Douglas Krantz - Technical Writer - Describing How It Works
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Should I Replace the Smoke Detector if it is 80% Dirty?

A smoke detector that's 80 percent dirty is already almost too dirty. You can replace it now, or wait just a little while for the detector to become 100 percent dirty. You then you have to rush out and replace a smoke detector that is now too dirty.

Should I Replace the Smoke Detector if it is 80% Dirty?


Should I Replace the Smoke Detector if it is 80% Dirty?


Greetings Douglas,

Question: Should I replace the smoke detector if it is 80% Dirty?

Thank you, HD



The EST system gives you a warning: once a smoke detector has gotten 80% dirty, sometime soon the detector may become 100% dirty.

Smoke Detection Chamber

Inside the smoke detector is a black chamber. For the detector to go into alarm, particles in the air reflect "enough light" from a Light Emitting Diode (LED) to a photo-pickup for the detector to consider that to be dangerous smoke. The detector then goes into alarm.

The trouble with this kind of sensing is that some of the particles sometimes cling to the wall of the chamber that they will add to the particles that are naturally in the air. That makes the smoke detector more sensitive. If enough particles cling to the wall of the detector that "enough light" is always reflected into the photo-pickup for the detector to consider that dangerous smoke, the detector goes into false alarm.

This is a "Dirty" detector.

Smart (Intelligent) Smoke Detectors

The EST detectors are intelligent. Inside the detector is a computer that senses how much light is reflected over a 24 hour period, and averages that amount. It then adjusts the smoke detector to be less sensitive, so only dangerous smoke suddenly entering the chamber will be considered dangerous smoke.

The detector has limits to how much it can adjust the sensitivity of the detector. When the detector is saying 0% Dirty, the detector is not adjusting the sensitivity at all. When the detector is saying 50% Dirty, the detector has reduced the sensitivity 50% of the way to the limit. When the detector is saying 80% Dirty, the detector has reduced the sensitivity 80% of way to the limit, and is now warning you that soon, the detector will be past the adjustment range.

When the detector reaches 100% Dirty, the detector will take itself off-line so it will not go into false alarm. This is a trouble on the panel, the detector has become useless, and has to be replaced right away.

Free PDF Excerpt from the book

"Make It Work - Addressable Signaling Line Circuits"

Excerpt is from the headings labeled "Addressing", Power Supply", and "For Troubleshooting, What is an SLC?"

Save the PDF to your files

Don't Try Blowing the Dirt Out Using Compressed Air

  1. Blowing compressed air into the smoke detector voids the warranty; EST won't stand behind the detector.
  2. That 24-hour averaging period takes 24 hours to average. The detector will show dirty for 24 hours, even if it could be cleaned with compressed air.
  3. There are two air-stream barriers inside the smoke detector, an insect screen and light baffles. These barriers prevent a compressed stream of air from dislodging particles clinging to the side of the chamber. Compressed air is useless for cleaning.

Replace the Detector at 80% Dirty

The detector is still functioning, but you can replace the detector now, or you can replace the detector once it reaches 100% Dirty.

If you replace it now, you get rid of that "Monitor" yellow light, which fire marshals and other AHJs consider to be "Trouble". You also don't have to make another service call later to replace the smoke detector when it does go into trouble.

Douglas Krantz
Post this by your fire alarm panel -- It shows the in-house fire alarm system and how it calls the fire department.


facpdoug@douglaskrantz.com
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