# Bovnar — Unit Ambiguities

> **Spec version:** 1.1
> **Status:** Reference — companion to the unit registry and the cheat sheet
> **Scope:** Every token that could plausibly mean two things — what Bovnar reads it as, and how to write the other meaning.

Companion to [Unit & Currency Reference](05_bovnar_unit_system.md) (the registry) and
[Unit & Currency Cheat Sheet](04_bovnar_unit_cheatsheet.md) (the symbol tables). Every row here was checked
against the reference parser; where a token is refused, it really is `error_unit_illegal`.

---

## Table of Contents

1. [How a token is resolved](#1-how-a-token-is-resolved)
2. [A bare unit outranks a prefixed reading](#2-a-bare-unit-outranks-a-prefixed-reading)
3. [Two prefixed readings compete](#3-two-prefixed-readings-compete)
4. [Refused outright](#4-refused-outright)
5. [Prefix symbol vs. base unit symbol](#5-prefix-symbol-vs-base-unit-symbol)
6. [Case decides](#6-case-decides)
7. [Look-alike characters](#7-look-alike-characters)
8. [Temperature scales vs. electrical units](#8-temperature-scales-vs-electrical-units)
9. [Same dimension, different quantity](#9-same-dimension-different-quantity)
10. [An affine scale inside a compound has no value](#10-an-affine-scale-inside-a-compound-has-no-value)
11. [A named unit is not its compound](#11-a-named-unit-is-not-its-compound)
12. [Unit vs. currency](#12-unit-vs-currency)
13. [Looks like a unit, is not one](#13-looks-like-a-unit-is-not-one)
14. [Water hardness: six scales, one quantity](#14-water-hardness-six-scales-one-quantity)
15. [Same name, different definition](#15-same-name-different-definition)
16. [Quick index: if you mean X, write Y](#16-quick-index-if-you-mean-x-write-y)

- [See also](#see-also)

---

## 1. How a token is resolved

A unit component is resolved in this order. Knowing the order explains every table below.

| Step | Rule | Consequence |
|------|------|-------------|
| 1 | A `$` anywhere selects the **currency** table | `$USD`, `k$EUR`, `k~$EUR`. A bare code is never a currency: `USD` is `error_unit_illegal` |
| 2 | Otherwise the base symbol is the **longest alias suffix** of the token | `min` matches the alias `min` (minute) before it could match `in` (inch) |
| 3 | Whatever precedes the base is the **prefix**, with `~` optional | `k~g` and `kg` are one unit; the writer always emits `k~g` |
| 4 | The **prefix policy** of that unit must allow it | `Ki~m` and `Kim` both fail; IEC prefixes are for `b`/`B` only |
| 5 | A handful of spellings are **refused by name** | `usb`, `kt` — see §4 |

Rule 2 is the load-bearing one: **a token that is itself a unit is always that unit**, never a
prefixed reading of a shorter one. That is what made the compact form (`kg` for `k~g`) safe to
introduce — it can only ever be accepted where the parser previously raised an error, so no
document written before it existed changed meaning.

Rule 3's `~` is never wrong to write. When in doubt, or when a reader of your file might be in
doubt, use it: `m~in` cannot be misread, `min` can be misremembered.

---

## 2. A bare unit outranks a prefixed reading

These 19 tokens are units in their own right. The prefixed reading a reader might expect is
reachable only with the `~`.

| Token | Bovnar reads it as | Might be mistaken for | Write that as |
|-------|--------------------|-----------------------|---------------|
| `min` | minute | milli-inch | `m~in` |
| `cd` | candela | centi-day | `c~d` |
| `ft` | foot | femto-tonne | `f~t` |
| `pt` | pint | pico-tonne | `p~t` |
| `qt` | quart | quecto-tonne | `q~t` |
| `ct` | carat | centi-tonne | `c~t` |
| `yd` | yard | yocto-day | `y~d` |
| `rd` | rod | ronto-day | `r~d` |
| `ch` | chain | centi-hour | `c~h` |
| `at` | technical atmosphere | atto-tonne | `a~t` |
| `au` | astronomical unit | atto-dalton | `a~u` |
| `kat` | katal | kilo-(technical atmosphere) | `k~at` |
| `nmi` | nautical mile | nano-mile | `n~mi` |
| `PS` | metric horsepower (Pferdestärke) | peta-siemens | `P~S` |
| `ph` | phot | pico-hour | `p~h` |
| `pH` | acidity (pH scale) | pico-henry | `p~H` |
| `mph` | mile per hour | milli-phot | `m~ph` |
| `kph` | kilometre per hour | kilo-phot | `k~ph` |
| `dB` | decibel | deci-byte | *(impossible: an SI prefix below kilo is illegal on `B`)* |

