Units
and how to use them
Introduction
This page covers the proper use of units in reports and documents. US units are not covered, although the principles for their use should be similar. If you correspond with anybody outside of the USA, please use metric units only! See conversions.The metric units shown below are those of the International System of Units (SI).
What not to do
| M3/HR | should be | m3/h |
| hrs | should be | h (no plural in units unless spelled out) |
| ppb | should be | μg/kg or μg/L in dilute solutions |
| sqm | should be | m2 |
| gr/lit | should be | g/L |
| m/second | should be | m/s (or spelled out meter per second) |
| F | is a | farad (capacitance). Degree Fahrenheit (not SI) would be °F |
| cm | is not a | cubic metre! |
Correct abbreviations
| Gram | should be | g | (not gr, not gm) |
| Kilogram | should be | kg | (not KG, not Kgr, not Kg) |
| Litre (US: liter) | should be | L | (l also correct, but can cause confusion) |
| Grams per litre | should be | g/L | (or g/l, but not gpl) |
| Metre (US: meter) | should be | m | (just m, not mt, and certainly not M) |
| Cubic metre | should be | m3 | (not cubm, cum or cm) |
| Cubic centimetre | should be | cm3 | (not cc, but can also be ml or mL) |
| Square metre | should be | m2 | (not sqm) |
| Hour | should be | h | (just h, not hr) |
| Second | should be | s | (just s, not sec) |
| Degree Celsius | should be | °C | (not just C, not degC) |
See multiples and sub-multiples below.
Which unit to use
As far as possible you should use metric units from the International System of Units (SI). For any scientific publication, you must use the SI units. However, some other units are tolerated in areas where they have been used traditionally, such as °dH for German degree of hardness, °f for French degree, bar for pressure values, curie for radioactivity (1 Ci = 3.7 · 1010 Bq), etc. In ion exchange, we really need equivalents (eq) as moles are not acceptable for multivalent ions. See units of capacity and concentration.
| Use | Instead of |
|---|---|
| meq/L | ppm CaCO3 |
| eq/L | g/L as CaCO3 |
| kPa or MPa | bar or kg/cm2 or atm |
| °C | C (which means coulomb) |
| k$ | m$ (which would be 0.1 cent) |
| k$ | M$ (unless you mean one million $) |
| M$ | MM$ |
Multiples and sub-multiples
| Power | Name | Abbr | Power | Name | Abbr | USA | |
|---|---|---|---|---|---|---|---|
| 1024 | Yotta | Y | 10–1 | Deci | d | ||
| 1021 | Zetta | Z | 10–2 | Centi | c | ||
| 1018 | Exa | E | 10–3 | Milli | m | ||
| 1015 | Peta | P | 10–6 | Micro | µ | ppm | |
| 1012 | Tera | T | 10–9 | Nano | n | ppb | |
| 109 | Giga | G | 10–12 | Pico | p | ppt | |
| 106 | Mega | M | 10–15 | Femto | f | ppq | |
| 103 | Kilo | k | 10–18 | Atto | a | ||
| 102 | Hecto | h | 10–21 | Zepto | z | ||
| 101 | Deca | da | 10–24 | Yocto | y |
The units ppm, ppb etc. should not be used in official documents. Use 1 mg/L or 1 mg/kg instead of 1 ppm, 1 µg/L or 1 µg/kg instead of 1 ppb, 1 ng/L or 1 ng/kg instead of 1 ppt.
Plural and spelling
Abbreviated units never have the mark of plural. Wrong: 14 kgs. Right: 14 kg.
Units are spelled out in literary text: he ran fifteen kilometers to reach the village. In technical text, one should write: he ran 15 km...
In principle, when the unit is spelled out the number should also be spelled out: 15 kilometers is not recommended. However, I consider that this particular rule can be bent in technical documents to make the text easier to read: I would recommend 135 kilopascals rather than one hundred and thirty five kilopascals, but 135 kPa would be better if it can be assumed that the reader understands the abbreviation.
Plural unit names are used when they are required by the rules of English grammar. They are normally formed regularly, for example, "henries" is the plural of henry. The siemens always has a final s, even in singular.
