Which of the following is a special piece of equipment that you must use with flammable gases?

Using a flashback arrestor is essential when working with flammable gases because it serves as a safety device that prevents flames from traveling back into the gas supply lines. This is particularly important in laboratory settings where flammable gases might be accidentally ignited. The device provides a barrier that can stop a flame at the weld or connection point, protecting both the equipment and personnel from potential fire hazards.

In contrast, while a gas mask is useful for respiratory protection in situations with harmful gaseous exposures, it does not specifically address the risks associated with flammable gases. A thermocouple measures temperature but has no direct function in preventing fire risks associated with flammable gases. A pressure gauge monitors the pressure within gas lines or equipment but, like the thermocouple, does not mitigate the risk of fire or explosion. Therefore, the flashback arrestor stands out as the critical safety equipment specifically designed to manage the dangers posed by flammable gases.