> `pH`, `mph` and `kph` are the reason three quantities became units: while they were not in the
> table, the compact form resolved them as the picohenry and the milli-/kilophot. Naming the
> quantity is the durable fix — rule 2 then keeps the prefixed reading out on its own.

---

## 3. Two prefixed readings compete

When a token can be split in two places, rule 2 decides: the **longer base symbol** wins.

| Token | Bovnar reads it as | The other split | Write that as |
|-------|--------------------|-----------------|---------------|
| `dat` | deci-`at` = deci-(technical atmosphere) | deca-tonne | `da~t` |
| `dau` | deci-`au` = deci-(astronomical unit) | deca-dalton | `da~u` (canonical `da~Da`) |

Both are deterministic, not ambiguous — but neither is guessable. Write `d~at` / `da~t` and
`d~au` / `da~u` explicitly; the separated form is unambiguous in both directions.

---

## 4. Refused outright

Two tokens are `error_unit_illegal` on purpose, listed in `.compact_exceptions` in
`src/gendata/units.bvnr`. Accepting them would turn a parse error into a quietly wrong unit.

| Token | Would have resolved as | Actually means, in the wild | Write |
|-------|------------------------|-----------------------------|-------|
| `kt` | kilo-tonne | kilotonne **or** knot, depending on the field | `k~t` (mass) or `kn` (speed) |
| `usb` | micro-stilb | the bus | `u~sb` (or `µ~sb`) if you really mean microstilb |

`kt` is the harder case: both readings are units Bovnar models, so no table lookup can settle it.
Reading a speed as a mass is exactly the failure this format exists to prevent, so the author is
asked to say which.

---

## 5. Prefix symbol vs. base unit symbol

Eight symbols are both a prefix and a unit. Bare, they are always the unit; before another unit
they are always the prefix.

| Symbol | As a bare unit | As a prefix | Example of each |
|--------|----------------|-------------|-----------------|
| `m` | metre | milli | `m` = metre, `ms` / `m~s` = millisecond |
| `d` | day | deci | `d` = day, `ds` / `d~s` = decisecond |
| `h` | hour | hecto | `h` = hour, `hPa` / `h~Pa` = hectopascal |
| `T` | tesla | tera | `T` = tesla, `TB` / `T~B` = terabyte |
| `G` | gauss | giga | `G` = gauss, `GHz` / `G~Hz` = gigahertz |
| `P` | poise | peta | `P` = poise, `PB` / `P~B` = petabyte |
| `R` | roentgen | ronna | `R` = roentgen, `Rm` / `R~m` = ronnametre |
| `u` | dalton (ASCII alias of `Da`) | micro (ASCII alias of `µ`) | `u` = dalton, `us` / `u~s` = microsecond, `uu` = micro-dalton |

`da` (deca) is a two-character prefix: `dam` = decametre, `dag` = decagram, `das` = decasecond.
It is not related to `Da` (dalton) — case separates them (§6).

---

## 6. Case decides

Symbols are matched byte for byte. These pairs differ only in case and mean unrelated things;
several are one keystroke apart in everyday data.

