In this paper, a portable instrument based on electrochemical sensors is developed to detect the concentration of volatile organic compounds of XLPE power cables, in which an air chamber is designed to reduce the dissipation rate of the gas. In order to increase the detection efficiency and gas concentration, a permanent magnetic brushless micropump is used to accelerate the gas flow rate along the gas path. Considering the portability of the instrument, a sliding mode controller is designed to estimate the speed of the rotor without the speed sensor to accomplish the speed closed-loop control algorithm. To improve the signal-to-noise ratio of the detection system, the ping-pong control algorithm is employed to reduce the influence of PWM noise on the gas sensing circuit. Both the experimental and on-field results showed that the detection time and the gas concentration of the instrument whose micropump with the speed closed-loop algorithm are reduced by at least 10 times, and increased by 20% respectively compared by the open-loop control algorithm.