Units derived from person's names lose their capital letter when spelled out: correct: 3 amperes; incorrect: 3 Amperes. However, the abbreviation usually has a capital letter: milliampere is abbreviated as mA, kilopascal as kPa, microsiemens as μS. When you spell out or pronounce microsiemens, it has always a final s, because it is a proper noun, so there is no "one siemen, many siemens".
Spaces
A non-breaking space (thin space if available) should always be put between the value and the unit:
Correct: 1650 L of Amberjet 1200 Na
Wrong: 1650L of Amberjet 1200 Na
This is also required for temperature: 25 °C (not 25°C or 25C), and for percentages: 25 % (not 25%).
The only exception if for angular values: 118°17'48".
Comments
- You can write µ (Greek small mu for micro) using Alt-0181 on your (Windows*) keyboard
- You can write π (Greek small pi) using Alt-960 on your keyboard
- You can write Ω (Greek capital omega for ohm) using Alt-937 on your keyboard
- You can write ° (degree sign) using Alt-0176 on your keyboard
- You can write ± (plus/minus sign) using Alt-0177 on your keyboard
- You can write ‰ (per mille) using Alt-0137 on your keyboard
- The multiplication sign should preferably be a "middle dot" · that you can write using using Alt-0183
Example: MΩ·cm for megohm-centimeter.
Another option is the multiplication sign × that you write using Alt-0215: MΩ×cm
If you are really lazy, a normal x can be tolerated... - You can write ≤ (smaller than or equal to) and ≥ (greater than or equal to) by underlining the < or > signs; it will look like this: < and >; or use the "character map" described below.
- If you need to write other symbols, you can find them in the "Character map" of Windows.
Access is via: Start...Programs...Accessories...System tools...Character map. From there you can copy a character and paste into your document. From within MS Word, you can use the menu Insert...Symbol that opens a window from which you can select your character and insert it directly into your document. - Don't use the ± sign to indicate "approximately equal", use the ~ tilde instead (or ≈ to copy from the character map)
- ppm, ppb and ppt should be avoided in formal writing. In water treatment, use mg/L, µg/L and ng/L respectively (although the absolutely correct form is mg/kg etc.).
- Careful with ppt: some people use this abbreviation for "parts per thousand", others for "parts per trillion"! Another reason to express concentrations only in g/L (g/kg) or ng/L (ng/kg) instead of the ambiguous ppt!
- Multiples and submultiples (see above) of all current units are permitted, e.g. µeq/L, MPa, GL (for a billion litres = 109 L).
- The litre (liter in US English) has now usually the abbreviation L to avoid confusion between the letter l and the number 1. The cursive
is not an approved symbol.
- The tonne is equal to 1000 kg, called "metric ton" in the USA and sometimes in the UK to avoid confusion with previous short and long tons, and must be abbreviated as t.
- Gauge pressure: it is the difference between the absolute pressure and the atmospheric pressure. This is why you often see pressure values expressed in bar g or kPa g. A value of 0 bar g corresponds thus to atmospheric pressure.
- Pressure in general: many different units have been used such as Torr, cm Hg, m water column, Atm, At, kg/cm2, bar and others. Only pascal (usually kilopascal or megapascal kPa and MPa) should be used.
- Conductivity values: we must be very careful, because sometimes we see mS/m (millisiemens per metre) instead of the more usual µS/cm (microsiemens per centimetre). The relationship is 1 mS/m = 10 µS/cm.
- Ångström: if you don't know how to write the Å (alt-0197), you better convert to nm (nanometre) which is an approved unit whilst Å is not; and A (without the ring) is an ampere...
Because 1 Å = 10-10 m and 1 nm = 10-9 m you have 1 Å = 0.1 nm.
Bed volumes
...are an exception in ion exchange and filtration technologiesAs you know, we use bed volumes to indicate a volume of water or solution in relation to a volume of resin. This unit has no dimension, because it is L/L or m3/m3. This unit is however useful in our technology, and the abbreviation BV or bv is therefore tolerated. We are using BV in these pages and in the sales literature. In scientific articles, the unit is usually spelled out as "bed volumes".
Sometimes the specific flow rate that we usually write in BV/h is written in h–1 or in m3/(m3·h).