Start with the **bare** symbols, because the prefixed rows below are built from
them and they are the three most common symbols in the whole table:

| Lower | Means | Upper | Means |
|-------|-------|-------|-------|
| `s` | second | `S` | siemens |
| `h` | hour | `H` | henry |
| `g` | gram | `G` | gauss |
| `kn` | knot | `kN` | **kilonewton** — the same trap as `gn`/`GN` and `fn`/`fN` below |
| `grad` | gradian (also `gradian`, `gon`) | `Grad` | **gigaradian** — and German writes *Grad* for the degree, so a German-language source is the likely way to type this by accident. Worse than the others: both are angles, so it *converts*, by a factor of 10⁹ |

Then the prefixed pairs:

| Lower | Means | Upper / mixed | Means |
|-------|-------|---------------|-------|
| `ms` | millisecond | `mS` | millisiemens |
| `pa`* | — | `Pa` / `pA` | pascal / picoampere |
| `kg` | kilogram | `kG` | kilogauss |
| `mt` | millitonne | `Mt` / `MT` | megatonne / megatesla |
| `gt`* | — | `Gt` / `GT` | gigatonne / gigatesla |
| `mb` | milli-bit → **illegal** (sub-kilo prefix on `b`) | `MB` / `Mb` | megabyte / megabit |
| `kb` | kilobit | `kB` | kilobyte |
| `gib` | — | `Gib` / `GiB` | gibibit / gibibyte |
| `gal` | gallon | `Gal` | galileo (`mgal` = milligallon, `mGal` = milligal) |
| `st` | stone | `St` | stokes (`cst` = centistone, `cSt` = centistokes) |
| `ha` | hectare | `hA` | hectoampere |
| `hp` | horsepower | `hP` | hectopoise |
| `gn` | standard gravity | `GN` | giganewton |
| `gr` | grain | `GR` | gigaroentgen |
| `dr` | dram | `dR` | deciroentgen |
| `fn` | fortnight | `fN` | femtonewton |
| `da` | *(prefix deca)* | `Da` / `dA` | dalton / deciampere |
| `np`* | — | `Np` / `nP` | neper / nanopoise |
| `ev`* | — | `eV` / `EV` | electronvolt / exavolt |
| `ph` | phot | `pH` | acidity |
| `ps` | **picosecond** — a valid unit, silently | `PS` / `pS` | metric horsepower / picosiemens |
| `cv`* | — | `CV` / `cV` | metric horsepower (French *chevaux*) / centivolt |

\* not a valid token in that casing — listed because the valid neighbours are easy to mistype.

Common miscasings that are **errors**, not silent variants — `K` is the kelvin, not the kilo prefix:

| Written | Result | Intended | Write |
|---------|--------|----------|-------|
| `dH` | decihenry — a valid unit, silently | German hardness degree | `°dH` |
| `cF` | centifarad — a valid unit, silently | hydroponic conductivity factor | `CF` |
| `fau` | femto-astronomical-unit — a valid unit, silently | formazin attenuation unit | `FAU` |
| `Kg`, `KG` | `error_unit_illegal` | kilogram | `kg` or `k~g` |
| `KB` | `error_unit_illegal` | kilobyte | `kB` or `k~B` |
| `NM` | `error_unit_illegal` | nautical mile | `nmi` |
| `Cal` | `error_unit_illegal` | kilocalorie (food calorie) | `kcal` or `k~cal` |

---

## 7. Look-alike characters

Several symbols exist in more than one Unicode code point. Bovnar accepts the variants on input
and always emits one canonical form.

| Meaning | Accepted spellings | Canonical output |
|---------|--------------------|------------------|
| micro prefix | `µ` U+00B5 · `μ` U+03BC (Greek mu) · `u` (ASCII) | `µ` U+00B5 |
| ohm | `Ω` U+2126 · `Ω` U+03A9 (Greek omega) · `ohm` · `Ohm` | `Ω` U+2126 |
| ångström | `Å` U+212B · `Å` U+00C5 · `Å` U+0041 U+030A (A + combining ring) · `angstrom`, `angstroms` | `Å` U+212B |
| degree | `°` U+00B0 · `deg` · `degr` · `degree` · `degrees` | `°` U+00B0 |

A document that has been through Unicode normalisation (NFC/NFD/NFKC/NFKD) still parses: every
variant above maps to the same unit, which the normalisation test sweep enforces over the whole
table.

