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A Day (Or Rather, A Night) In The Life Of A Small Greenhouse.


I’ve been after a thermohygrograph for months – no, make that years (you know, one of those clockwork-driven paper charts with pens on levers tracing the temperature and humidity) – so that I can see what’s REALLY happening in my greenhouse. Unfortunately, new ones were unaffordable (about £500) and used ones non-existent.

Then, in rummaging the web in futile hope once again, I happened upon the Lascar EL-USB-2, and bought one right away. For about £50 (+ VAT & shipping) I got a waterproof data logger like a tubby memory stick that I just plug into my PC when I want to see what’s been going on. So, for an affordable sum I’ve got a “thermohygrograph” that is just so easy to use, takes no space, hasn’t got messy paper charts and pens, and even works out and plots the dew point for me as well!

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Below is my very first trace for a day (or rather, night, as it starts and ends in the afternoon). From left to right, this is what it showed me:

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It starts in the afternoon with the temperature high and the air dry (it had been a sunny day), but the sun has just gone behind the trees so there is a rapid fall of temperature and rise of humidity as I mess around watering and misting. About three o’clock I finish and go indoors, while in the greenhouse the temperature keeps dropping and the humidity in consequence rising. The flat green dewpoint line assures me nothing else is going on (this shows that the AMOUNT of water in the air isn’t changing).

It was a cold day, and by about 7pm the temperature had dropped to the point that heating was needed. Sure enough, the red line shows my primary heater switching on for about ten minutes then off for about ten minutes right through the night. Very occasionally it is joined by my back-up heater, showing a bigger spike when it got particularly cold.

I am delighted that it shows my temperature control to be really stable, and that the heating and cooling rates are very well balanced. With a 600 watt heater on half the time for 11 hours, that means I’ve used about 3 units of electricity, costing about 50p, on this particularly cold night – not bad, I reckon.

The blue line shows the humidity going up and down in the reverse of the temperature, as you would expect. It also shows a slight fall as moisture is lost from the air by condensation on the greenhouse shell. This is also shown as a very slight fall in the green dewpoint line. The wiggles in this line are actually fibs - they arise because the humidity sensor is less responsive than the temperature sensor. When it’s working out the dewpoint it’s therefore using a humidity figure that hasn’t yet stabilised (is a bit wrong) and so the temperature shows through in the calculation. The true dewpoint would actually be very stable in such a closed environment. Come 7am (6am GMT), it’s time for my “dawn boost“ to kick in – a happy hour of lights on and 1200watts of heating (about 20p of electricity). The (air) temperature shoots up to the low twenties while the humidity correspondingly drops like a stone, although this is offset because I have a wet-floor greenhouse and the heater is directed at the floor to pick up some moisture. I am delighted to see that this works well, as shown by the fairly dramatic rise in dewpoint.

By 8am the “boost” is over, and as the plants warm up, they cool the air and absorb some moisture, but on balance the humidity rises, all as expected. By 9am, conditions have become stable, and so it continues for the best part of an hour until the sun gets above the trees and switches on my “sun switch” which gets my humidification going. The dewpoint and temperature both zoom up, and the humidity falls (but far less than it would without the humidifiers) to a little under 50%. I am pleased to see that once the roof vent opens, my humidification is good enough to make up for ventilation losses of moisture and keep the dewpoint fairly stable. From midday onward (not shown – I was too keen to see my first results to let it run a full 24 hours!) things are fairly stable until the start of the next cycle.

I’ve learnt one important thing straight away – my dawn boost is a bit stronger than it need be, and I will be better running it at 600watts for two hours rather than 1200watts for one hour – that will also give the sun more of a chance to get up before the boost ends.

All in all, I can’t recommend this thing too highly – it was delivered instantly, was easy peasy to set up, and the instructions are in decent English with clear diagrams. It is also very well made and the program for looking at the data starts automatically and is a dream to use. Find out more at www.audon.co.uk/elusb2.html

RML, 21 Oct 2010 - May be reproduced without request.

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