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After the flood: dehumidifiers and drainage

Last week, I wrote about the flood that affected our basement stores following a sudden and violent thunderstorm. As I said then, the initial response went very smoothly, and we were able to clear all of the standing water out of the basement within a couple of hours. We didn’t stop there, however!

A flood can continue to be a problem for museum collections even after all of the water has been mopped up, and even if none of the objects got soaked directly. Having water all over the floor will raise the relative humidity (RH) of a room significantly, and if the water is not removed quickly and thoroughly, the humidity can stay high for days. An RH over 65-70% is enough to grow mould in the right conditions, especially for objects that have been affected by mould in the past.

The graph below shows data from our main basement store before and after the flood (click on the picture to see a larger version):


The point marked A (about 3.30am) is when the flood waters entered our basement; you can see the RH jump sharply from 55% to 65%. At this point, the water had only penetrated a couple of metres into the storeroom (despite covering the whole floor in other rooms), so you can see that even a little bit of water can make a big difference to the environmental conditions.

Point B is at 8am on the same morning, when the museum team started clearing water out of the basement. The RH falls over the following few hours, until it is back around 55%, which is where we would like it to be. Some of the fall in RH is because we pumped out all the standing water, but some of it is due to this machine:


This is a portable dehumidifier, and is one of the most useful bits of equipment we have in our stores. If we spot that the humidity is creeping up in one of our storerooms, we can move the dehumidifier in there and run it until the humidity is back within the safe range. We left it running for a few days in the main store, and it helped dry out the room completely.

In areas that had been more severely flooded, a more aggressive approach was needed. Even though we’d mopped up all the water from the floors, there was water trapped underneath cupboards and shelving, as well as in the atmosphere. We hired industrial dehumidifiers and ran them continuously to dry out these areas as quickly as possible.

The dehumidifier below (in the Library’s Map Room store) has an external water tank of 20 litres (about 4.5 gallons). While the room was drying out, the tank filled up completely every couple of days – that’s a lot of water that we were removing from the atmosphere!

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To monitor the drying process, I moved one of our Eltek transmitters into the Map Room:


This transmitter collected information about the temperature and RH every half hour and sent it to my computer in the museum office. The data was displayed on a graph like this one (again, you can click on the image to see a bigger version):

Map Room 2

Point A is when we moved the dehumidifier into the Map Room and started monitoring the environment. The RH was 70% at that point – definitely not good news for the collections! After a week, the humidity had settled down and was in danger of becoming too low (point B), so we switched the dehumidifier off and waited to see what would happen. The only way to tell if an environment is stable is to turn everything off and see what happens without the dehumidifiers, heaters or fans. After a few days, the RH had crept back up to 60% again (point C), so we turned the dehumidifier on again to remove the last few bits of moisture from the room. Our basement stores are now all stable once more, and we’ve avoided any secondary damage to objects from the flood.

Mopping up was not confined to the inside, however! Outside the Institute is a try pot (a metal cauldron used in the whaling or sealing industries). The thunderstorm dislodged so many leaves and twigs from the tree above that the drainage hole drilled in the bottom became blocked and the pot filled with water. The result was a rather unpleasant stagnant, leafy soup.

The picture below shows Grahame with a “wet vac” – it’s exactly like the vacuum cleaners that we all have at home, but is designed to suck up water as well as dirt.


Grahame used the wet vac to remove all the water from the try pot, and then cleared out all the leaves so it could drain freely again.

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The try pot is now lovely and clean once more, and everything is back to normal after the flooding. I think we can say that we well and truly weathered that storm, and are (hopefully) well prepared for the next one…


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