> **The canonical output is deliberately not NFC.** U+2126 (ohm) and U+212B (ångström) both have
> *singleton* NFC decompositions — to U+03A9 and U+00C5 — so a document this library writes is not
> Unicode-normalised, and running it through NFC **changes its bytes**. It still parses, and
> re-serialising returns U+2126/U+212B, so the round trip is a semantic fixed point but not a byte
> fixed point. That matters if you hash, sign or byte-diff bovnar output (the conformance suite's
> byte-for-byte comparison included): normalise before hashing, or do not normalise at all — but do
> not assume the two are the same file.

`u` is both the ASCII micro prefix and the bare symbol for the dalton — rule 2 keeps them apart:
`u` alone is the dalton, `us` is a microsecond, `uu` is a micro-dalton.

---

## 8. Temperature scales vs. electrical units

The degree sign is not decoration: without it the token is a different quantity entirely.

| With `°` | Means | Without `°` | Means |
|----------|-------|-------------|-------|
| `°C` | degree Celsius | `C` | coulomb |
| `°F` | degree Fahrenheit | `F` | farad |
| `°N` | degree Newton (historical scale) | `N` | newton |
| `°Ra` | degree Rankine | `Ra` | *(accepted alias of `°Ra`)* |

ASCII aliases avoid the question: `degC`, `degF`, `degRa`, `degN`, `degDe`, `degRe`, `degRo`.
`K` (kelvin) is absolute and needs no degree sign.

The water-hardness degrees carry the same warning, and one of them is genuinely dangerous:

| With `°` | Means | Without `°` | Means |
|----------|-------|-------------|-------|
| `°dH` | German hardness degree | `dH` | **decihenry** — a valid unit, so this misreads silently |
| `°e` | English (Clark) hardness degree | `e` | `error_unit_illegal` |
| `°fH` | French hardness degree | `fH` | femtohenry |
| `°rH` | Russian hardness degree | `rH` | rontohenry |
| `°aH` | American hardness degree | `aH` | attohenry |

Their ASCII long forms avoid the question entirely: `german_hardness`, `english_hardness`
(or `clark_degree`), `french_hardness`, `russian_hardness`, `american_hardness`.

---

## 9. Same dimension, different quantity

The SI dimension vector cannot tell these apart — they are all dimensionally equal — yet they are
not interchangeable in meaning. Bovnar's compatibility check is dimension-based, so it **accepts**
conversions in the first group; the second group is protected by an explicit quantity-kind rule.

**Dimensionally equal, and Bovnar will convert between them (factor 1) — the distinction is yours to keep:**

| Units | Shared dimension | The distinction Bovnar cannot see |
|-------|------------------|-----------------------------------|
| `J` and `N·m` | kg·m²·s⁻² | energy vs. torque |
| `Gy` and `Sv` | m²·s⁻² | absorbed dose vs. equivalent dose |
| `W`, `VA`, `var` | kg·m²·s⁻³ | active, apparent and reactive power |
| `Hz`, `Bq`, `Bd` | s⁻¹ | frequency, radioactivity, symbol rate |
| `mph`, `kn`, `m/s` | m·s⁻¹ | genuinely interchangeable — just different scales |

> Offering the conversion is not the same as rewriting the annotation.
> `BVN_UNIT_REDUCE` leaves every unit in this table alone: a `Sv` serialises as
> `Sv`, never as `Gy`. The collapse folds a *compound* into the named unit it
> spells out (`A·s` → `C`); it never substitutes one named unit for another.

