Your First Weighing Practice

Choosing a scale

For study-sized batches of a few grams, a jewelry-style scale reading to 0.01 g is the usual starting point, and our equipment list gives the full picture. A scale reading to 0.001 g handles smaller amounts, at a higher price and with more sensitivity to drafts and vibration. Whichever you buy, look for one with a capacity above your largest container plus contents, and a check weight, so that you can confirm it is reading correctly.

What the display does not tell you

A scale's readability is the smallest step it displays. It says nothing certain about accuracy. Cheap scales can drift, and very small amounts are the least reliable. As an example, a 0.01 g error is 1% of a 1 g addition, but 20% of a 0.05 g addition. So a sensible rule of thumb is to avoid weighing right at the limit of your scale. On a 0.01 g scale, we suggest treating 0.2 g or so as a practical lower limit for a single addition, and the exercise below will show you whether that holds for your own equipment. That figure is our judgment, not a standard.

What about smaller amounts? You make a dilution and weigh more of a weaker solution, as the next lesson shows. That is one of the main reasons perfumers keep dilutions on the shelf.

The basic technique

1. Set up. Put the scale on a firm, level surface, away from drafts and vibration. Let it warm up if the manual says to.
2. Check it. Weigh your check weight, and note any difference.
3. Tare. Put your clean, labeled vial on the scale and press tare. The display should read 0.00.
4. Add slowly. Use a clean pipette and add drop by drop near the end, holding the pipette above the vial without touching it.
5. Wait. Let the reading settle. Many cheap scales lag behind, or they may not register very small additions until you have added more.
6. Write it down straight away: the actual reading, not the target.

The full reference is how to weigh perfume ingredients, including what to do when you overshoot. The principle to keep is this: the weight you record is the weight you actually added.

Weighing and drops

Counting drops is tempting and is not a reliable measure, since drop size changes with the liquid, the pipette and your angle. The guide on drops to grams shows how much it varies. You will measure your own drop size in the exercise below, to see this for yourself.

If you overshoot

Do not try to take liquid back out with the same pipette, or tip it out. Either accept the amount and adjust the rest of the recipe by calculation, or start again. Later on, the guide to adding too much of an ingredient shows how to rescue a formula using arithmetic. For now, overshooting on water costs you nothing, so learn to approach a target slowly.

Common mistakes

• Confusing readability with accuracy. A second decimal place does not make a reading correct.
• Weighing near the limit of the scale. Make a dilution instead.
• Recording the target rather than the reading. Your record has to say what really happened.
• Reusing a pipette. Traces transfer between materials. Use a clean one for each material.
• Working by volume because it is easier. Materials differ in density, and the guide to weight and volume explains how far that error can go.

Key terms

Readability

The smallest step a scale displays, such as 0.01 g. It is not the same as its accuracy.

Tare

To zero the scale with a container on it, so that only what you add is counted.

Minimum weight

The smallest amount you can weigh on a given scale with acceptable uncertainty. It is well above the scale's readability.