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Weatherproofing External Alarm Sensors

Would you recommend conformal coating on sensor boards that are going to be used in moist external environments? What are the key issues with DIY conformal coatings?

A: We’d start by purchasing sensors with IP67 ratings if we were going into install sensors fully exposed – there aren’t many of them, so this will require a search and a preparedness to pay for the best quality. You want your sensors to have sunshades, as well as appropriate seals.

Whether or not you undertake a conformal coating of sensor boards is up to you – you’ll need to check with the manufacturer to ensure this does not breach warranty – the boards may already be coated and not need an additional layer.

You’ll need to mask the connectors, switches and sockets – failing to do so may impact on circuits. You can mask switches with a little gob of silicone that you pop out again after the conformal coat dries. Another way to handle this is to make sure all the connections are plugged in while you are spraying. You can check the integrity of the conformal coating using a blacklight (UV light).

Polyurethane sprays like Humiseal are ideal. When applying the spray be careful not to apply heavy coats that might sneak past masking and onto terminal screws, programming plug-in contacts, battery connections or tamper switches and between light coats let the board sit for 5 minutes.

Something else to think about with intrusion detectors is spraying anything onto boards which have unmasked PIR sensors on them. Conformal coatings are electrically insulating, so a spraying a film over a piezoelectric sensor is going to blind the device. You’ll need to carefully mask the sensor and then walk test to ensure normal operation.

In SEN’s opinion, the best way to protect external sensors is to keep them away from direct sunlight, either by protecting them with integrated sunshades, locating them under eaves or whatever cover you can find, or by customising a simple protective cover that can be positioned above them.

If possible, ensure the lens of an externally mounted sensor is never exposed to direct sunlight.


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