**Dimensionless but *not* interchangeable — Bovnar refuses these conversions:**

| Unit | Kind | Refused against |
|------|------|-----------------|
| `b` | information (bit) | `B` — 8 bits are not silently a byte |
| `B` | information (byte) | `b` |
| `rad`, `°`, `grad`, `rev`, `arcmin`, `arcsec` | angle (one shared kind, so `°` → `rad` works) | plain numbers, `Hz`, `%` |
| `sr` | angle, weight 2 (`sr` = `rad²`) | `rad` at exponent 1 |
| `lm`, `lx`, `ph` | angle, weight 2 — each carries one steradian (`lm` = `cd·sr`) | `cd`, `cd/m²`, `sb`: luminous flux is not luminous intensity, illuminance is not luminance |
| `dB` | logarithmic | `Np`, `%`, plain numbers |
| `Np` | logarithmic | `dB`, `%`, plain numbers |
| `pH` | logarithmic | `%`, `ppm`, plain numbers |
| `NTU` | turbidity, white light 90° | every other turbidity scale, `%`, plain numbers |
| `FNU` | turbidity, near-infrared 90° | ditto |
| `FTU` | turbidity, geometry unstated | ditto |
| `FAU` | turbidity, attenuation 0° | ditto |
| `JTU` | turbidity, visual candle | ditto |
| `PSU` | practical salinity | `‰`, `g/kg`, plain numbers |
| `%`, `‰`, `‱`, `pcm`, `ppm`, `ppb` | pure ratios | *(freely interconvertible, and with a plain number: 1 % → 0.01)* |

Water chemistry calls the American hardness scale "ppm" (milligrams of CaCO₃ per litre). Bovnar's
`ppm` is the **dimensionless** 10⁻⁶ and the hardness scale is `°aH`, an amount concentration — the
two carry different dimensions and do not convert into one another. If your source says "ppm
hardness", write `°aH`.

The logarithmic kinds are separate because no factor can relate them: 20 dB is a ratio of 100, not
twice 10 dB, and a pH one unit lower is a tenfold concentration. `rpm` is a *cycle* rate and
`rev/min` an *angular* rate — they differ by 2π and do not convert into one another either.

---

## 10. An affine scale inside a compound has no value

`°C`, `°F`, `°De`, `°N`, `°Re` and `°Ro` are *offset* scales: 0 is not nothing.
That is fine for a lone temperature and meaningless in a product.

| Written | Parses | Converts | Why |
|---------|--------|----------|-----|
| `°C` | yes | yes — `20 °C` → `293.15 K` | the offset has somewhere to go |
| `°C/h` | yes | **no** — `error_unit_mismatch` | the offset is a number of kelvin; `K·s⁻¹` has nowhere to put it |
| `°C·m` | yes | **no** | ditto |
| `°C²`, `°C·°F` | yes | **no** | an offset scale is meaningful at exponent 1 only |

The annotation stays legal, because it is a reasonable thing to *write* — people
do record a rate of change in °C/h. What it means is a temperature **difference**
per hour, and the difference is what Bovnar cannot infer from the unit: `20 °C/h`
as a rate is `20 K/h`, while `20 °C` as a reading is `293.15 K`. Rather than pick
one, the library refuses to produce a number and leaves the components for a
consumer that knows which was meant. Write `K/h` when you mean the rate — the
kelvin has no offset and the ambiguity disappears.

---

## 11. A named unit is not its compound

A named unit is a single component; the compound that means the same thing is several. Both are
valid and both convert to the same SI value, but annotation-vs-inline reconciliation is
**structural**, so mixing the two spellings in one value is `error_unit_mismatch`.

| Named | Equivalent compound | Same SI factor? | `<float:64,named> 1.0 compound;` |
|-------|---------------------|-----------------|----------------------------------|
| `mph` | `mi/h` | yes (0.44704 m·s⁻¹) | `error_unit_mismatch` |
| `kph` | `k~m/h` | yes (5/18 m·s⁻¹) | `error_unit_mismatch` |
| `kn` | `nmi/h` | yes | `error_unit_mismatch` |
| `Hz` | `s^-1` | yes | `error_unit_mismatch` |
| `Pa` | `k~g/(m·s²)` | yes | `error_unit_mismatch` |

Pick one spelling per document and stay with it. `bvn_units_compatible` and
`bvn_unit_convert_factor` relate them, so a consumer can still convert freely.

