Question 540General Subjects - Assistant Engineer

The correct answer is D) Bubble tube system. A bubble tube system uses a supply of compressed air to sense the liquid level in a tank or vessel. The compressed air is released through a submerged tube, and the back pressure on the air flow is proportional to the depth of the liquid, which can then be measured to determine the liquid level. This type of system requires a compressed air supply to function, unlike the other options which use different liquid level sensing methods. The other answer choices are incorrect because: A) A differential pressure transmitter does not require a compressed air supply. B) A float type level indicator uses a mechanical float, not compressed air. C) A diaphragm box system measures changes in pressure, but does not require compressed air.