Why So Many Units?

Historical reasons and being 'understandable'- the same argument as for Imperial units.

Out of those only one is part of SI. The medical people are mostly using cmH2O around here, but that's conviniently close enough to 1 hPa /

Part of design co-operation should be defining and enforcing the units to reduce possible mistakes.

Gases are compressible and the correct unit should be g/s ;-)

And then there are absolute and gauge pressures, which you also need to take into account with ventilators and code!

Fully agreed. But I find that fight one I can't win and thus propose minimally error prone common units. Of course sometimes the 2% error can make a difference..

It is a custom and practice thing. mmHg is still used for blood pressure since conversion to pascals resulted in too many errors. Mercury manometers have not been used to do the measurement for many decades.

I don't recall ever seeing mmH2O or cmH2O before.

mBar and Torr persist in my field and there are still good textbooks in cgs units so you have to be reasonably adept at living with a few orders of magnitude here and there.

Or mass flow controllers which are calibrated by amount delivered.

I've seen cmH2O used with ventilators and anaesthesiology in Finland. I've understood that there's some difference even between the Nordics in this field. mmHg for blood pressure - forgot that.

Discussing about gas mixing and physiology with anaesthesiologists and other docs was an interesting one for an engineers. Fortunately I've learned to disregard things like non-SI units - learned a lot both ways!

People cannot even agree what a ton is.

Do you express fuel consumption of cars in square meters, as it should have always been? This is the cross-section area of the virtual stream of fuel running along your car when you are driving. Self-normalised, doesn't need any "per 100km".

Mechanical horsepower is fun as well. Maybe we should consider specifying the power of SMPSes that way.:-)

Best regards, Piotr

Around here, domestic gas pressure is measured in inches of water, or just "inches", roughly six.

It used to be distributed at that pressure, but now it's some higher pressure with a regulator at each house.

"Inch" is a funny sounding word.

American aviation is using inches of mercury for air pressure and piston engine mainfold pressure. Other pressures are in psi, pounds per square inch.

Weather maps and reports use inches of mercury here. A bad hurricane can get down to something like 28.

Shop and tire pressures are in PSI.

How do Europeans measure tire pressure?

We use all SI for engineering, except PCB layout is in decimal inches. At least we don't work in fractions. Much.

We cook in cups, pounds, ounces, degrees F, pinches, tablespoons, all sorts of fun stuff.

Millibars here.

Bars - a bar is defined as 100kPa which is pretty much sea-level atmospheric pressure.

In the UK psi are often used too, though for tyres, not tires.

It's a shame an inch isn't 25.6mm instead of 25.4mm.

Older people would use most of those, except that 'cups' aren't used here. If they're not metric, shoe size increments still use barleycorns, but few people know that.

The difference between Celsius and Fahrenheit was recently explained on a radio programme. "Celsius is a temperature scale where zero degrees is the freezing point of water and one hundred degrees is the boiling point of water, whereas Fahrenheit is wrong."

Euro paper sizes are neat. A0 is one square metre, so A4 is 1/16 m^2 which means a single sheet of 'normal' 80gsm printer paper is 5g which happens to be the same weight as a 20p piece. (And three pre-decimal

Trivia.

In the early days of Rubylith/Xacto IC design, it was agreed that 1" =

What scares me is that our aerospace customers design aircraft, jet engines, helicopters, and satellites in pounds, inches, feet, slugs, poundals, LFPM, BTUs, horsepower, and degrees F. Their thermal

In "atmospheres" or bars. My car has the correct pressure hint plaque defined in atm. This is the same up to ~1% accuracy, but much clearer to laymen.

Best regards, Piotr

Is that absolute pressure, or relative to atmospheric? Is a flat tire

Nope. The purpose of SI was to honor dead scientists before they were forgotten.

Add to your list, Torr (for vacuum) atm (atmospheres) hPa (hectopascals or 100 Pa)

Don't forget about drop factor (gtts/min) in IV flow rate: or the "miners inch":

mandag den 20. juli 2020 kl. 19.54.06 UTC+2 skrev John Larkin:

relative

Ah. Hard metric vs soft metric. Commonly found in the building trade, where 6" = 150mm.

That can be a problem where you are mixing two different types of "6in" tiles.

Space probes have missed (or was it hit too hard) because of that.

Like the famous professors Kilogramov and Metrovich? ;-)

LOL! The attoparsec happens to be about an inch. :->

Best regards, Piotr

That's what I have been accustomed to hearing European coastal weather forecasters using. Not sure that SI allows for any multiplier other than powers of 1000...

mandag den 20. juli 2020 kl. 20.41.08 UTC+2 skrev Mike Coon:

During the cold war, the Soviets copied a lot of Western chips, except that the pin spacing was exactly 2.50 mm (not 2.54 mm). This also applied to card edge connectors.

Yeah, but 'agreed' goes out the window if you print up a PCB for a 28-pin DIP with 2.5mm pin spacing. That last pin misses its hole by two diameters...

Postscript printers can fine-tune their output, it's possible to adjust a computer graphic to the right size. I've done small boards with MacDraw... the alternative was to beg access to the big copy camera.