---

## 12. Unit vs. currency

A currency is recognised **only** with the `$` sigil, so the two namespaces cannot collide. The
classic cases:

| Token | Bare (no `$`) | With `$` |
|-------|---------------|----------|
| `cup` / `CUP` | `cup` = US cup; `CUP` = `error_unit_illegal` | `$CUP` = Cuban Peso |
| `BTU` | British thermal unit (`Btu`, `btu` too) | *(not an ISO 4217 code)* |
| `SOL` | `error_unit_illegal` | `$SOL` = Solana |
| `BAR` | `error_unit_illegal` (use lowercase `bar`) | *(not an ISO 4217 code)* |
| `USD` | `error_unit_illegal` | `$USD` |

A prefix goes before the sigil, `~` optional: `k~$EUR` and `k$EUR` are both thousands of euro.
`kUSD` is an error — the sigil is not optional.

---

## 13. Looks like a unit, is not one

Common abbreviations that are **not** in the table. All are `error_unit_illegal`, which is the
point: a wrong unit is worse than a refused one.

| Written | Intended | Write instead |
|---------|----------|---------------|
| `kWh` | kilowatt-hour | `kW·h` or `k~W·h` |
| `mAh` | milliampere-hour | `mA·h` or `m~A·h` |
| `mcg` | microgram | `µg`, `ug` or `µ~g` |
| `cc` | cubic centimetre | `cm³`, `cm^3` or `c~m³` |
| `sqft` | square foot | `ft²` or `ft^2` |
| `ksi` | kilopound per square inch | `kpsi` or `k~psi` |
| `kts` | knots | `kn` |
| `hr`, `hrs` | hour | `h` |
| `Cal` | food calorie | `kcal` or `k~cal` |
| `NM` | nautical mile | `nmi` |
| `KB`, `Kg` | kilobyte, kilogram | `kB`, `kg` |
| `kt` | kilotonne or knot | `k~t` or `kn` (§4) |

Two that *are* accepted but rarely mean what the writer intended:

| Written | Bovnar reads | If you meant… |
|---------|--------------|---------------|
| `fl` | femtolitre | fluid ounce → `fl_oz` |
| `mt` | millitonne (= 1 kg) | metric ton → `t`; megatonne → `Mt` or `M~t` |

---

## 14. Water hardness: six scales, one quantity

All six measure the concentration of dissolved alkaline-earth ions, and Bovnar converts freely
between them and `m~mol/L`. What differs is the reference compound each scale counts, which is why
their *mass*-per-litre definitions are not comparable and the amount concentration is.

| Scale | Symbol | Defined as | 1 mmol/L equals |
|-------|--------|------------|-----------------|
| German | `°dH` | 10 mg CaO / L | 5.6077 °dH |
| English / Clark | `°e`, `°Clark` | 1 grain CaCO₃ / imperial gallon | 7.0217 °e |
| French | `°fH` | 10 mg CaCO₃ / L | 10.0086 °fH |
| Russian | `°rH` | 1 mg Ca / L | 40.078 °rH |
| American | `°aH` | 1 mg CaCO₃ / L (called "ppm") | 100.086 °aH |
| US grains | `gpg` | 1 grain CaCO₃ / US gallon | 5.8468 gpg |
| Equivalents | `m~val/L`, `m~eq/L` | ½ mmol/L (divalent ions) | 2.000 mval/L |
| Amount | `m~mol/L` | — | 1 |

The traps, in order of how easily they bite:

| Trap | Why | Do this |
|------|-----|---------|
| `dH` instead of `°dH` | `dH` is the decihenry — an accepted unit, so nothing errors | keep the `°`, or write `german_hardness` |
| "ppm" for hardness | Bovnar's `ppm` is the dimensionless 10⁻⁶ | write `°aH` |
| `mval/L` for a monovalent species | `val` here is the *water-analysis* equivalent: 1 val = ½ mol | write `m~mol/L` for the amount directly |
| Expecting an exact conversion | the degrees are derived from molar masses, so they carry `.exact = false` | ordinary conversion works; a lossless one reports `error_unit_inexact` |
| `mmol/l` as a "unit" | it is the compound `m~mol/L`, not a base unit | write `mmol/L`, `m~mol/L` or `mmol/l` — all the same |
| `gr/gal` for `gpg` | `gr/gal` is a *mass* concentration, `gpg` an *amount* concentration; they do not convert | write `gpg` for the hardness scale |

