The Viscosity of Water Increases as Temperature Drops

This week we had a drop in temperatures from a range of 70 high/50 low oF to a range of 60/40 oF. I didn’t think much of it other than worrying about how my new cucumber, pepper, winter squash, and okra seedlings would fare in the cold weather. Much to my dismay, yesterday morning when I went out to see how my plants fared, the water level was really low. I had no idea what had happened, but I quickly filled my rain barrel and added the de-chlorinating agent to make it safer for the fish, and then drained all 50 gallons into the Aquaponics sump.   whew!

That evening when I got home, I checked it again – the water level was even lower than it was that morning!   Again I frantically added 50 gallons, and while doing so, found that the cause was not a terrible leak, but the new strawberry tubes!  They were overflowing; the drain tube was not draining the tubes very fast and there was an inch of freestanding water over the standpipe.  What in the world?   I quickly lowered the standpipe and it drained again somewhat.

This morning I checked it again – the water had not overflowed, but was fairly high and near the overflowing limit, so I took the standpipe out completely, which should leave the strawberry tubes only about half full of water.   This should still be fine, as I think all of my plants still have roots in the water, and the shale tends to wick up water anyway.

Thinking about it some more, I went online today and googled the viscosity of water and its changes at various temperatures – as this was the only explanation of why the water was not draining well now when it did just fine previously.   Viscosity is pretty much the measurement of internal resistance in a liquid.  The higher the viscosity, the slower it flows.  For example, honey has a higher viscosity than water, and flows much more slowly.   For water at various temperatures, I found the following data:

Temperature
t –
(oF)
Dynamic Viscosity
µ –
(lb s/ft2) x 10-5
Kinematic Viscosity
ν –
(ft2/s) x 10-5
32 3.732 1.924
40 3.228 1.664
50 2.730 1.407
60 2.344 1.210
70 2.034 1.052
80 1.791 0.926
90 1.580 0.823
100 1.423 0.738
120 1.164 0.607
140 0.974 0.511
160 0.832 0.439
180 0.721 0.383
200 0.634 0.339
212 0.589 0.317

The above is fairly apparent – water gets more viscous as temperature drops.   Hence it makes sense that the strawberry tubes require a greater head to drain out the 5/8 inch tube to the sump, and resulted in overflowing.  The change in Dynamic Viscosity from 50 oF to 40 oF is fairly significant: 2.73 to 3.22.

The lesson learned here is that I’ll need to either keep the standpipe off all the time, or at least remove/lower it when temperatures get low. When it gets warmer, I should be able to have a fairly high standpipe with good flow, which is better for new seedlings that don’t have roots long enough to reach the lower half of the strawberry tube.

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