Exam Question
(a) Sketch and describe crank case oil mist detector and the principle of its operation. (8)
(b) Draw crank case explosion relief door and name the parts. State pressure setting of the door. (8)
Class 4 (4th Engineer)
MEKM
📅 Aug 2022
Reference Answer
### (a) Crankcase Oil Mist Detector
#### Principle of Operation
The fundamental principle of a crankcase oil mist detector (OMD) is the measurement of light obscurity caused by oil mist particles suspended in an air sample. It operates on a differential measurement system, comparing the optical clarity of an air sample drawn from the crankcase against a sealed reference chamber containing clean air. A common light source illuminates both a 'measuring tube' (containing the crankcase sample) and a 'reference tube' (containing clean air). Photoelectric cells at the end of each tube measure the intensity of the light that passes through. An imbalance in the electrical output from these cells, caused by light scattering or absorption by oil mist in the measuring tube, indicates the presence of oil mist and triggers an alarm. This detection occurs at a concentration well below the Lower Explosive Limit (LEL) of the oil mist, providing an early warning.
*(Reference: "The detector consists of two parallel tubes of equal size, each having a photoelectric cell at one end... One tube is sealed to contain clean air and is termed the reference tube. The other, the measuring tube, has connections through which samples of the crankcase vapour are drawn... If a concentration of oil mist is present in the sample, light will be obstructed... Electric balance between the two cells will be disturbed and an alarm will be operated.")*
#### Principle of Operation
The fundamental principle of a crankcase oil mist detector (OMD) is the measurement of light obscurity caused by oil mist particles suspended in an air sample. It operates on a differential measurement system, comparing the optical clarity of an air sample drawn from the crankcase against a sealed reference chamber containing clean air. A common light source illuminates both a 'measuring tube' (containing the crankcase sample) and a 'reference tube' (containing clean air). Photoelectric cells at the end of each tube measure the intensity of the light that passes through. An imbalance in the electrical output from these cells, caused by light scattering or absorption by oil mist in the measuring tube, indicates the presence of oil mist and triggers an alarm. This detection occurs at a concentration well below the Lower Explosive Limit (LEL) of the oil mist, providing an early warning.
*(Reference: "The detector consists of two parallel tubes of equal size, each having a photoelectric cell at one end... One tube is sealed to contain clean air and is termed the reference tube. The other, the measuring tube, has connections through which samples of the crankcase vapour are drawn... If a concentration of oil mist is present in the sample, light will be obstructed... Electric balance between the two cells will be disturbed and an alarm will be operated.")*
Full answer on MeoMock — surveyor-grade reference answers for 7,000+ written exam questions.
Prepare on Android — MeoMock on Google Play