists giving up what's familiar.  As the world becomes more "global," we'll all eventually use the same language, and units are just language--words to define concepts.

Why do I want to keep the "outdated English units"?  Because I know that Young's Modulus for steel is 30 million psi, Aluminum is 10 million.  Could I learn those values in MPa?  Sure I could.  But I'm used to the old numbers.

In 1960, US conversion to metric was only a few years away.  We're gaining on it, just not very quickly.

[AlanStevens]

I sympathise Fred!  When I started work at Rolls-Royce way back when, I used units like BTU/hr, lb/ft^2/hr etc., but we changed to SI within a couple of years of my starting.  The pain of switching was short-lived and now I wouldn't want to go back to those old units.  Clearly, the pain of switching is greater the longer you have been using the old units though!

Correct me if I'm wrong, but I've a feeling that US scientists use SI by and large - it's just the US engineers lagging behind.

Alan

[RichardJ]

I've a feeling that US scientists use SI by and large

As far as I know, entirely. I have not used anything other than SI since high school.

[Fred_Kohlhepp]

Richard Jackson wrote:

I've a feeling that US scientists use SI by and large

As far as I know, entirely. I have not used anything other than SI since high school.

I believe you are both correct; we old engineers are being dragged kicking and screaming to metric.  (One of the undesirable side effects is the creation of "new" metric units.  Can you say "kgf"?  Isn't that wonderful?  One of the primary reasons for metric when I was in school was the ability to drop "g" from so many of the formulas in the engineering handbook.)

Ten years ago (or so,) we were required to track helicopter weight in metric units.  The "in-place" computer system was in English units, and we were tasked with converting the entire data base (a #10-32 SHCS weighs 0.0x lb) to kg.  Amazingly, the converted helicopter had "gained" about 50 kg.  Can you say, "rounding error accumulation?"

We're stubborn, but we're still moving.  It's good to hear that some (most?) scientists are converted to metric.  Clearly the trend is in that direction.

[AlvaroDíaz]

Hi. All. Thanks for the translations and comments.

I guess that one thing is what scientist or engineer do with units, and other very different what "the" others do; even they can  to be scientist to, like doctors, or any worker. They use feets, inches, pounds, etc. Motors still came in HP and air conditioners in BTU/hr, but not in kW., which is the same unit for both. New books have a set of problems for ENG units, and other for SI.

Another thing, in both problems, is the difficult to differentiate mass from force for that almost every engineer student have, when actually that understanding must to came from secondary school.

Best regards.

Alvaro.

[tietjee]

I was in a model basin in Ireland doing some testing.  The test engineer was trying to decide which load cell.  One load cell was listed in kg and the other in nt.  Being from the US, I was looking for slugs and lbf.  So much for a uniform system.

[AlvaroDíaz]

Hi David. Load cell are pressure sensors. But, because the surface S is know by the designer of the load cells, can be used it as mass or force sensors too. Given that F = P/S = m*a, with a = g = 10m/s2 (ten, not 9.88 for those applications where that approx don't affect measures), it's ok the vendor information: kg for m and Newtons.for F.

For me the best is in kgf because people use kg (f) for express weights. But as Fred Kohlhepp points that have some undesiables side effects.Actually, this "unit system" is called KMS: kilogram, meter, second. Equivalent to FPS tor english units.Or CGS or KMS with Celsuis, different of the system KMS with Kelvins (my prefered) . Or FPS with Rankines. Or .... ups. To many, isn't? (forgoting abVolt, abAmpere, ....)

Mathcad is great for this tasks because if you don't use the correct unit, one of Newtons, kgf (kilopounds in civil engineering) or lbf, your result is bad as it must to be.

Best regards

Alvaro.

[ValeryOchkov]

[tietjee]

Reynolds number is Velocity (ft/sec) times Length (ft) divided by Kinematic Viscosity (ft^2/sec).  You are using Dynamic Viscosity.

Cheers

[Fred_Kohlhepp]

There are two equations for Reynold's Number (since there are two viscosities:

[ValeryOchkov]

There is only dynamic viscosity or viscosity. Kinematic viscosity is one calculation phantom.

[rpayne]

Thank you. It shouldn't surprise you that it worked.