Can a dry box and a protective case replace each other?

Answer: Above 50% RH, yes: a case with 1.5 g/L of silica and a colour card costs under 40 CNY. Below 35% RH the integrated module holds better, or passive media is changed monthly.

How long does 100 g of silica last in a 20 L box?

Answer: Twelve months at four openings a week and 60% RH ambient. Double the openings and it falls to 7; a box that stays shut reaches 18.

Can colour-changed silica be reused?

Answer: Yes, 2 h at 120 °C or 3 min on high in a microwave. After 20 cycles uptake falls to 70% and the charge is spent; spread it 20 mm deep.

Why does an IP67 case still need desiccant?

Answer: IP67 stops liquid, not vapour. At 0.55 m², 6–10 g a year crosses the seal, enough to push a 20 L cavity from 40% to 55% RH, the range electronics dislike most.

How far apart can two gauges read?

Answer: Within 5% RH at the same spot. Past 8% RH, change the battery and calibrate against a saturated salt solution; a dial drifts 1–2% RH a year and should be replaced after two.

Dry Box vs Protective Case: Dehumidifying

Quick answer: Dry box vs protective case is a question of moisture balance, not shell strength. Drying a 20 L cavity from 60% to 40% RH costs only 24 g of a 100 g silica charge; how long that charge lasts depends on openings and seal permeation, not on volume.

Target Humidity: Four Bands and Dosing
Desiccant Dosing: Is 100 g Enough for 20 L
Sealing and Breathing: IP67 Stops Water, Not Vapour
Electric vs Passive Drying: Where the Cost Line Falls
Humidity Indication: Card, Gauge, Logger
FAQ

1. Target Humidity: Four Bands and Dosing

Set the humidity target first. Both hold below 50% RH; the difference is who absorbs the swings. An integrated dry box carries its own module and gauge, a protective case supplies an IP67 shell and leaves the desiccant to the user.

Target RH	Typical load	Desiccant	Dose	Check interval
50–60%	Hardware, paper files	Silica gel	1 g/L	6 months
40–50%	Boards, connectors	Silica gel	1.5 g/L	3 months
20–40%	Optics, gauges	Molecular sieve	2 g/L	1 month
Below 10%	Coated optics, long storage	Sieve plus foil bag	3 g/L	2 weeks

The bands follow measured thresholds: steel corrosion slows below 60% RH, sulphiding of copper and silver is suppressed below 40%, optics grow mould above 30%. Silica holds 24 g per 100 g at 40% RH but 12 g at 20%, which is why low bands switch to sieve.

2. Desiccant Dosing: Is 100 g Enough for 20 L

Run the water balance. A 20 L cavity at 25 °C and 40% RH holds 0.18 g of vapour; coatings, foam and labels release 3 g as they cool; the seal passes 0.2 g a month. First-year ingress is near 5.6 g; the rest is held for openings.

Dry box vs protective case seal close-up: off-white shell corner, blue catch pressed home, thin white gasket line between lid and rim

Cavity	Silica	Capacity at 40% RH	Openings	Recharge interval
10 L	50 g	12 g	3 per week	8 months
20 L	100 g	24 g	4 per week	12 months
50 L	250 g	60 g	5 per week	15 months
80 L	400 g	96 g	6 per week	16 months

One opening is easy to size: a 20 L box held open 30 s exchanges about 6 L of air, which at 25 °C and 60% RH carries 0.08 g. Four a week is 17 g a year, plus 5.6 g of first-year ingress, so 22.6 g of a 24 g charge goes in twelve months. The 50 L box lasts longer because it travels closed.

3. Sealing and Breathing: IP67 Stops Water, Not Vapour

IP67 means no ingress after 30 min at 1 m depth, and it governs liquid water only. A water molecule is 0.28 nm across and an EPDM gasket passes 0.02–0.05 g/(m²·day) at 38 °C and 90% RH; at 0.55 m² that is 0.02 g a day, 7 g a year.

Seal build	Vapour transmission, g/(m²·d)	Annual ingress, 0.55 m²	Desiccant needed
Single lip, 25% compression	0.05	10 g	Yes
Double lip, 30% compression	0.03	6 g	Yes
Double lip plus 0.2 bar valve	0.05	10 g	Yes, valve included
Silicone plus foil liner	0.01	2 g	Depends on target

Temperature swings are louder than permeation. Seal a box at -10 °C, carry it to 25 °C: 101 kPa across 35 K implies 13 kPa of differential, so the lid needs over 40 N, or 12 N with a valve. Each cycle admits 0.3 mg.

The real leak is inspection. A 0.5 mm grain in the groove or a 2 mm crease in the lip takes decay from 0.5 kPa to over 3 kPa and triples permeation; both box types need a visual check before closing and a periodic decay test.

4. Electric vs Passive Drying: Where the Cost Line Falls

An integrated dry box sells convenience; a passive setup sells no power draw. Over five years a 20 L box runs about 2120 CNY electric against 165 CNY passive, a twelve-fold gap:

Dry box vs protective case open: white base, blue rim, white gasket ring in the lid, blue and white folded cloth, clear bag with sachet

Peltier module: 35 W, 250 mL per 24 h at 30 °C and 80% RH, 1200 CNY, about 184 CNY of power a year at 0.6 CNY/kWh
Passive silica: two 100 g charges rotated, 30 CNY, regenerated at 120 °C for 2 h, 30 CNY a year plus 3 CNY of power
Molecular sieve: below 10% RH only, regenerated at 200 °C, three times the low-humidity uptake of silica
Hybrid: Peltier plus 50 g of silica as outage buffer, 15 CNY extra

Two lines settle it. Below 35% RH with over 100 L, go electric, or passive media is changed monthly. At 50 L or less with a 35% RH target, go passive: 184 CNY of electricity buys six years of silica.

5. Humidity Indication: Card, Gauge, Logger

A dry box without indication turns desiccant failure into a guess.

Option	Accuracy	Price	Reading	Use
Five-spot card	±10% RH	2 CNY	Colour comparison	One-way storage, shipped with goods
Dial gauge	±5% RH	25 CNY	Direct read	Boxes opened routinely
Data logger	±2% RH	300 CNY	10 min sampling, exported curve	Validation and traceability

The three are not substitutes: a card shows trend, a gauge the present, a logger the path. One trap with dials: on the lid the body runs 3 °C above cavity temperature on a summer afternoon and reads 4% RH low, which looks healthy. Fit it mid-wall, 100 mm from the desiccant.

Conclusion

Dry box vs protective case comes down to four bands (60%, 50%, 40%, 10% RH), 1.5 g/L of silica, and 6–10 g a year of permeation. Passive wins under 50 L and above 35% RH at 30 CNY a year; past 100 L or below 20% RH, 184 CNY of electricity ends the monthly recharge.

This article discusses only the container (case body) side of specifications and design intent, and does not cover any handling, loading or use of the contents.