# Winter Carbon Dioxide Measurement in Honeybee Hives
###### Abstract
Carbon dioxide has long been known as a narcotic for the honeybee and is used to immobilize them during scientific manipulation or transfer, while higher levels of exposure cause permanent damage or result in death [35; 36; 37]. While there have been many laboratory studies on gas levels, such as oxygen, humidity, and carbon dioxide, the availability of small, low-cost, and gas-specific sensors yielding true parts per million (ppm) has previously limited the in-hive measurements. There are a range of small and inexpensive metal oxide (MOx) sensors that respond to yield an estimate for CO\({}_{2}\), which are derived from hydrogen measurements and termed eCO\({}_{2}\) or CO\({}_{2}\)eq. One of the problems with these sensors is that the calibration of a non-aged sensor will change significantly during the first hours of operation, and it can take several hours to reach a stable value in stable environmental conditions. In practice, these do not satisfy the requirements for calibrated CO\({}_{2}\) measurements. Recent advances in non-dispersive infrared (NDIR) sensor technology mean that such devices for carbon dioxide are now available in small and relatively affordable commercial implementations, such as the Telaire T6713 (Amphenol Thermometrics, Inc., St. Marys, PA, USA) [38], the SCD41 from Sensirion (AG, Stafa, Switzerland) [39], and the Figaro (Rolling Meadows, IL, USA) CDM7162 [40]. NDIR-type sensors typically consist of an infrared source, a wavelength filter, a sample gas chamber, and an infrared detector. The infrared beam passes through a sample cell containing CO\({}_{2}\), and the amount of infrared absorbed by the sample is measured by an infrared detector. The sensitivity of NDIR sensors is directly proportional to the optical beam path. This means that most of the commercial NDIR sensors are in packages that typically measure 5 or 6 cm. While some measurements using these sensors have been reported [41; 42], this has been a limitation for hive use, as it is a significant package size. A reduction in the path usually compromises the performance, so a variation using photoacoustic technology has been developed in which the sensor sensitivity is independent of the size of the optical cavity. In the photoacoustic method, a modulated infrared radiation at 4.26 \(\upmu\)m, corresponding to the absorption bands of CO\({}_{2}\) molecules, is emitted into a small enclosure. The CO\({}_{2}\) molecules in the measuring cell absorb part of this radiation, as it mainly excites molecular vibrations, causing a measurable periodic change in pressure in the measuring cell at the modulation frequency. This is at an acoustic frequency that can be measured with a MEMS microphone, with the amplitude of the sound being proportional to the CO\({}_{2}\) concentration. This is the principle used in the Sensirion SCD4x range of sensors, allowing a footprint of around 1 cm by 1 cm to be achieved. The devices include a built-in processor algorithm to calculate the CO\({}_{2}\) concentration and an i2c digital interface for outputting the data. These types of devices can provide multiple carbon dioxide measurements per minute over a wide range of concentrations. Many of the Sensirion range, such as the SCD41 used in this work, also provide temperature and relative humidity measurements within the same package. This device will measure CO\({}_{2}\) from the ambient value in air of 400 ppm up to 40,000 ppm, with different errors for different ranges. An example of this from the data sheet for the range of 2001 ppm to 5000 ppm is provided as: \(\pm\)(40 ppm + 5% of reading), with a quoted repeatability of \(\pm\)10 ppm. The SCD41 relative humidity is quoted as \(\pm\)9% and the temperature as \(\pm\)1.5 \({}^{\circ}\)C.  
For external temperatures below 10 \({}^{\circ}\)C, honeybees will not leave their hive, and this means that in climates where this is common during the winter months, beekeepers do not open their hives to inspect their colonies, as this will be damaging to the bee health.