Intro Flashcards
What does a barometer measure?
Atmospheric pressure
How does an aneroid barometer work?
Inside an aneroid barometer, there is a thin metal capsule filled with a partial vacuum that expands and contracts with changes in pressure. This movement is converted mechanically to a pointer on the dial.
How should an aneroid barometer be stored?
The barometer must be kept away from heat sources and shielded from gusts/draughts.
Why must barometer readings be corrected for height?
Because most barometer readings will be taken at a height above sea level, which could affect the atmospheric pressure. The readings must be reduced to sea level so that they can be compared.
Why does a barometer need to be calibrated?
A barometer needs to be calibrated regularly to maintain accuracy.
What units is pressure measured in?
Hectopascals (hPa) or millibars (mb)
What is barometric tendency?
The rate of change in atmospheric pressure over 3 hours
Describe the function of a precision aneroid barometer.
A PAB contains three aneroid capsules together. The movement of these capsules alters the position of a pivot bar that carries an electrical contact. A mechanically operated micrometer drum is then rotated until the circuit is completed and a reading can be taken.
How do you take a reading from a PAB?
Press the black switch button. Turn the micrometer handle until the indicator lights connect, then wind back and forwards until you are sure it is just at the point of connection. Then release the switch button and take a reading. If the tenths reading is equally between two figures, the odd number should be taken.
What is a damping cap?
A damping cap can be fitted to the entry tube of a PAB to reduce the effect of fluctuations in pressure.
Where should a PAB be kept?
It should be mounted on a fore-and-aft bulkhead rather than an athwart-ships bulkhead to reduce the effect of rolling.
It should be kept away from direct heat sources and draughts/gusts.
What are the advantages of a PAB over an aneroid barometer?
A PAB is more accurate than an aneroid barometer as it has more aneroid capsules.
The simple mechanism in a PAB reduces the amount of errors from friction and wear.
Describe the function of a barograph.
A barograph has a stack of about five aneroid capsules, which expand and contract with the pressure this movement is converted through a series of levers to move a pen over a recording chart (barogram).The barogram is on a drum which revolves once every seven days. The observer can then see the barometric tendency over the week.
What are the advantages and disadvantages of a barograph compared to a PAB?
The barograph produces a continuous, permanent record of pressure changes over time. This allows the user to see the barometric tendency at a glance.
A PAB is more accurate than a barograph, however accuracy is not necessary to the function of the barograph as it is mainly to show tendency rather than individual readings.
How is a barograph protected from the effect of vibration?
It is mounted on thick foam feet and the aneroid capsules are housed in a drum filled with damping oil.
How is a barograph stored?
A barograph should be housed horizontally on a flat, rigid surface and kept away from heat, vibration, gusts and direct sunlight.
What needs to be done daily to a barograph?
A mark should be made every day at a specific time so that is the clockwork runs slow, the readings can be adjusted. There is a push button fitted to do this.
How often does a barometer need to be wound?
A barometer should be wound every week.
How is the barogram changed, and how often?
Once a week, the barogram needs to be exchanged for a new one. This is done by opening the instrument and moving the pen away from the barogram. The new barogram needs to be fitted tight to the drum, with the end of the chart overlapping the start correctly. The drum is then rotated to a time slightly faster than the correct GMT and turned backwards to the correct time. The pen is then replaced to the correct position and the instrument closed.
Why does the barograph need to be compared with a PAB?
The barograph readings are compared with readings from the PAB so that the error can be noted, and if it is to big it can be removed by the adjusting screw.
What does a masons hygrometer measure?
relative humidity, and as a result, the risk of fog
