Put the humidity-sensing element outside the sealed electronics compartment when you need ambient humidity measurements. Use a protected remote probe or a purpose-designed through-wall probe, with a sealed connection into the enclosure. A pressure-equalization vent can reduce pressure differences, but it does not turn the box into a representative sampling chamber. Separate three requirements on the drawing: protection of the electronics, exposure of the sensor, and pressure equalization. Each needs its own component specification and acceptance check.
What the humidity sensor actually measures
A humidity sensor measures conditions at its sensing element, not automatically the conditions outside its enclosure. Inside a sealed box, it measures the internal microclimate. That reading can be useful for monitoring condensation risk, but it is not an ambient measurement.
Internal conditions depend on trapped moisture, electronics heating, enclosure temperature and moisture absorbed by internal materials. Even after water vapor crosses a membrane, the internal relative humidity can differ from the outdoor value.
Relative humidity depends strongly on temperature. For example, air at 77 °F [25 °C] and 80% RH has approximately the same water-vapor pressure as air at 86 °F [30 °C] and 60% RH. Those values assume no moisture is added or removed. Electronics warming the sensing element can therefore produce a substantially lower reading without making the surrounding environment any drier.
Keep the humidity element and its temperature measurement exposed to the same representative air. Avoid warming either through a metal probe stem, an adjacent power supply or direct sunlight. Software temperature compensation cannot reliably correct an unknown temperature difference between the sensing element and the air being measured.
What a membrane vent passes
A suitable hydrophobic membrane vent allows gas exchange while resisting liquid-water entry under its specified operating conditions. Water vapor can also pass through an appropriate membrane. The membrane does not selectively admit dry air while keeping humidity outside.
Pressure equalization and humidity sampling are different jobs:
- Pressure equalization: gas moves through the vent as internal and external pressures differ.
- Humidity equilibration: water vapor moves according to the vapor-pressure difference, with additional effects from gas movement.
- Representative sampling: the sensing element encounters air that reflects the intended measurement location within the required response time.
A vent can perform the first job without adequately performing the third. When pressures are nearly equal, there may be little bulk airflow through the enclosure. A small opening, a large internal volume and moisture-absorbing materials can make humidity changes slow or uneven.
Do not assign an enclosure response time from the bare sensor's data sheet. That value may assume airflow and exposure unlike the finished assembly. Membrane area, probe-cap construction, contamination, internal volume and temperature all affect the result.
Vent performance also depends on its condition. Water covering the membrane can restrict gas exchange. Dirt, oils and cleaning residues can change performance or damage the membrane. Check compatibility with the actual wash chemicals, concentration and temperature.
A pressure vent is not a cooling device. Calculate internal temperature using the enclosure heat calculation guide, rather than assuming that pressure equalization removes meaningful electronics heat.
What ingress testing does not establish
An IP67 designation addresses dust ingress and temporary immersion under specified test conditions. It does not establish ambient-humidity accuracy, measurement response time or reliable readings while the sensing surface is wet. IEC 60529 ingress protection and sensor metrology answer different questions.
Our enclosure ratings are manufacturer ratings for the specified design; the enclosures are not UL listed.
An accessory's rating is not automatic evidence that the modified enclosure retains that protection. The vent, probe, gland, sealing surface and installation must be suitable together. Confirm panel thickness, mounting orientation, seals and tightening requirements against the accessory manufacturer's instructions.
Treat the probe's electronics protection separately from its measurement capability. A probe may tolerate a wet environment yet return unreliable values until its sensing element dries. A water film changes the immediate sensing environment and can slow recovery after washdown.
Likewise, a vapor-permeable vent does not prevent condensation. Moisture entering during warm conditions can condense when an internal surface falls below the dew point. Use the condensation guide for dew point, heaters and moisture management. Keep any heater's thermal influence away from the ambient sensing location.
How to separate the probe from the electronics
The simplest arrangement is usually a remote humidity probe connected through a sealed cable entry. The probe is selected for the ambient exposure, while the enclosure protects the controller, power supply and signal connections.
Locate the probe where air represents the process or environment being measured. Avoid enclosure exhaust, hot walls, direct sunlight and stagnant recesses. Do not place it directly in a spray path unless the complete probe is intended for that exposure. A protective cap or shield must also allow the response required by the application.
A through-wall probe is another option. Its sensing end projects into ambient air, while a flange or purpose-designed fitting seals the enclosure boundary. Specify the probe's installation method rather than improvising a seal around a circuit board or bare sensor package. Confirm that heat conducted through the fitting will not create an unacceptable measurement bias.
Keep these boundaries distinct:
- The probe cap protects the sensing element from the environmental exposure it is designed to withstand.
- The probe flange or cable gland seals the enclosure penetration.
- Internal terminals and signal electronics remain within the protected compartment.
- A separate pressure vent serves the electronics compartment when pressure equalization is required.
