Checking the ventilation before a session

Purpose

To establish, before anything is opened, that the room takes air in at the clean end and puts it out where the vapour is made — and to say what the session becomes if it does not.

When to use it

Before any session that will make a dust, a vapour or a mist: weighing a powder, opening an acid, working a fixer, warming a gelatin bath, or mixing anything from solids. Dry work — cutting paper, loading a tank, filing negatives — does not need it.

Run the whole of it once when the room is first commissioned, after any change to the room, and after anything that moves the air: a new fan, a door that now stays shut, a window painted closed, a season that changes which way the draught runs.

Before you start

You should already have drawn your own room with the two areas, the air path, the waste station and the eyewash marked on it, as the layout lesson asks, and read why a fan in the wrong place makes matters worse rather than better.

In your hand: a strip of paper torn from the notebook, a tape measure the first time, and the fan’s rating in m³/h or l/s if you have one — 1 l/s is 3.6 m³/h.

!The arithmetic, done once and written on the wall

Room volume in cubic metres is height by length by width. Multiply it by five and by ten, and you have the band the published figures converge on: Kodak’s darkroom design sheet gives five changes an hour for a chemical handling area, its environmental guidance gives ten room volumes an hour for open-tray processing, HSE’s control sheet gives more than five with a through draught, and CIBSE, as ILFORD quotes it, gives six to eight. None of the four is a measurement of a kitchen with three trays in it. What they agree on is the order of magnitude: a room this size needs its air replaced several times an hour, and a domestic extract fan is in that range.

Procedure

1. The first time only, measure the room and write the two numbers — volume times five, volume times ten — somewhere they will still be legible in a year. Compare them with the rating of whatever fan you have.
2. Open the extract or the window at the wet end, where the vapour is made.
3. Open the make-up air path at the far end of the room: a door, a trickle vent, a gap under the door. A fan with nothing to draw is a fan straining against the room.
4. Hold the torn strip at the doorway. It should lean towards the wet end. Air runs in at the clean end and out where the vapour is made, never the reverse.
5. Hold the strip again over the tray or the bench where you will actually stand. It should lean away from your face and towards the extract, because the point of extraction is to capture a vapour before it mixes with the air you are breathing rather than after.
6. Check that the outlet is higher than the inlet, so that the room runs slightly below the pressure of the rest of the house and vapour does not travel into it.
7. Put the covers on. Lids on the trays and caps on the bottles between pours do more than a fan does, and they come before extraction in every control hierarchy the course cites.
8. Rule out the three arrangements that do not work, before you rely on one of them. A sealed cupboard has no supply and no path out and is excluded at Level A. An internal room whose only path is a shower extractor draws its make-up air from the rest of the house, which is the direction you least want vapour to travel — if it is the only room you have, work with the door open, run the fan throughout, keep the trays covered and keep the session short. A room with a fuel-burning appliance in it is a question for whoever is qualified to look at the appliance, because an extract fan can pull combustion products back down a flue.
9. Decide the session’s ceiling. If steps 4 and 5 cannot both be made true, the session is dry work only: nothing is opened that makes a vapour, a mist or a dust.
10. Leave the extract running for the whole session and for a while after the last container is capped, and note in the log that you did.

How you know it worked

• The strip leans towards the wet end at the door and away from your face at the bench.
• The make-up path is open and stays open — a door that will swing shut has been wedged.
• Every tray that is not in use has a cover on it and every bottle a cap.
• The two arithmetic figures are written where you can find them, next to the fan’s own rating.
• You can say, in a sentence, what the session becomes if the fan stops.

If a step fails

Step 3, there is no make-up path. The fan will not move the air it is rated for, whatever the label says. Open a door, drill a vent, or accept the lower rate and reduce what you open — a smaller batch and a covered tray are steps 3 and 4 of the hierarchy and they cost nothing.

Step 4, the strip leans away from the wet end. The extract is blowing rather than drawing, or the make-up air is on the wrong side. Reverse or reposition the fan. Until it is right, this is a dry work session.

Step 5, the strip leans towards your face at the bench. The capture point is behind you. Move the tray towards the extract, or stand on the other side of it, and test again. A room can pass step 4 and fail step 5, and step 5 is the one that describes what you breathe.

Step 7, the trays have no covers. Anything rigid, non-reactive and larger than the tray works. This is the cheapest control on the page and the one most often skipped because it is not equipment.

Step 8, the only room available is internal. Work the mitigations in step 8 and shorten the session. Do not treat a shower extractor as a substitute for the arrangement in steps 2 to 6; treat it as the reason to open fewer things.

Step 10, the fan stops mid-session. Cap everything, cover the trays, leave the room and let it air. Come back to a room that has changed its air rather than working on in one that has not.

What to record

Once, when the room is commissioned: the volume, the two rates, the fan’s rating, and where the inlet and outlet are. Then, per session, only what was not true — a wedged door that came unwedged, a strip that leaned the wrong way, a fan that stopped — and what the session became as a result. Those lines are what tell you, a year later, whether the room works or whether you have simply been lucky with the weather.