Precision Aneroid Baromoter.
This is used to give pressure readings accurate to 0.1 millibar. It is supplied to Met observing vessels and is often examined in MCA oral examinations.
- Press black button
- Turn wheel on the right and watch the exclamation mark window
- When exclamation mark flickers you are at the correct readout, mb in white numbers on black dial. Decimals are in red on silver dial.
- Use plastic damper to left of machine when in a seaway to counter sudden changes in pressure due to pitching.
- Apply correction, this is usually +ve and corrects pressure for height above sea level.
This is used to determine pressure tendency and is divided into mbs across and 3hr slots vertically.
The marine hygrometer consists of two thermometers contained in a wooden Stephensons Screen. One of the thermometers has a muslin sock on the mercury bulb and this is wicked into a distilled water reservoir which must be topped up every few days. As the moisture on the muslin sock evaporates it cools the wet bulb and the difference between this reading and the dry bulb is known as the wet bulb depression.
Comparison of dry bulb temperature and wet bulb depression, when compared to NP100 Mariners Handbook table can determine dew point. This is usually needed for determining the onset of fog. For example when a vessel is approaching the English Channel from the Atlantic the Dewpoint will be rising as the Sea Temp is falling. These coincide at the point of Advection Fog.