Chemistry: moles and equivalents
The mole (symbol: mol) is an accepted unit. Concentrations can be expressed in mol/L. However, in our field of ion exchange, the concept of equivalent is essential, even if it is not an approved unit. The common symbol is eq, although in some countries (e.g. Germany and several Eastern European countries) the symbol val is used. The relation is:
One equivalent = one mole divided by valence
One litre of Amberjet 1200 H can load two moles of sodium (2 eq) but only one mole of calcium (2 eq) because calcium is divalent. Therefore, concentrations given in mmol/L are very confusing.
If ionic concentrations have to be expressed in mol/L or mol·kg–1, then the valence of the ion should definitely be shown, such as Zn2+ or PO43–.
In short, I recommend to use
| 0.50 meq/L Ca (or Ca2+) | instead of 0.25 mmol Ca2+ |
| 0.64 meq/L SO4 (or SO42–) | instead of 0.32 mmol SO42– |
The nucleon number (mass number) of a nuclide is indicated in the left superscript position: 7Li.
The number of atoms in a molecule of a particular nuclide is shown in the right subscript position: 16O2.
The proton number (atomic number) is indicated in the left subscript position: 29Cu.
The state of ionisation is indicated in the right superscript position: Al+++ or Al3+.
See the conversion table of concentration units used for water analysis.
The unit for molecular mass (molecular weight should not be used any longer) is the dalton, abbreviated Da.
Example: this UF membrane has a molecular mass cut-off of 10'000 daltons (or 10'000 Da).
Grouping digits
Because the comma is widely used as the decimal marker outside the United States, it should not be used to separate digits into groups of three. The International Union of Pure and Applied Chemistry (IUPAC) suggests to never use a comma or a point as thousands separator. Instead, digits should be separated into groups of three, counting from the decimal marker towards the left and right, by the use of a thin, fixed (non-breaking) space.
Correct: 76 473 254.8
Wrong: 76,473,254.8
In those languages where the decimal separator is a comma, the same rule should be applied:
Correct: 76 473 254,8
Wrong: 76.473.254,8
The "Swiss method" uses an apostrophe instead of a space or a comma:
Example: 76'473'254.8
Dollars and sense
It is unacceptable to use Roman numerals to express the values of quantities. In particular, one should not use M and MM as substitutes for 103 and 106 respectively. I find it very irritating to see most financial reports expressed in MM$, sometimes m$.
M should strictly be used as a prefix for one million (M for mega). In Europe, the most current use is k€ for thousands and M€ for millions of Euros, which makes sense and is correct.
Dates
Your local format:
Example 1: We have met on 13/12/06 and we plan to meet again on 14/04/05. Read once more. Does it make sense to you? If it does, there is a good chance you are Japanese or Chinese. For most other people, it is a puzzle.
Example 2: Please make sure to attend our meeting of 11/06/2015. Well, Europeans will show up in June (on a Thursday), Americans only in November (on a Friday)...
Conclusion: as long as we are writing in a known language, we should express dates in a way that does not leave any doubt. Example: 11 Jun 2009. If you write <2009 Jun 11> or <Jun 11 2009>, the date will still be understandable. In some languages however, the abbreviation for the month may not make sense to us (Ene in Spanish, Lug in Italian, Ara in Turkish), but even so there is less risk that somebody miss the date (they can look up the appropriate dictionary).
I recommend to use the international standard called ISO 8601 with the format yyyy-mm-dd (like "Japanese format"). The only way to incorporate dates in text so that a computer will order the pieces of text chronologically is using this ISO 8601 format. So if you have file names with dates, use Report19990624 and Report20080113 rather than Report240699 or Report062499 which will never sort properly, even with a four digit year.
Links
- Guide for the Use of the International System of Units (SI) with many details. It lists the SI units, the non-SI units accepted for use with the SI, those tolerated temporarily and those not accepted (e.g. poise or calorie). It also gives the detailed rules for proper use of the units, from which I have excerpted some the present page.
- See also the summary of abbreviations and units.
- Conversion of US to metric units.
- Tout sur les unités de mesure (en français) avec à peu près les mêmes informations que les deux premiers liens ci-dessus.
- Eine erbauliche und z.T. unterhaltsame Seite auf Deutsch über Maßeinheiten.
- This page with volume conversions (German and English) will tell you how many fluid ounces you have in a bushel or in a cubic metre... All English units including the most unfamiliar ones (dram, firkin, minim, peck, tablespoon and more) are listed. Other conversions also available in the same site.
