| Many articles have been written about the | | | | using an infrared sensor will be able to keep the |
| negative effects of carbon dioxide. Sick Building | | | | concentration of gas constant at all times.Rate of |
| Syndrome, loss of concentration due to high levels | | | | consumption varies with crop, light intensity, |
| of carbon dioxide, asphyxiation in breweries or | | | | temperature, stage of crop development and |
| wine cellars, all these things spring to mind when | | | | nutrient level. An average consumption level is |
| we hear the magic phrase carbon dioxide. Yet, | | | | estimated to be between 0.12 - 0.24 kg/hr/100 |
| perhaps today when Venus passes across the | | | | m2 of greenhouse floor area. The higher rate |
| face of the Sun, we should remember that our | | | | reflects the typical usage for sunny days and a |
| original atmosphere consisted of nitrogen and | | | | fully-grown crop. This equates to roughly 150 litres |
| carbon dioxide. Free oxygen is something that is | | | | of carbon dioxide per hour.There are many |
| not really chemically possible. Yet we have it as a | | | | processes that naturally and unavoidably produce |
| result of plant life busily photosynthesising and | | | | carbon dioxide: Fermentation and combustion are |
| converting carbon dioxide into oxygen during | | | | two classic examples. In temperate zones it is |
| daylight hours. This is the original use of solar | | | | necessary to heat a greenhouse (glasshouse is |
| energy!Plants require carbon dioxide to grow and | | | | just another word for the same thing), and this |
| why not help them by increasing the level of | | | | heating will almost always involve the burning of |
| carbon dioxide? Normally, this is something that is | | | | fossil fuels, producing carbon dioxide. This leads to |
| undesirable, since carbon dioxide is the original | | | | the natural urge to re-circulate the exhaust gas |
| greenhouse gas, as our neighbouring planet Venus | | | | from the heating system into the greenhouse and |
| can testify. But in a controlled, genuine greenhouse | | | | so achieve a double advantage for the plants. This |
| environment, there is no real reason why the | | | | will require careful monitoring of the flue gas to |
| level of carbon dioxide should not be enhanced in | | | | ensure that there are at the most only traces of |
| some way.Indeed, tests have shown that | | | | carbon monoxide being passed into the |
| increasing the level of carbon dioxide in a | | | | greenhouse. This is not only bad for the plants but |
| greenhouse to 550 ppm will accelerate plant | | | | also potentially lethal to the people working there! |
| growth by 30 - 40 %. The natural level of carbon | | | | Such technology is available with gas monitors |
| dioxide in the atmosphere is around 450 ppm, | | | | that will measure the carbon monoxide |
| having increased from about 250 ppm in the last | | | | concentration continuously and have analogue |
| ice-age, so this slight increase may not appear | | | | outputs that can be used to regulate the burners |
| significant at first sight. The point of the matter is | | | | or operate a trip to switch the unit off should |
| that the level of carbon dioxide in the average | | | | problems occur. The combination of breweries |
| greenhouse with the ventilation system closed will | | | | with greenhouse systems is also serious business |
| drop sharply due to uptake by the plants and will | | | | in some areas. Generally, these methods are to |
| lie around 150 - 200 ppm if nothing is done about | | | | be approved and should really be worthy of |
| it. In summer the ventilation system will be open | | | | government support. Not only are they producing |
| and the fresh air circulation will augment the level | | | | crops, they are removing a pollutant that would |
| to a useful degree. But what about those long, | | | | otherwise be vented into the |
| cold, dark northern winters? Most commercial | | | | atmosphere.Monitoring of the added carbon |
| greenhouses will have lighting and heating systems | | | | dioxide is essential, however, since high |
| to encourage plant growth, but you still cannot | | | | concnetrations of carbon dioxide can lead to |
| open the ventilation and allow the cold outside air | | | | dizziness or even unconciousness of the personnel. |
| into your heated greenhouse without losing all the | | | | Some plants will require higher levels of nutrients |
| early crops. The only real solution is to augment | | | | to compensate for some of the changes that |
| the natural level of carbon dioxide in some way. | | | | occur. Particularly tomatoes and violets are |
| Where it is used, the general rule of thumb is to | | | | sensitive to increased levels of carbon dioxide, |
| augment by about 1000 ppm when the sun is | | | | hence the need for constant monitoring of the |
| shining (or all the lights are on!) and keep the level | | | | ambient concentration.Simon Fowler studied |
| around 400 ppm during times of darkness. This | | | | Mechanical Enineering at City University in London, |
| will require monitoring, since there are so many | | | | UK and now works in Vienna, Austria for madur |
| variable factors involved and a simple control unit | | | | electronics, a manufacturer of infrared sensors. |