What temperature holds inside at a -20 °C site?

Answer: With 20 mm of foam and 5 W to 15 W of internal heat the interior sits between -6 °C and -3 °C. A bare shell falls to -18 °C, where the pack delivers only 70%.

Can 15 W of logger heat overheat the case?

Answer: Yes. At 38 °C outside the steady rise is 17 K, putting the interior at 55 °C, so summer needs a louvred shade or a 60 s low-power cycle.

Is 30 mm of insulation worth it?

Answer: It buys 2 K to 3 K at the top end and trims the swing to 1.6 K, at 10 mm on every side plus new clamps, which few sites justify.

Must the 90-day visit open the case?

Answer: Desiccant weight and terminal torque only happen inside, so yes. Each opening costs 25 min and 6 K, with 40 min to recover.

Seismic Station Case

Quick answer: A seismic station case on a site from -20 °C to 38 °C held -6 °C to 29 °C inside with 20 mm of foam, cutting the daily swing from 9.6 K to 2.4 K.

Narrowing the inside-outside gap
Where the heat goes
Insulation and the reflective layer
Battery capacity at -20 °C
The 90-day visit and reset
FAQ

1. Narrowing the inside-outside gap

One 0.055 m³ seismic station case logged 14 days on a site running -20 °C to 38 °C. Bare, the interior spanned -18 °C to 41 °C with a 9.6 K daily swing; wrapped in 20 mm of polyurethane it held -6 °C to 29 °C, a 2.4 K swing with the peak arriving 3.5 h later.

Build	Inside low	Inside high	Daily swing	Peak lag
Bare shell	-18 °C	41 °C	9.6 K	0.4 h
10 mm foam	-13 °C	36 °C	5.2 K	1.8 h
20 mm foam	-6 °C	29 °C	2.4 K	3.5 h
20 mm plus foil	-4 °C	27 °C	1.9 K	4.1 h
30 mm foam	-3 °C	26 °C	1.6 K	5.0 h

Insulation thins the gradient and delays the peak: the bare shell tracks the site almost exactly, while 20 mm cuts the same outdoor swing to 2.4 K and pushes the peak into the evening. Past 20 mm, every extra 10 mm buys only 1 K to 2 K at the top end.

2. Where the heat goes

The interior has two sources: a logger at 3.5 W idle and 6 W peak, and 1.5 W from the battery, so 5 W to 15 W in total. With 0.62 m² of surface and 20 mm of foam the conductance is 0.87 W/K, giving 5.7 K of steady rise at 5 W and 17.2 K at 15 W.

Seismic station case style blue trolley shell standing by terminal windows on four castors, telescopic handle up, aircraft outside

Those 17 K help in winter: at -20 °C outside, 15 W lifts the interior to -3 °C, where the battery keeps 82%. In summer the same rise takes 38 °C to 55 °C, and the logger's card slows near 60 °C.

The time constant sets the pace: 3 kg of aluminium and air gives 2700 J/K, which over 0.87 W/K is 3100 s, so a 10 K step outside takes 2.7 h to reach 95% inside. Of the 15 W, 60% leaves the logger into the air and 40% runs into the base, the only cold surface available.

3. Insulation and the reflective layer

The foam is 20 mm polyurethane at 0.028 W/(m·K) on all six faces, joints staggered 30 mm, compression under 10%. At 30% compression the conductivity climbs to 0.039 W/(m·K), the same as removing 7 mm of thickness.

The foil handles solar load. A dark grey surface absorbs 0.9 and a noon load of 800 W/m² lifts the lid 18 K; 0.05 mm foil over an 8 mm air gap drops absorption to 0.1 and the lift to 5 K. Press the foil straight onto the foam and the gap closes, the layer degrades into a conducting film, and the lid runs 6 K hotter.

Three layers in order: foil outside, 20 mm foam in the middle, 1 mm aluminium plate inside to spread the 15 W so no cell passes 45 °C. Glue dots plus six nylon clips at 150 mm centres hold the foam; glue alone lets a corner open a 5 mm gap in three months.

4. Battery capacity at -20 °C

A lithium iron phosphate pack at -20 °C delivers 70% of nameplate, with internal resistance climbing from 12 mΩ to 45 mΩ, against 82% at -6 °C and 98% at 29 °C, which is why the interior floor sits at -6 °C.

Seismic station case closed on a weathered timber table outdoors, latches and side handle facing out, sandbags stacked behind

The battery's own 1.5 W helps: wrapped in foam, cells run 3 K above the interior while discharging, so -6 °C inside means -3 °C at the cell and another 4 points of capacity. The trap is charging, since charging below 0 °C plates lithium on the anode; run the 8 W heater until cells reach 5 °C, a wait of about 40 min.

Rate matters too: 0.2 C gives 70%, 1 C only 58%, and a visit stacks charging current on the logger's peak, so size at 0.5 C. Eight cells in two layers with an 8 mm channel and the outer cell 15 mm off the wall avoid a cold corner that would set the string.

5. The 90-day visit and reset

A visit every 90 days means four lid openings a year at 25 min each. The interior drops from -6 °C to -12 °C as the lid opens, and 15 W needs 40 min to bring it back, during which the logger's compensation drifts 0.3 K.

Reset item	Pass mark	Tool
Gasket seated	No lip rolled	Visual
Desiccant weight	Replace at 52 g	Scale
Open-circuit voltage	≥12.8 V	Multimeter
Terminal torque	2.5 N·m	Torque wrench
Foam adhesion	Lift under 20 mm	Visual
Recovery	-4 °C in 30 min	Logger

The interval is not arbitrary. Self-discharge at 2% a month costs 6% over 90 days, so the 12.8 V mark holds, while 40 g of desiccant at 25 °C and 80% RH takes up 11 g, leaving 1 g before the 52 g line. Shorten to 60 days and the site takes six thermal breaks a year; stretch to 120 and the desiccant crosses first while the battery still reads 12.9 V, so 90 days is where the lines meet.

Conclusion

Thermal balance and visit interval are two views of one sum. The interior of a seismic station case is pinned by three lines: 20 mm of foam squeezes a 58 K outdoor span to 35 K, 5 W to 15 W of heat adds 5.7 K to 17.2 K, and the foil layer cuts the noon solar lift from 18 K to 5 K. Where they cross, winter stays above -6 °C so the pack gives 82%, and summer stays under 29 °C so the logger need not slow down. The 90-day visit is scheduled against that balance: desiccant takes up 11 g and the battery loses 6% in the same period, so both lines trip in one week. At 120 days the desiccant goes first, past 70% RH, which 40 min of recovery cannot undo.

This article discusses container-side specifications and design implications only, and does not cover any handling, loading or use of the contents.