For a cable entry, select the gland by the actual cable outside diameter and jacket construction. The gland's cable range, thread, sealing washer and permitted panel thickness must all match. A gland that seals its mounting hole can still leak around an unsuitable cable.
Route the cable so water does not run directly toward the entry. Provide suitable strain relief and observe the cable's minimum bend radius. Follow the cutout and cable-entry guide for the penetration details rather than choosing a hole from the thread's nominal size alone.
A removable entry plate can simplify probe replacement or later cable changes. The gland-plate selection article explains when that extra sealed joint is worthwhile.
How to verify the finished measurement
Test the complete probe, cap, mounting arrangement and enclosure together. A bare sensor check will not reveal thermal coupling, a restrictive shield or moisture retained around the installed probe.
Begin with a suitable reference instrument placed close to the sensing location. Keep both instruments exposed to comparable air without allowing one to obstruct the other. Allow temperatures and readings to stabilize before comparing them. Include the reference instrument's uncertainty when judging acceptance.
Then check the operating conditions that can produce misleading readings:
- Electronics energized: compare performance with the normal internal heat load present, not only with the controller unpowered.
- Changing humidity: record the time required to meet the application's response criterion after an environmental change.
- Changing temperature: look for measurement errors during warm-up, cool-down and transitions between operating states.
- Expected wetting: use only exposure and verification procedures permitted by the component manufacturers, including the specified recovery period.
- Installed shielding: verify that sun shields, splash shields and nearby structures do not create a different microclimate.
Define the response criterion before testing. For example, the process may require the reading to enter a stated tolerance band within a specified time. Use requirements appropriate to the process and the complete probe specification; do not invent a universal waiting period.
If the application requires readings during active wetting, obtain documented performance for that condition. Otherwise, define when readings must be flagged as unreliable and when normal measurement can resume. Do not interpret a saturated or slowly recovering sensor as proof that the broader ambient air remains at saturation.
What to put on the enclosure drawing
Specify the measurement arrangement before placing holes. A generic request for a waterproof box with a humidity vent leaves the measurement boundary undefined.
| Drawing item | What to specify |
|---|---|
| Measurement target | Ambient humidity, internal humidity, or two separate measurements |
| Environmental exposure | Temperature and RH range, condensation, spray, immersion, dust and cleaning chemicals |
| Probe assembly | Exact probe, protective cap, mounting method and documented operating limitations |
| Probe location | Sensing position, orientation, shielding and separation from heat sources |
| Cable entry | Cable outside diameter, gland model, thread, seal and strain-relief requirements |
| Pressure vent | Exact model, purpose, placement, panel-thickness limits and environmental compatibility |
| Cutouts | Face, datum, center coordinates, dimensions, tolerances and required sealing surfaces |
| Enclosure protection | Required rating for the completed configuration and any project listing requirement |
| Acceptance criteria | Accuracy requirement, reference uncertainty, response time and wet-recovery behavior |
| Maintenance access | Probe replacement route, cap inspection and access to internal connections |
Choose enclosure material from the actual corrosion exposure, not the humidity percentage alone. Chlorides, cleaning chemistry and deposits matter alongside moisture. Keep the probe and vent accessible for inspection without requiring unnecessary disturbance of their seals.
Use the cutout designer to position the approved probe opening, cable entry and vent. Include the component drawings so clearance checks cover locknuts, connectors and internal cable bends, not just visible hole edges.
We build custom enclosures with customer-defined material, width, height, depth and cutouts, from one piece. Send the measurement arrangement with the enclosure requirements so the approved drawing defines both the protected compartment and the exposed sensing location.
Questions
Can a humidity sensor inside a sealed enclosure measure ambient humidity?
An internal sensor measures the enclosure's own air, which may differ in temperature and moisture content from ambient air. A membrane vent can allow water-vapor exchange, but it does not guarantee representative readings or a suitable response time. An externally exposed, properly protected probe is usually the better arrangement.
Does a pressure-equalization vent keep humidity out?
No. A suitable hydrophobic vent resists liquid-water entry under specified conditions, but water vapor can pass through it. Internal surfaces can still fall below the dew point and collect condensation. Pressure equalization, condensation control and ambient-humidity measurement need separate design checks.
Will an IP67 humidity probe read accurately during washdown?
Not necessarily. The rating addresses specified ingress exposures, not humidity-measurement accuracy during wetting. Water on the sensing surface or protective cap can distort readings and delay recovery. Check the complete probe's documented limits and define how measurements are treated during washdown and drying.
How should the probe connection pass through the enclosure wall?
Use the probe manufacturer's specified through-wall fitting or a suitable cable gland for a remote probe. Match the cable diameter, panel thickness, mounting thread and seals. Include the exact components and installation requirements on the enclosure drawing, then verify the completed configuration against the required protection.
