Relative humidity is the number that matters, not 'wetness'
Relative humidity (RH) is the amount of water vapor in the air expressed as a percentage of the maximum the air can hold at that temperature. It is the single most important variable in moisture protection because cargo damage — corrosion, mold, mildew, caking — is driven by RH inside the package, not by the absolute amount of water. The same gram of water makes a hot container humid but barely registers in a cold one. This is also why condensation ('container rain') happens: warm humid air hits a cold surface, the air's capacity drops, RH hits 100%, and water condenses. A desiccant's job is to hold the package RH below the threshold where the specific cargo is damaged.
- RH = current water vapor ÷ maximum the air can hold at that temperature.
- Cargo damage tracks RH inside the package, not absolute water mass.
- RH rises as temperature falls (same water, less capacity) — hence container rain.
- Desiccant goal: keep package RH below the cargo's damage threshold.
What an adsorption isotherm is
An adsorption isotherm is the curve that shows how much water a desiccant holds (as a percentage of its own weight) at each equilibrium RH, at a fixed temperature. For silica gel the curve rises steeply at low-to-mid RH and continues climbing toward its ~33% capacity near high RH. The crucial buyer insight: capacity is not a single number — it depends on the RH the gel is working at. A silica gel sachet 'rated 33%' only reaches that near high humidity; at 20% RH it might hold 8–10%, at 40% RH around 20%. Clay desiccant has a flatter, lower curve. Molecular sieve has a steep early curve that plateaus (great at very low RH, limited total). The isotherm is the honest spec; the headline percentage is just one point on it.
- Isotherm = water held (% of own weight) vs equilibrium RH, at fixed temperature.
- Silica gel: rises steeply then climbs toward ~33% near high RH.
- Capacity at 20% RH ≪ capacity at 80% RH — it is not one fixed number.
- Clay: flatter, lower curve. Molecular sieve: steep early, plateaus low.
- The headline '33%' is the top of the curve, not its everyday value.
Type A vs Type B silica gel on the isotherm
Pore size changes the isotherm shape. Fine-pore grades (often Type A) adsorb strongly at low-to-mid RH, making them ideal for packaging where you want to pull RH down and hold it there. Wider-pore grades (Type B) pick up more water at high RH but hold less at low RH — useful for humidity buffering in damp environments. For most export packaging the fine-pore behavior is what you want: aggressive moisture capture in the 20–60% RH band where cargo sits. When a manufacturer quotes a grade, the relevant question is 'what does its isotherm look like in my RH range?' rather than just 'what's the peak capacity?'.
- Fine-pore (Type A): strong adsorption at low-to-mid RH — best for packaging.
- Wider-pore (Type B): more capacity at high RH, less at low RH — humidity buffering.
- Match grade to the RH band your cargo actually experiences.
- Ask the manufacturer about isotherm behavior in your range, not just peak %.
Using the isotherm to size desiccant honestly
Correct sizing uses the isotherm value at your target RH, not the headline capacity. If you want to hold a carton at 40% RH and your gel holds ~20% of its weight at that equilibrium, then you must base the calculation on 20%, not 33% — otherwise you under-dose by 40%. Practically: estimate the water that will enter the package over the voyage (from packaging permeability, trapped air, and any moisture in the goods), then provide enough gel that, at the target RH point on its isotherm, it can hold that water with margin. DryGelWorld's sizing guidance and the container dosage calculator bake this in, but understanding the isotherm is what lets you sanity-check a supplier's recommendation rather than trust it blindly.
- Size on the isotherm value at your target RH, not the peak %.
- Targeting 40% RH with a gel holding ~20% there? Calculate on 20%.
- Estimate water ingress (permeability + trapped air + goods moisture) over the voyage.
- Provide capacity to hold that water at the target-RH point, with margin.
- Use the container dosage calculator, then sanity-check against the isotherm.
Temperature shifts the whole curve
Isotherms are quoted at a fixed temperature (often 25°C). Raise the temperature and silica gel's capacity drops — the same gel holds less water when hot, which is why desiccant performance degrades in a sun-baked container roof and why high-temperature industrial drying needs more gel or a higher-temperature material. Lower the temperature and capacity rises, but that is also when RH spikes toward condensation. The takeaway for export buyers: voyages that cross big temperature swings (tropical load port, cold transhipment, warm destination) stress the desiccant more than a steady climate, so size up for routes with large day–night and port-to-port temperature ranges.
- Isotherms are temperature-specific (commonly quoted at 25°C).
- Higher temperature → lower silica gel capacity (hot containers underperform).
- Lower temperature → higher capacity, but also condensation risk.
- Big temperature swings on a route stress the desiccant — size up for them.