How much desiccant a crate needs

What desiccant does

Desiccant absorbs water vapour from the air around it. In a sealed bag, it dries the trapped air and holds the humidity low, so condensation does not form on the load when the temperature drops. Common desiccants include clay and silica gel, packed in breathable pouches, and each has a capacity that falls as the humidity rises.

It does not clean up a wet load. Wood, cardboard and fabric inside the bag hold moisture, and desiccant draws it out slowly. Pack dry materials in the bag, and tell the packer which parts of the load are damp, painted recently or made of wood.

What a unit is

A desiccant unit is a standard quantity defined by capacity: the amount of desiccant that absorbs a given quantity of moisture under set test conditions. The definition is published in the military specification MIL-D-3464, which is why suppliers describe bags and packs in units and not in grams.

The unit gives buyers a common measure, but it does not tell you how many to use. Ask the packer how many units are specified, and what assumptions they used for bag area and voyage. A figure with no stated basis cannot be checked.

What decides the count

Moisture gets into a sealed bag slowly through the film and through the seams. The more bag surface, the more ingress, and the longer the voyage, the more total ingress. So the bag's area and the transit time are the main inputs. Contents add to it, since a load with wooden blocking, paper wrap or recent paint gives off moisture of its own.

We give no formula figures here, since the count depends on the bag film's rated transmission and on the route. A packer working from the film maker's data and the voyage can calculate it. Our job is to build the crate with room for the bag and the packs, and to write the count into the spec.

Why the bag matters more than the packs

A perfect pack count in a leaky bag is wasted. Humid air flows in through any gap, and the desiccant saturates in days and not weeks. A continuous heat-sealed seam on a punctured-free barrier film is the foundation. Desiccant only has to handle the slow ingress through a sound bag.

That is why the bag is checked before closing. Packers look for pinholes, thin seams and chafe points, and the crate builder keeps sharp edges away from the film. If the bag fails, the desiccant did not fail first.

Placing and checking

Desiccant is placed where air collects and where it can be seen when the bag is opened, in breathable pouches that hang or sit clear of bare metal. Pouches that touch the load can hold water against it.

1. 01
   Seal the bag

   Heat-seal the barrier bag and check the seam over its full length before adding the packs.

2. 02
   Add the packs

   Put the specified number of units inside before the last seam closes, spaced around the load.

3. 03
   Fit a humidity indicator

   Place an indicator card where it can be read through a window in the bag, or at the seam for removal.

4. 04
   Record the count

   Write the unit count and bag details in the crate spec and on the packing list.

5. 05
   Check on arrival

   Read the indicator before opening. A card showing high humidity means the bag failed or the packs were spent.

Reading a humidity indicator card

A humidity indicator card carries printed spots that change colour at set relative humidity levels, typically blue when dry and pink when damp. Read it at the spot that matches its rated threshold, not the first spot you see. Cards are inexpensive evidence: they show the bag held its dry air from the packing bench to the receiving dock.

If a card reads high when the crate is opened, do not assume the desiccant was undersized. Check the seam first, look for punctures near blocking, and note whether the crate sat in standing water or under a leaking tarp. Photograph the card and the bag before disturbing either. That record tells the packer whether to change the film, the seal, the count or the handling instructions.

Mistakes that waste desiccant

Opening a foil pouch of packs and leaving it on a bench for an afternoon uses up capacity before the crate is even closed. Clay and silica gel begin absorbing the moment they meet room air, so packers keep them sealed until the last seam is welded. Stacking packs in one corner, or burying them beneath wrap where air cannot reach, also hobbles their work.

Another frequent slip is sealing wet material inside. Fresh paint, damp blocking, a wipe-down rag left behind or condensation on a cold machine arriving from an unheated shed all load the bag with water on day one. Let the load come up to shop temperature, dry it, then bag it.