Condensation in Electrical Enclosures: Why Dew Point Beats RH

Part of our iHaaS deep-dive series. New to HaaS? Start here.

Here's a pattern we hear a lot. The cabinet has a humidity sensor. The log says 80% RH, sometimes 88%. Nobody's alarm went off. Then a technician opens the door and finds water beads on the inside of the lid and fresh corrosion on a terminal block.

The sensor wasn't lying. It was answering a different question. Relative humidity tells you how close the air is to saturation at the air's own temperature. Condensation happens on a surface, at the surface's temperature. If you only watch RH, you're watching the wrong place.

At ATMOSIScience®, we make anti-condensation packs for sealed cavities, and we built the HaaS DEW condensation sensor to measure what those packs are up against. This guide walks through the physics, the math and the monitoring setup.

Condensation is a surface problem, not an air problem

Start with two definitions. The UK's National Physical Laboratory describes dew point as the temperature at which condensation would occur if a gas were cooled, and notes that it's an absolute measure of how much water vapor the gas holds. Relative humidity, by contrast, is "relative to temperature" (NPL Good Practice Guide No. 124).

That second point matters more than people expect. NPL's own example: at 50% RH, warming the air from 20 °C to 21 °C drops relative humidity by about 3% RH. Same water, different reading. Now picture a cabinet where the air near the electronics is 26 °C and the inside of the door is 20 °C. Same air mass, two very different stories.

So the useful question is never "what's the RH?" It's "how far is my coldest surface above the dew point of the air touching it?" We call that gap the dew point margin:

Dew point margin = surface temperature minus air dew point

Positive margin, the surface stays dry. Zero or negative, water forms. That's the whole game.

How to calculate dew point from temperature and RH

You don't need a lab instrument to get dew point. A temperature reading and an RH reading are enough, using the Magnus formula. The World Meteorological Organization's Guide to Instruments and Methods of Observation (WMO-No. 8, Annex 4.B) gives this form for saturation vapor pressure over water, valid from -45 °C to 60 °C with an uncertainty of up to 0.6% (WMO humidity calibration training unit):

e_w(t) = 6.112 × exp(17.62 × t / (243.12 + t)), with e_w in hPa and t in °C

Flip it around and you get dew point from air temperature T and relative humidity RH:

γ = ln(RH / 100) + 17.62 × T / (243.12 + T)
T_d = 243.12 × γ / (17.62 − γ)

That range covers where condensation actually happens, which is the cold side of the cycle.

Worked example: the cabinet that sweats at 88% RH

An outdoor cabinet closes up on a humid afternoon. The air inside sits at 30 °C and 70% RH.

• γ = ln(0.70) + (17.62 × 30) / (243.12 + 30) = -0.357 + 1.935 = 1.579
• T_d = 243.12 × 1.579 / (17.62 − 1.579) = 23.9 °C

At night the sky cools the lid and the door. The inside of the door reaches 20 °C. The air near the boards, warmed by the electronics, settles at 26 °C. Here's what each reading says:

The RH sensor said 88%. The door was already wet. That's why a humidity-only threshold can miss enclosure condensation entirely.

Why sealed enclosures sweat: day-night swings and breathing

Our anti-condensation packs are built for exactly these cavities: 5G outdoor AAU/RRU units and integrated cabinets, new-energy vehicle electronic controls, battery packs, motors and electric drives, cameras and radar, and automotive headlamps. They share three traits:

• Heat inside, cold outside. Electronics warm the air. The shell loses heat to the night sky. So the walls are the coldest thing in the box.
• Big swings. Outdoor 5G equipment faces sharp day-night temperature changes. Automotive headlamps go through repeated hot-cold cycling.
• Sealed, but not airtight. Gaskets, cable glands and vents let small amounts of air move.

The breathing effect, in numbers

When an enclosure that isn't perfectly airtight cools, the air inside contracts and outside air is drawn in to make up the volume. You can estimate how much with basic gas law arithmetic: the fraction drawn in is about 1 minus T_night / T_day, using absolute temperatures.

• Day: internal air at 55 °C (328.15 K). Night: 15 °C (288.15 K).
• Fraction drawn in: 1 − 288.15 / 328.15 = 0.122, or about 12% of the internal air volume per cycle.
• For a 100 L enclosure, that's about 12 L of outside air every night.
• Night air at 15 °C and 90% RH carries about 11.5 g of water vapor per m³, so 12 L brings in about 0.14 g.
• Over 30 nights that's about 4.2 g. Over a year, about 51 g.

A few grams doesn't sound like much until you remember where it goes: onto the coldest surface, which is often right above a circuit board. And it arrives every night, whether anyone checks or not. That's the case for both a material that absorbs moisture and a sensor that tells you how that material is coping.

How the HaaS DEW sensor reads condensation risk

Our DEW condensation sensors measure three things at the same moment: air temperature, air humidity and surface temperature. From those three readings, condensation inside the space can be calculated directly. Each sensor carries a surface temperature probe plus a temperature and humidity probe behind a waterproof breathable membrane, and mounts with double-sided adhesive tape. For a fuller look at the hardware, see Inside the HaaS Sensor Kit.

