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4 Jul 2021
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and how to use them


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/HRshould bem3/h
hrsshould beh (no plural in units unless spelled out)
ppbshould beμg/kg or μg/L in dilute solutions
sqmshould bem2
gr/litshould beg/L
m/secondshould bem/s (or spelled out meter per second)
Fis afarad (capacitance). Degree Fahrenheit (not SI) would be °F
cmis not acubic metre!

Correct abbreviations

Gramshould beg (not gr, not gm)
Kilogramshould bekg (not KG, not Kgr, not Kg)
Litre (US: liter)should beL(l also correct, but can cause confusion)
Grams per litreshould beg/L(or g/l, but not gpl)
Metre (US: meter)should bem(just m, not mt, and certainly not M)
Cubic metreshould bem3(not cubm, cum or cm)
Cubic centimetreshould becm3(not cc, but can also be ml or mL)
Square metreshould bem2(not sqm)
Hourshould beh (just h, not hr)
Secondshould bes (just s, not sec)
Degree Celsiusshould 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.

UseInstead of
meq/Lppm CaCO3
eq/Lg/L as CaCO3
kPa or MPabar or kg/cm2 or atm
°CC (which means coulomb)
k$m$ (which would be 0.1 cent)
k$M$ (unless you mean one million $)

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".


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".


* On an Apple keyboard, open the "Keyboard Viewer" or the "Character Viewer".

Bed volumes

...are an exception in ion exchange and filtration technologies

As 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.


ISO format:

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.


© François de Dardel