**Turbidity and salinity are instrument scales**, and the trap is that they look interchangeable.

| Token | Means | Not | Because |
|-------|-------|-----|---------|
| `NTU` | white-light turbidity, 90° (EPA 180.1) | `FNU`, `FTU`, `FAU`, `JTU` | equal on a formazin standard, different on real water — the optics respond differently to particle size and colour |
| `FNU` | near-infrared turbidity, 90° (ISO 7027) | the other four | same reason, from the other side |
| `FTU` | formazin turbidity, **geometry unstated** | the other four | it does not say which optics were used; that is its entire content |
| `FAU` | formazin *attenuation*, 0° (ISO 7027) | the nephelometric three | measures light removed from the beam, not scattered sideways — a different optical quantity, used above ~40 FNU |
| `JTU` | Jackson candle turbidimeter (historical) | the formazin four | "1 JTU ≈ 1 NTU" holds near 40 units and nowhere else — nonlinear and sample-dependent |
| `PSU` | practical salinity (PSS-78, a conductivity ratio) | `‰`, `g/kg` | *S*_P 35 is ≈ 35.165 g/kg — equating them is wrong by ~0.5 %; write `g/k~g` for absolute salinity |
| `CF` | hydroponic conductivity factor (EC in mS/cm × 10) | `cF` — that is the **centifarad** | uppercase only |

`JTU` and `PSU` take no prefix (the candle method cannot resolve below ~25 JTU; practical salinity
is bounded by construction). The other turbidity scales do — `m~NTU` is real in ultrapure-water
work.

Bovnar refuses every conversion in the "Not" column rather than implying a factor exists. `CF` is
the exception in the other direction: it really is a conductivity, so `1 CF = 0.1 mS/cm = 100 µS/cm`
converts exactly.

**Conductivity and dissolved solids** need no units of their own, and that is the trap: people look
for one.

| Quantity | Write | Not |
|----------|-------|-----|
| Electrical conductivity | `µS/cm`, `mS/cm`, `dS/m`, `S/m` | a base unit — there isn't one, and none is needed |
| … in the pre-SI spelling | `µmho/cm` (`mho`, `mhos`, `℧` are the siemens) | `mho` was unknown before this; it is an alias now |
| Total dissolved solids | `mg/L` | `ppm` — that is the dimensionless 10⁻⁶ |
| Resistivity | `MΩ·cm` | — |

For TDS the `ppm` question is softer than for hardness: at 1 kg/L, 1 mg/L *is* 1 ppm by mass, so
`ppm` states a real (dimensionless) quantity. It simply is not the same dimension as `mg/L`, so the
two never convert into one another. A meter's "TDS ppm" reading is EC times an instrument factor —
record the EC and the factor, or the TDS as `mg/L`.

---

## 15. Same name, different definition

Not parser ambiguities — real-world ones. Bovnar keeps these distinct, and the distinction is
usually where the money is.

| Family | Members and exact values |
|--------|--------------------------|
| Ton | `t` = 1000 kg · `tn_sh` (short ton) = 907.18474 kg · `tn_l` (long ton) = 1016.0469088 kg |
| Ounce | `oz` (mass) = 28.349523125 g · `oz_t` (troy) = 31.1034768 g · `fl_oz` (volume) = 29.5735295625 mL |
| Gallon | `gal` (US) = 3.785411784 L · `gal_uk` = 4.54609 L (US is 0.8327× the imperial) |
| Pint / quart | `pt`, `qt` (US) vs. `pt_uk`, `qt_uk` — same 0.8327× ratio |
| Pressure ≈ 1 atm | `atm` = 101325 Pa · `at` (technical) = 98066.5 Pa · `bar` = 100000 Pa |
| Millimetre of mercury | `Torr` = 101325/760 Pa ≈ 133.322368 Pa · `mmHg` = 133.322387415 Pa — **not** the same unit (they differ by 1.4×10⁻⁷ relative) |
| Horsepower | `hp` (mechanical) = 745.6998715822702 W · `PS` (metric) = 735.49875 W |
| Calorie | `cal` = 4.184 J (thermochemical); the food "Calorie" is `kcal` |
| Thou | `thou` = `mil` = 25.4 µm — `mil` is **not** a milliradian, which is `mrad` / `m~rad` |

