In today’s store-houses the ripening of fruit is controlled by managing the ethylene
concentration in the ambient atmosphere. Precise and continuous ethylene monitoring is very
advantageous since low ethylene concentrations are produced by the fruit itself and are
indicative of its ripeness, and on other occasions, ethylene is externally added when ripeness or
degreening of the product must be promoted.
In this Thesis an optical multi-channel non-dispersive mid-infrared (NDIR)
spectrometer for ethylene measurement is built and characterized for measuring fruit status in
apple’s store-houses. The corresponding optical components and signal processing electronics
have been developed, tested and integrated in a compact measurement system. In addition to the
ethylene channel, the spectrometer also features ammonia and ethanol channels to consider their
cross-sensitivities, and a reference channel to improve long-term system stability. Moreover,
these channels are useful for monitoring a potential malfunction of the cooling system and
possible fouling of the fruit. Therefore, the complete system can be considered as a
multipurpose instrument for controlled atmosphere management.
In the trend towards miniaturization, a novel detector module containing multiple IR
sensor channels is built and characterized. In its final form it contains thermopiles as IR
detectors, narrow band filters to select the absorption bands of the target gas, and a four Fresnel
lenses fabricated on the same silicon substrate in a combined multi-lens array to increase system
sensitivity. In order to reduce the number of photolithographic steps, a new design based on
sharing up to sixteen quantization steps by the four lenses is done.
However, it has been found that integration of filters too close to the IR detector may
lead to degraded performance due to thermal coupling. To avoid such detrimental effects two
possibilities have been considered: set the IR detector in vacuum conditions and increase the
solder joint height between the filter and the thermopile.
Specific signal conditioning electronics were designed. They contain an analogue preamplification
stage based on an instrumentation amplifier, and a digital lock-in amplifier
implemented on a commercial microcontroller as a signal recovery system.
The complete spectrometer has been successfully tested in laboratory and field
conditions. Assuming that ethylene and ammonia may not appear simultaneously, it has been
found an ethylene detection limit of 30ppm, which is low enough to detect when fruit is ripe and
prevent it to decline to senescence. The detection limit corresponding to ammonia is 160ppm,
which can be used to set an alarm if a leakage from the cooling system occurs.