Quick answer: To size fiber desiccant, estimate how much water your pack has to handle, choose the humidity your product must stay under, and divide the water by the desiccant's capacity at that humidity. Plant-fiber desiccant holds more than 10% of its own weight at 20% RH, more than 35% at 50% RH and more than 70% at 90% RH (25 °C), so 0.35 g of water at a 50% RH target needs about 1.0 g of fiber desiccant before margin. Then confirm the result with samples in your own pack.
Getting the mass right is the first step to reliable moisture protection. Too little and the product sees moisture before the end of its shelf life; far too much and you pay for capacity you never use. The method below is how we approach sizing with customers.
Step 1: estimate the water load
The desiccant has to hold two things:
- The moisture already inside at sealing, in the headspace air and, for some products, released by the product itself.
- The moisture that gets in over time, through the pack walls and seal (set by the material's WVTR and the pack area) and each time the pack is opened.
Your packaging supplier can give the WVTR of your material. Pack area, shelf life and opening pattern come from you. If those numbers are not to hand, send us the pack and we will estimate them with you.
Step 2: pick the humidity your product needs
This is the most important choice. A powder that cakes at moderate humidity, an electronic part kept dry, or a dried food that turns leathery each has its own limit. Size on the humidity the product must stay under, not on the highest figure on a datasheet.
Step 3: divide by capacity at that humidity
Fiber desiccant holds more than 0.10 g, 0.35 g and 0.70 g of water per gram at 20%, 50% and 90% RH (25 °C). Divide your water load by the figure for your target humidity:
| Water load | At 20% RH target | At 50% RH target | At 90% RH reference |
|---|---|---|---|
| 0.10 g | about 1.0 g | about 0.29 g | about 0.14 g |
| 0.25 g | about 2.5 g | about 0.71 g | about 0.36 g |
| 0.50 g | about 5.0 g | about 1.43 g | about 0.71 g |
| 1.00 g | about 10.0 g | about 2.86 g | about 1.43 g |
These are minimum-specification figures, so the masses are conservative before any margin. For a Fiber Desiccant Card at 1.0 mm, the comparison with silica gel at 25 °C and 90% RH is three to one under the same test conditions.
Step 4: add a margin and choose the format
- Add a margin for opening patterns, shipping through warm humid routes and line handling time.
- Choose the format that fits: a card at 0.7 or 1.0 mm, a fiber sheet at 0.5 or 1.0 mm, or a bagged sachet from 1 to 1,000 g.
- Split the mass if needed: two smaller pieces in different spots can suit long or awkward packs.
Step 5: confirm in your own pack
Seal packs with the sized desiccant, store them in your real conditions, and check the product. We send samples cut to your format for exactly this step. For handling during the trial, see how to use fiber desiccant.
A worked example
Example only: a supplement tub must stay at or below 50% RH, and the estimated water load over its life, including openings, is 0.25 g.
- Capacity at 50% RH: more than 0.35 g per gram of fiber desiccant.
- 0.25 g ÷ 0.35 = about 0.71 g of fiber desiccant.
- Add a margin for openings and shipping, and round up to the nearest card size.
- Confirm with samples in the real tub.
Your numbers will differ; send us yours and we will run them with you.
Frequently asked questions
How do I calculate how much desiccant I need?
Estimate the water your pack must handle, choose the humidity your product must stay under, divide by the desiccant's capacity at that humidity, add a margin and confirm in your own pack.
Can I size fiber desiccant from my silica gel packet?
Not gram for gram. Size on your target humidity. At 1.0 mm thickness, 1.0 g of Fiber Desiccant Card absorbs as much moisture as 3.0 g of silica gel under the same test conditions (25 °C / 90% RH). Across 20 to 40% RH on our measured 25 °C curve, plant-fiber desiccant holds about twice as much as silica gel.
What capacity figures should I use?
Fiber desiccant holds more than 10% of its own weight at 20% RH, more than 35% at 50% RH and more than 70% at 90% RH (25 °C). Use the figure for the humidity your product needs.
What if I do not know my pack's WVTR?
Ask your packaging supplier, or send us the pack details and we will estimate it with you.
Do you size it for us?
Yes. Use the form below and we will size the format with you and send samples.
Size it with us
Send the pack and the product. We will estimate the water load, recommend a format and mass, and send samples to confirm in your own pack.
Prefer email? info@atmosiscience.com
Figures from ATMOSIScience fiber desiccant product documentation and usage instructions, the Fiber Desiccant Card product sheet and the fiber desiccant MSDS.
