Why Measure the Evaporation Rate?
How many producers say "this diffuser lasts two months" having actually measured it? The evaporation rate determines three fundamental properties of a room fragrance at once: scent intensity, product life, and whether the customer will complain that it "does not smell" or "ran out straight away". The only instrument needed to measure it is a set of scales; the only other requirement is discipline.
The grams-per-day test measures how many grams of liquid a full diffuser (or any other evaporative room fragrance) loses per day. The result lets you compare different solvents, different fragrance oil ratios, different reed counts and different fragrance oils on the same scale. "This one smells better" is a subjective judgement; "this one evaporates 1.4 g a day, the other 0.6 g" is data.
This article explains how to set up the test, how to read the results and which values to aim for. It is explained through the reed diffuser; the same method applies to every evaporative product other than candles.
Test Set-Up: Step by Step
The test is meaningful when done in a controlled environment and comparatively. A single number from a single bottle says little unless there is something to compare it with.
- Fix the environment
Same room, same shelf, away from direct sun and air movement, temperature 20–25°C. Do not change the environment during the test; record temperature and humidity.
- Prepare the samples
One bottle for each variable to be compared; two of each if possible (replicates). Same bottle, same reed type and count, same fill weight. Only the variable being compared differs.
- Weigh on day zero
With the reeds inserted, on scales reading to 0.1 g. There is rapid absorption in the first hour after inserting the reeds; take the day-zero weighing an hour after the reeds have gone in.
- Weigh at the same time every day
7 days, ideally 14. Do not move the bottle or flip the reeds before weighing. If a flipping schedule is part of the test (on day 7, say), record it.
- Take a scent note
At each weighing, scent intensity from a distance of 1 metre: 0 (none) – 5 (very strong). Same person, same time, entering the room from outside.
- Calculate grams per day
(Day-zero weight − day N weight) ÷ N. Assess the first 2–3 days separately; the initial rate is usually high. The steady rate is the average from day 3 onwards.
- Calculate the estimated life
Fill weight ÷ steady grams per day. 95 g fill ÷ 1.2 g/day ≈ 79 days. The real life is somewhat longer because of reed clogging, and weaker in scent.
Reading the Results: Target Ranges
Values vary with product type and target life; the ranges below are a practical framework for a 100 ml reed diffuser.
| Grams per day (100 ml) | Estimated life | Interpretation |
|---|---|---|
| < 0.5 | > 6 months | Very slow; most likely a weak scent. Solvent too heavy or viscosity high |
| 0.5–1.0 | 3–6 months | Slow to medium; acceptable for a small room and a light scent |
| 1.0–2.0 | 1.5–3 months | Target range; medium room, noticeable scent, reasonable life |
| 2.0–3.0 | 1–1.5 months | Fast; strong scent but short life. Reduce the number of reeds or move to a heavier solvent |
| > 3.0 | < 1 month | Very fast; solvent too volatile. The product draws "ran out straight away" complaints |
The evaporation rate is not enough on its own; it is read together with the scent score. If two products share the same 1.2 g/day value while one scores 4 and the other 2, what is evaporating from the second is mostly solvent. This shows that the fragrance oil is built on mid-to-heavy notes or that the ratio is low.
The real strength of the test is comparison: three solvents with the same fragrance oil, three fragrance oil ratios with the same solvent, 4, 6 and 8 reeds with the same liquid. Each comparison goes into a table, and the product decision is made from the table.
Variables That Change the Rate
If the test result falls outside the target range, one of five variables is adjusted.
Solvent. The biggest effect. A more volatile solvent (isoparaffin, some glycol ethers) increases the rate; DPM and Augeo are medium; DPG is very slow. Blending solvents gives intermediate rates, and this is the cleanest way to tune the product.
Reed count and diameter. The rate is directly proportional to the evaporation surface. Going from 6 reeds to 8 increases the rate by roughly a third. This is the only adjustment in the consumer's hands; it should be explained on the product card.
Fragrance oil ratio and character. As the fragrance oil ratio rises, the rate usually falls (fragrance oil is heavier than solvent) but the scent increases. Light, citrus- and green-led fragrance oils evaporate quickly; woody-musk-led fragrance oils evaporate slowly and build up in the reed.
Bottle neck. A narrow neck reduces evaporation from the bottle itself; it limits loss other than through the reeds and extends life. A wide-necked bottle loses quickly even without reeds.
Environment. Temperature and air movement. These are held constant in the test environment; the warning "runs out faster in a draughty spot" on the product card is accurate.
Frequently Asked Questions
The most common questions about the evaporation rate test.
Which scales are good enough for grams per day?
How many days should the test run?
What is the target grams per day for a 100 ml diffuser?
Evaporation is high but the scent is weak, why?
Does flipping the reeds spoil the test?
Is the same test done for room sprays or gel fragrances?
What temperature should the test environment be?
Is a test with a single bottle meaningful?
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