Analysis of the Use of Thermocouples in Oxygen Deficiency Protection Devices

Gas heaters are commonly used indoor heating equipment in households, which require oxygen consumption during operation. Long term operation without ventilation can cause indoor oxygen deficiency, leading to discomfort and even suffocation, endangering personal safety. Therefore, oxygen deficiency protection design is necessary. The function of the hypoxia protection device is to immediately close the control valve and display an alarm indicating hypoxia when the device detects low oxygen content in the indoor air, reminding the user to ventilate and exchange air. It is a very common temperature sensor widely used in measuring temperature in daily life. Its principle is to connect the two ends of two different conductors into a closed circuit, and due to the unequal temperature at both ends, a thermoelectric potential is generated in the circuit. This phenomenon is called thermoelectric phenomenon in physics. This article mainly utilizes the thermoelectric phenomenon to design an oxygen deficiency protection device.

1, Working principle of hypoxia protection device

The working principle of this protection is that when the gas supply and oxygen supply are constant, the flame combustion temperature is also constant. When the oxygen content in the air is sufficient, the flame temperature can reach around 700 ℃; When the oxygen content in the air decreases, the flame temperature also decreases. According to American standards, the temperature of a flame in an oxygen deficient state should not exceed 450 ℃, and the thermoelectric potential at a relative temperature of 450 ℃ is 1815mV. If the temperature of the constant flame measured by the thermocouple is not enough, it can be determined that the oxygen content in the air is insufficient, and an alarm needs to be triggered and the control valve needs to be closed. The hypoxia protection function of this scheme is implemented as follows: a wide temperature microcontroller AT89C2051 is used, which comes with an analog comparator inside. P110 and P111 are the input pins of the comparator. Connect the 1815mV potential signal to pin P111, where the voltage signal at the reference terminal needs to be stable, so connect a voltage regulator at VCC. The flame temperature signal measured by the thermocouple is amplified and connected to pin P110, and then compared with the two electrical signals using its internal analog comparator. If the signal of pin P110 is lower than that of pin P111, it indicates that there is oxygen deficiency in the room. At this time, the control valve closes and an alarm is displayed.

2, Hardware composition

The device mainly consists of a microcontroller, thermocouple, and potentiometer. The microcontroller used is the high-efficiency AT89C2051 microcontroller launched by the American company ATMEL. It comes with an analog comparator inside, and P110 and P111 are the input pins of the comparator. This analog comparator can be used to achieve hypoxia protection function. The instructions of AT89C2051 are fully compatible with Intel's MCS-51 series microcontrollers, and it comes with a 2K byte flash programmable erasable read-only memory. Using this chip can greatly simplify the circuit design, and it also realizes other functions of the heater, which will not be elaborated here. The thermocouple adopts a K-type thermocouple, and the thermoelectric potential signal it receives is amplified and processed by an amplifier before being connected to a microcontroller. This thermocouple also plays a role in collecting constant fire temperature signals in the heater. The potential signal of the potentiometer is 1815mV, and its reference terminal is connected to a voltage regulator 358B-25.