# A Review of Hydrogen Sensors for ECLSS: Fundamentals, Recent Advances, and Challenges
## 3 Heat Conduction Hydrogen Sensors
### Principle of Concentration Measurement  
The heat conduction-type hydrogen sensor belongs to the crossover class of electrical and physical thermodynamics sensors; its detection circuit is based on the design theory of a balanced bridge, while its detection principle is based on thermodynamics [34; 35; 36; 37; 38]. Figure 3 shows a heat conduction-type hydrogen sensor, which is designed based on the theory that each gas has a different thermal conductivity. Its most important feature is the constant temperature measurement, which is determined via the current flowing through the thermal material. The thermal material is usually a Pt wire with good thermal conductivity. When the target gas flows through the Pt wire, some of the heat from the wire is transferred to the gas. For different concentrations of the same gas and two gases with different levels of thermal conductivity, the amount of heat transferred in the process is not the same. The heat transferred is, therefore, a reflection of the type and concentration of the target gas. In the detection circuit, a balanced bridge design is used, in which the thermal element is used as one of the measuring bridge arms. The loss of heat from the Pt wire causes a change in its temperature, the balance of the bridge is broken at the initial operating current, and the bridge outputs a differential pressure signal to the outside. After the two processes of heat conduction and conversion of the bridge, the amount of heat transferred is converted into the size of the output voltage signal. Therefore, by measuring the differential voltage signal of the bridge, the concentration of the target gas can be reflected. The concentration detection is achieved by relying on a Wheatstone bridge built with a Pt wire resistor and divided into a measurement arm and a reference arm. A reference gas is encapsulated in the reference arm, which is designed to weaken the effect of ambient temperature changes on the bridge. When the measured gas flows through the measuring arm, the temperature of the Pt wire of the measuring arm changes, and the bridge loses its balance and outputs a voltage difference signal. Once the composition of the target gas is determined, its thermal conductivity only changes with gas concentration. To make the heat conduction-type sensor work stably, a thermostatic circuit is needed to ensure that the detector works properly. Figure 4 shows the principle circuit for thermostatic detection of heat conduction-type gas sensors. Resistors \(R_{1}\), \(R_{2}\), \(R_{3}\), and r form a Wheatstone bridge to convert the unbalanced voltage output. Here, \(R_{2}\) and \(R_{3}\) form the reference bridge arm, \(R_{1}\), while r forms the measurement bridge arm and is the heat conduction-type sensor. When the target gas concentration in the environment is 0, the bridge is in equilibrium, i.e., \(R_{2}R_{3}=rR_{1}\). The operating current of the sensor is reasonably adjusted to provide a constant temperature for the normal operation of the sensor. When the target gas detection is carried out, as the thermal conductivity of the target gas increases, the temperature of the thermal conductivity sensor decreases, causing its resistance to decrease and breaking the equilibrium of the original bridge. The signal output outputs an unbalanced voltage \(u_{0}\), and after differential amplification and subsequent circuit processing, the value of \(u_{0}\) can be measured to detect the concentration of the target gas. When the gas concentration becomes 0, the bridge also restores the balance; thus, the resistance of the sensor and the temperature remain unchanged, which is regarded as constant temperature detection. Figure 4: Heat conduction-type hydrogen sensor detection circuit. Figure 3: A type of heat conduction hydrogen sensor. A quantitative analysis of the thermostatic detection principle of the heat conduction-type gas sensor is herein carried out. We assume that the operating current of the sensor is \(I\) and the internal resistance is \(r\).