---

## 16. Quick index: if you mean X, write Y

| If you mean | Write | Not |
|-------------|-------|-----|
| millisecond | `ms`, `m~s` | `mS` (millisiemens) |
| millisiemens | `mS`, `m~S` | `ms` |
| kilogram | `kg`, `k~g` | `Kg`, `KG` |
| kilobyte / kilobit | `kB` / `kb` | `KB` |
| kilobyte (binary) | `KiB`, `Ki~B` | `kB` (decimal, 1000 B) |
| megatonne | `Mt`, `M~t` | `MT` (megatesla), `mt` (millitonne) |
| kilotonne | `k~t` | `kt` (refused) |
| knot | `kn` | `kt`, `kts` |
| milli-inch | `m~in` | `min` (minute) |
| milliradian | `mrad`, `m~rad` | `mil` (thou) |
| picohenry | `p~H` | `pH` (acidity) |
| acidity | `pH` | `p~H` (picohenry) |
| phot | `ph` | `pH` |
| millilitre | `mL`, `m~L` | `ml` is fine too; `fl` is a femtolitre |
| microgram | `µg`, `ug`, `µ~g` | `mcg` |
| kilowatt-hour | `kW·h`, `k~W·h` | `kWh` |
| cubic centimetre | `cm³`, `cm^3` | `cc` |
| miles per hour | `mph` (or `mi/h`) | mixing both in one value |
| kilometres per hour | `kph`, `kmh` (or `k~m/h`) | mixing both in one value |
| degree Celsius | `°C`, `degC` | `C` (coulomb) |
| thousand US dollars | `k$USD`, `k~$USD` | `kUSD` |
| a plain count | omit the unit, or `no_unit` | `%` (that is 10⁻²) |
| German water hardness | `°dH`, `german_hardness` | `dH` (decihenry) |
| hardness "in ppm" | `°aH` | `ppm` (dimensionless 10⁻⁶) |
| hardness in equivalents | `m~val/L`, `mval/l` | `m~mol/L` (that is twice the value) |
| hardness in millimoles | `m~mol/L`, `mmol/L` | — it needs no unit of its own |
| hardness in US grains | `gpg` | `gr/gal` (a mass concentration) |
| conductivity | `µS/cm`, `mS/cm`, `dS/m` | — no base unit exists or is needed |
| conductivity, older US data | `µmho/cm` | — `mho` is the siemens |
| dissolved solids | `mg/L` | `ppm` (dimensionless 10⁻⁶) |
| turbidity, white light 90° | `NTU` | `FNU`, `FTU` |
| turbidity, near-IR 90° | `FNU` | `NTU`, `FTU` |
| turbidity, method unstated | `FTU` | `NTU` or `FNU` — those claim a geometry |
| turbidity, attenuation | `FAU` | `FNU` (a different optical quantity) |
| turbidity, candle method | `JTU` | any formazin scale |
| practical salinity | `PSU` | `‰` or `g/kg` |
| absolute salinity | `g/k~g` | `PSU` |
| hydroponic EC scale | `CF` | `cF` (centifarad) |

---

## See also

- [Unit & Currency Reference](05_bovnar_unit_system.md) — the registry these readings come from
- [Unit & Currency Cheat Sheet](04_bovnar_unit_cheatsheet.md) — every symbol in one place
- [Specification §11 — Units System](03_bovnar_spec.md#11-units-system) — the normative unit rules
- [FAQ §4 — Units](02_bovnar_faq.md#4-units) — common unit questions

---

*Every claim in this document was verified against the reference implementation. If you find
a token whose behaviour surprises you and it is not listed here, that is a documentation bug —
please report it.*

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*End of Bovnar — Unit Ambiguities (Bovnar spec 1.1).*