In this paper we address the stability of a low-dissipation (LD) heat engine (HE) under maximum power conditions. The LD system dynamics are analyzed in terms of the contact times between the engine and the external heat reservoirs, which determine the amount of heat exchanged by the system. We study two different scenarios that secure the existence of a single stable steady state. In these scenarios, contact times dynamics are governed by restitutive forces that are linear functions of either the heat amounts exchanged per cycle, or the corresponding heat fluxes. In the first case, according to our results, preferably locating the system irreversibility sources at the hot-reservoir coupling improves the system stability and increases its efficiency. On the other hand, reducing the thermal gradient increases the system efficiency but deteriorates its stability properties, because the restitutive forces are smaller. Additionally, it is possible to compare the relaxation times with the total cycle time and obtain some constraints upon the system dynamics. In the second case, where the restitutive forces are assumed to be linear functions of the heat fluxes, we find that although the partial contact time presents a locally stable stationary value, the total cycle time does not; instead, there exists an infinite collection of steady values located in the neighborhood of the fixed point, along a one-dimensional manifold. Finally, the role of dissipation asymmetries on the efficiency, the stability, and the ratio of the total cycle time to the relaxation time is emphasized.

Traditionally, the estimation of water quality is realized through laboratory analysis which is time-consuming and requires specialized installations, equipment, and personnel. Nowadays, it is possible to make real-time water monitoring through electrochemical sensors, microcontrollers, and central processing units to detect water pollutants. This work proposes a system based on the Arduino platform for monitoring parameters associated with water quality, such as oxidation-reduction potential, pH, total dissolved solids, turbidity, temperature, electrical conductivity, and dissolved oxygen. A critical criterion for the sensor selection was its cost and availability, resulting in sensors from different companies. They were integrated without much complexity, thanks to the selected platform. In addition, a proposal is made for a signal conditioning circuit for the oxidation-reduction potential electrode. A stage of filtering is added to the pH and turbidity commercial circuits to improve their performance. Remote access to the data is done through a mini-PC with WIFI connectivity and a MySQL database. All the sensors were calibrated with reference solutions or against other commercial meters. Through the proposed system, time series having a sampling period of 20 s of all parameters were recorded for more than a week-long exhibiting circadian patterns for the same water sample. Pearson correlation for the parameters was carried on. The results show that the system successfully monitored the seven physicochemical variables through low-cost sensors. It also has remote access capabilities.

In the present paper we study the connection between local stability and energetic properties in low-dissipation heat engines operating in the maximum-power and maximum-compromise ($\mathrm{\Omega }$) regimes. We consider two different feedback regulatory pathways: (1) one in which restitutive forces linearly depend on the deviations from the stationary values of the heat exchanges with the hot and cold reservoirs and (2) another where restitutive forces depend on the deviations from the stationary values of the power output and the heat outflux into the cold reservoir. The first dynamics leads to an isolated stable point while in the second one the system is metastable. Further analysis of random perturbations from the steady state gives valuable information about the dynamic behavior of thermodynamic properties like entropy, power, and efficiency in both operation regimes.