si units

4 Articles

New Metric Prefixes Get Bigger And Smaller

November 22, 2022 by 72 Comments

It always fascinates us that every single thing that is made had to be designed by someone. Even something as simple as a bag and box that holds cereal. Someone had to work out the dimensions, the materials, the printing on it, and assign it a UPC code. Those people aren’t always engineers, but someone has to think it out no matter how mundane it is before it can be made. But what about the terms we use to express things? Someone has to work those out, too. In the case of metric prefixes like kilo, mega, and pico, it is apparently the General Conference on Weights and Measures that recently had its 27th session. As a result of that, we have four more metric prefixes to learn: ronna, quetta, ronto, and quecto.

Apparently, the new prefixes are to accommodate “big data” which is rapidly producing more data than there are atoms in the Universe. There were actually proposed earlier in a slightly different form but accepted at the conference. Apparently quecca is too close to a Portuguese swear word. So what do these actually mean? A QB (quettabyte) would be 1030 bytes while an RB (ronnabyte) is only 1027.  So 1 QB would be 1,000,000 yottabytes (YB) the previous top of the scale.

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Naming Names

January 23, 2021 by 100 Comments

We have all heard that good variable names are important for creating readable programs — advice that will serve you well when you come back to your code two years later, or even twenty. Sometimes, when you are so deep in the zone and begin to question a variable name that you made three levels up the calling hierarchy, it can be helpful to take a step back and review your variable naming conventions. The wikipedia article on computer program naming conventions is a good starting point, where you can dig into the nitty gritty of Hungarian notation like bFlag, case separated names like camelCase, and so on. But sometimes you have to go meta, and need names to describe the names themselves.

For example, in everyday usage the terms parameter and argument are often used interchangeably. But strictly speaking, a parameter is a placeholder, the variables in a function declaration for example (see image above). An argument is the actual value itself, say the number 50.334 in the example.

We recommend that units of measure should always be clearly specified in your comments, perhaps even in the variable names if you’re mixing different systems in the same program. At Hackaday we prefer to use SI units, check out NIST SP 330 if you’re interested. But invariably, there are going to be exceptions for years to come. We like to deal with those at the edges during I/O if possible, thus keeping a consistent set of units in the core of the program.

What about the terms number and value? We like to think of a number as being a kind of value. For example, a function’s return value could be a number, say a velocity. Or it could return an enumeration or a boolean. Sometimes the perfect meta-name for something will collide with a reserved keyword in your programming language, names like string or data for example. In these cases you have to be creative and find suitable synonyms, perhaps text and payload.

Using consistent and precise language can be tedious, but it can be just as helpful as good commenting and variable naming practices. Do you have any examples where precise terminology such as this has been been helpful or perhaps tripped you up? If so, share in the comments section below.

 

 

The Kilogramme Will Cease To Be A Physical Entity

November 16, 2018 by 72 Comments

One of the most illuminating high school courses no doubt for many readers as much as for your scribe, was the series of physics lessons during which the SI units were explained. That glorious sense of having the order of the universe unlocked into an interlocking series of units whose definitions could all be derived in terms of a series of base units was mind-blowing in those early teen years, and even though the explanations might have been at a for-the-children level that has been blown out of the water by later tiers of learning it’s still a bedrock that will serve an engineer or scientist life-long.

The definitions of the SI base units have evolved with scientific advancement to the point at which they are no longer tied to their original physical entity definitions. Of all the base units though there is still one that has resisted the urge to move away from the physical: the kilogramme (giving it its French spelling to preserve context) is still defined in terms of a metal cylinder in a laboratory just outside Paris. Kg diehards have not much time left to cling onto their platinum-iridium alloy though, for a new definition has been adopted in which it is derived from Planck’s Constant. From next May this will become the official kilogram, at which point concerns over microscopic erosion of the metal standard become irrelevant, and an SI kilogram can be replicated by any laboratory with the means to do so.

The piece of apparatus that makes this definition possible is the Kibble balance, a balance in which the force required to overcome the effect of gravitational force on a given mass is measured in terms of the electrical power required to do so. The gravitational force at a given point can be measured accurately and is defined in terms of the other SI units, while the electrical power can be derived from a Josephson junction, a superconducting junction whose current is defined in terms of Planck’s constant. As a result, the kilogram can be measured solely in terms of the constant and other SI units, consigning the metal cylinder to history.

This high-end metrology and physics make for interesting reading, but it’s fairly obvious that the de facto kilogram we all use will not change. Our everyday measures of everything from sugar to PLA filament will be the same today as they will be next May. But that’s not the point, everyday measurements do not need the extreme accuracy and reproducibility of a laboratory. The point of it all comes in as yet unforseen applications, as an example would the ability to synchronise timing to create GPS or digital radio have been possible were the second to be still defined in terms of astronomical movements rather than atomic states?

Standard kilogramme replica picture: Japs 88 [CC BY-SA 3.0]