The data then goes to our Condensation Analysis Platform, part of the HaaSMind anti-condensation validation model. You pick the devices, set a time window and run the analysis. Two settings show how carefully it treats the data:

• Surface-temperature correction. You can apply a correction value to the surface reading before the analysis runs.
• Missing surface temperature. If a device has no surface reading, the system can approximate it with air temperature, and it flags the result with a warning so nobody mistakes it for a measured value.

More on how the models work in HaaSMind: AI models for shelf life, condensation and moisture diffusion.

How much margin is enough?

Every reading has an uncertainty, so the dew point you calculate does too. Using our spec sheet figures (temperature ±0.2 °C, humidity ±1.5% RH in the 60 to 90% RH band) at 30 °C and 70% RH:

• RH off by 1.5% RH shifts the calculated dew point by about 0.35 °C.
• Air temperature off by 0.2 °C shifts it by about 0.19 °C.
• Assuming the surface probe carries a similar ±0.2 °C, that adds another 0.2 °C.
• Combined (root sum of squares): about 0.45 °C.

So an alert margin of at least 1 °C keeps you clear of measurement noise. Go wider if you want time to react before the surface actually gets wet.

Choosing the right condensation sensor

We make three versions. All are IP65, store 260,000 records at a settable logging interval, and report straight to a phone or through a gateway.

Rule of thumb: if the cavity runs hot, go high-temperature. If space is the constraint or you need minute-by-minute data for a year or more, go external battery. If it's a standard-temperature cabinet, the rechargeable version keeps things simple.

Prevention: let the pack absorb first, then prove it

Monitoring tells you when the margin is shrinking. It doesn't remove water. That's the job of the material.

Our anti-condensation packs use reversible absorption and desorption. Inside a sealed cavity, the pack absorbs moisture first, before it can condense on a cold wall or a board. When conditions swing back, it can release moisture again. That's the "material" half of what we call material plus sensing: the pack holds the baseline, and the DEW sensor shows you, in real data, whether it's holding.

We validate this in a climate room that simulates real operating conditions, so a pack and a sensor plan can be matched to your actual duty cycle rather than a generic one.

What the material has delivered: 5G base stations

Our anti-condensation packs have been applied in 5G outdoor AAU/RRU units and integrated cabinets, where high power draw, dense integration and sharp day-night swings create internal condensation risk. In that benchmark case, the packs retained at least 80% of performance after hot-cold shock, with an expansion rate of 10% or less, and a 10-year effective service life. That life figure holds under the agreed conditions of use and operating conditions, with performance decay of no more than 20% over 10 years.

Ready to size packs for a specific enclosure? Start with our Anti-Condensation Fiber Pack.

A monitoring plan for one enclosure

Here's how we'd set up a single cabinet, step by step:

1. Find the coldest surface. Usually the lid, the door, or the wall facing open sky. Stick the surface probe there with the adhesive tape.
2. Put the air probe where the electronics are. That's the air you're protecting, and its dew point is what the surface has to beat.
3. Pick a logging interval that sees the swing. Night cooling can happen fast. Shorter intervals catch the minimum. The external-battery version is rated for 18 months at 1-minute sampling, so you don't have to trade resolution for battery.
4. Alert on margin, not RH. Set the trigger on dew point margin (at least 1 °C, per the uncertainty math above). Alarms can go out by SMS, voice call or email through our HaasLink platform.
5. Run before and after. Log a baseline cycle, add the anti-condensation pack, and compare. Same weather window if you can, or a climate room if you can't wait for the weather.
6. Review the cycles, not just the alarms. A margin that shrinks a little more each week, in similar weather, is your early cue to review pack sizing or placement, long before water shows.

Frequently asked questions

Why does my enclosure condense when the humidity sensor reads below 90%?

Because the sensor reads RH at the air's temperature, and condensation forms on surfaces that are colder than the air. In our worked example, the air near the boards read 88% RH while the door was 3.9 °C below the dew point and already wet.

How do you calculate dew point from temperature and relative humidity?

Use the Magnus formula. With the WMO constants, γ = ln(RH/100) + 17.62 × T / (243.12 + T), then dew point = 243.12 × γ / (17.62 − γ). At 30 °C and 70% RH, that gives 23.9 °C.

Does sealing an enclosure stop condensation?

It slows moisture in, but most enclosures still breathe through gaskets, glands and vents as they heat and cool. Our example shows a 100 L box drawing in about 12% of its air volume on a 55 °C to 15 °C night, which would bring in roughly 51 g of water vapor over a year if every night looked like that. Moisture that's already inside when you close the door stays inside, too.

Where should I mount a condensation sensor?

Put the surface probe on the coldest inside surface (often the lid or door) and let the air probe sample the space around the electronics. Then alert on the gap between surface temperature and dew point.

Do anti-condensation packs replace a heater or vent?

They work on a different part of the problem. The pack absorbs moisture inside the sealed cavity first, so less of it is available to condense. Tell us your enclosure, duty cycle and site, and we'll suggest a pack and sensor plan that fits it.

Sources: National Physical Laboratory, The Beginner's Guide to Humidity Measurement (Good Practice Guide No. 124); World Meteorological Organization, Calibration of humidity instruments, Part 1 (citing WMO-No. 8, Annex 4.B). Worked examples are our own calculations.