Humidity distributions measured in the greenhouse with glass fibres

Published on
February 16, 2016

Saving energy in greenhouses often means a wetter climate, causing climate inequalities and giving additional risks of fungal diseases. Therefore Wageningen UR Greenhouse Horticulture and TNO investigated in the past year in a TO2 cooperation which potential distributed accurate measurement of humidity and temperature in conjunction with a custom control from the climate can have on energy savings.

The focus was on developing a dense monitoring network of fibre optic sensors and evaluating them under semi-field conditions in a tomato greenhouse at Wageningen UR in Bleiswijk.

The fibre optic sensors are based on the Fibre Bragg Grating principle, in which a small grid of parallel carvings are made into the optical fibre, that selectively reflect a particular wavelength of light. A coating to the glass fibres can absorb the moisture from the air, so that the glass fibre will expand. The wavelength shift that arises is measurable and can be converted into relative humidity. Temperature can be measured directly even without a coating. The glass fibre may contain several gratings, so that in many places in the greenhouse a measurement can be carried out with only one central light source. A monitoring network that can contain hundreds of data points for humidity and temperature.

The evaluation indicated that the fibre optic sensors perform comparable to the current electronic sensors. But, a fibre optic monitoring network with a high density will be much cheaper per sensing point. For now, installation of fragile fibres in a glass greenhouse still is problematic, and is also waiting for the development of simpler and cheaper light sources.