8 replies to this topic

[Art Montemayor]

Guest:

Since 1 centipoise = 0.00100 N s./m2, I suspect your "m" is meant to mean "milli".

I have personally found some of the metric and SI unit to be absurd in dimensional (size) terms. I also believe that's one reason why USA engineers still persist in gravitating to US units when confronted by identifying heat transfer units such as "kW", a long-time and conventional electrical term. Additionally, it takes 0.00029 kW to make one Btu/hr.

In my opinion, the European traditions of pomp and ego have ingrained some very ridiculous sizes of "numbers" in the SI system - merely for the sake of using historical names. What originally started out as a simple, common-sense sytem of measurement has resulted in what seems as ego trips by some scientists trying to incorporate their national pride by establishing "standard" names based on their favorite or national scientists' names. This kind of mess always results when scientists and engineers get involved in political "agendas".

Why anyone is interested in "standardizing" a viscosity unit that is 1/1,000 the size of the nominal, dynamic viscosity of water in centipoise at conventional temperatures is something I don't understand. Why not just stick with "centipoise"? What is really alarming is to discover an "engineering" text book that resorts to forgetting common sense and continues to print ridiculously small, hard-to-manage units such as N s./m2. And then they compound the problem by not identifying their "edited" version of mNs/m2. That is unconventional, dumb engineering! All this results in perpetuating the struggle to establish the SI metric system. Logical and common-sense engineers find it more practical to use simpler, and shorter terms like "cP" and not be forced to employ exponential numbers with longer titles like mNs/m2. I still remember what my first engineering mentor, Alf Newton, told me when I reported for my initial engineering assignment: "K.I.S.S. - Keep It Simple, Stupid!".

[siretb]

Art,
I do not quite agree with you. Indeed there is something annoying as the SI units system leads to very large or very small numbers, and this is why we use multiples and submultiples. But the great merit of the SI is that it is fully consistent, and that this is very convenient for a chemical engineer that manipulates dimensionless numbers (like Re,...) .
I use also the american set of units, but I cannot be satisfied to see gas volumes in cubit feet and liquid volumes in gallons. After all a volume is a volume.
The one clever thing to do is to always clearly state the units useed, make no assumption that a temperature will be °F or °C or °K, but state it. As well as a reference state if needed.

[gvdlans]

The unit mN·s/m2 is consistently used throughout Coulson & Richardson's Volume 6. It is indeed "milli Newton second per square meter" and is equivalent to cP (centi Poise). For example in section 8.7.1 they give an experimental value for the viscosity of toluene at 20°C as "0.6 cP = 0.6 mN·s/m2". I am more used to see this unit being written as "mPa·s".

[Art Montemayor]

Guest:

Please don't get my comments mis-interpreted in meaning or as my political views. I don't believe in engineers expounding their politics in technical or scientific matters. My opinions are based on solely what I have lived through and witnessed in the 45 yrs that I have practiced engineering on a world-wide basis. I have lived in a "metric system" in South America and in Europe in an era when there was no so-called "Systeme International". It was called the Metric system, and it was logical, OK, made good common sense, and seemed to be on its way to being accepted as a viable, practical and reasonable answer to improved & good communications between Europe and the USA.

Some thing happened in Europe in the late 1960's until now. What was the original, proposed answer to a common scientific and engineering measuring system somehow began to be "improved" and "better" by the addition of various definitions and units of measurement. All of a sudden, what had been described world-wide as a logical and stable measurement system started to be modified and re-defined! And the basis for the re-definitions was based on "political correctness" as envisioned by certain Europeans. This is very confusing and only creates a lot of chaos.

As siretb himself agrees, some of the SI units seem to dwell on the ridiculous. And it didn't "use to be that way". The metric system was sound, logical, and made a lot of sense. It only started getting ridiculous when certain individual, nationalistic persons started to use it as a political platform to expound their ego trips. I'm certain that siretb, gvdlans (Guido), and myself couldn't care less about the ego part. However, we are affected when we are made to read, interpret, and sometimes employ dimensions and units that don't make for common sense. The metric system is no more "French" than it is USA, Japanese, or Nauhuatl. It shouldn't make any difference what language "predominates" and the system (not "systeme") should relate to the entire world in general.

I am a fervent follower of any system that employs common sense and simple and established procedures and standards. The metric system started that way, but lately it seems to have become a platform to expound nationalistic interests on the part of certain countries. I lived, breathed, and employed the metric system during the years 1963 to 1973. I consider this a great era where I learned and assimulated a lot of useful learnings. I thought and dreamed in metric system - and enjoyed the simplicity and common-sense related to it. However, lately I am given the impression that if I don't employ French words (Systeme International, metre, kilogramme, etc. ) I am not a "real engineer" nor am I using the "Systeme" correctly. I believe the metric system was meant to be used world-wide, with its rightful descriptions in Spanish, Farsi, Hindu, Japanese, Chinese, etc., etc. I'm afraid my training as a practical engineer does not allow for such flights of fancy.

I'm 100% for a universal units measurement system. But when our proposed solution starts to generate absurd and impractical proposals, I have to try to expose what I regard as a detriment to improvements in engineering communications. I realize that I offer a pact to agree on what we seem to disagree on, but that is the beauty of engineering: we are (& hopefully continue to be) practical and have traditionally recognized the failings and shortcomings in many past "solutions". Lets all hope that the Metric System will prevail - but in a sensible and logical manner without any politics or agendas.

[JMW]

Personally i prefer centipoise and centistokes and, because i am often working with hydrocarbons, it isn't a matter of SI units, its cst because those are the units commonly used in the various ASTM standards in use.
However, i do get concerned that there are so many different units in common use such as Engler and Saybolt, especially when some are now supposed to be obsolete.
Worst problem? US literature that finds its way overseas and quotes "gallons" but doesn't expressly state "US gallons" and, to be even handed, UK literature makinbg the same mistake.

[mbeychok]

To a goodly extent, I agree with Art. As an American living and working in Europe in 1960-1970, I found the metric system in use at that time to be consistent and easy to use. Basically, the metric system in use at that time was the cgs (centimeter-gram-second) system. The pressure unit was kg/cm² and the heat unit was primarily the kilocalorie.

But the SI system differs quite a bit from the metric system. It was developed by physicists rather than engineers and SI system has Pascals, Newtons and other inconveniently sized units ... where atmospheric pressure, for example, is 101,325 Pascals and 1 bar is 100,000 Pascals ... and a kilowatt-hour is 3,600,000 joules.

The SI system wasn't foisted upon us only by the French ... the USA and many other countries all had a hand in it. In the early 1900's, the metric system in use was the cgs system (centimeter-gram-second) which then evolved into the MKSA system (meter-kilogram-second-ampere). Then in 1960, the degree Kelvin (as the temperature unit) and the candela (as the luminous intensity unit) were added to the MKSA system and that formally became the SI system. In 1970 the Pascal was introduced and I'm not quite sure when the joule entered the picture. About 36 countries, including the USA, participated in the evolution of today's SI system.