## 3 Sensors Used in Boluses and for Related Experiments  
The rumen bolus is also suitable for lifelong monitoring due to its stable placement and the long-life and small-sized batteries available today. In such a case, it is reasonable to measure as many physiological characteristics as possible with sensor fusion, which raises further questions. A rumen bolus is a heavily constrained device in terms of the available battery power, space, and bandwidth. It is usually watertight, which makes chemical sensors difficult to use. Finally, the price is also a limiting factor. On the other hand, during preliminary or short-time experiments, virtually any possible kind of sensors can be used. Here, the otherwise very important energy consumption, size, and price have secondary interest.  
Rumen boluses are usually targeted to measure a subset of three main modalities; the ambient temperature, the motion activity (including rumination), and the pH. Temperature is one of the first non-electrical physical quantities, which could be transformed into an electrical signal and measured digitally. The most straightforward method utilizes the temperature dependence of different properties of PN junctions of semiconductors, making it possible to integrate the whole digital sensor into a single chip. Single-chip temperature sensors are available from numerous vendors in a wide range of accuracies for a moderate price. Other sensor types use the thermoelectric effect of a thermocouple or resistive thermal sensors, both combined with an analog-digital converter. These solutions provide a much wider thermal scale (way above 100 \({}^{\circ}\)C), which does not exist inside the rumen. Examples for using ambient temperature sensors can be seen in [32; 47].  
Measuring any factor related to motion is carried out by a MEMS (micro-electromechanical system) [48], or possibly gyroscopes. These sensors are often integrated in a single device, called an inertial measurement unit. These devices contain MEMS sensors, analog-digital converters, and the supporting logic to provide a standard digital interface. There are only a few companies providing cost-effective MEMS accelerometers or inertial measurement units with high sensitivity, selectable sample rate, and low power consumption. These versatile sensors can serve as a basis for several data-processing algorithms. Papers [7; 12; 48; 49; 50] show good examples for using accelerometers in reticulo-ruminal boluses.  
Rumen pH is still very challenging to measure. Sensors appropriate for pH measurement use electrodes in direct contact with the rumen fluid; therefore, they are prone to corrosion, which dramatically shortens their lifetime to a few months. Experimental solutions are mentioned in [51; 52].  
During preliminary or short-time experiments, the goals, conditions, and available measurement methods are different from that given for a bolus. As measuring motion can provide meaningful information not only inside the rumen but in the body surface of the animal, accelerometers and IMUs are often used in preliminary experiments as well, a typical application being a neck collar or fitted to the animal's leg. Accelerometer-based data loggers attached to the leg can provide information about the lying behavior of the animals [53].  
Finally, we can mention the special sensors, which are primarily used in medical diagnostics, such as ECGs [8; 48], pulse oximeters, and respiration transducers.