Technical Info about Weather Instruments

Technical info about weather instruments, barometers, thermometers and hygrometers

Frequently asked questions (Copied for my website with kind permission of the BARIGO Company in Germany) some more hints from my years of practice mentioned as well, T.N.

  1. How does a barometer work ?
    The sensor element of a barometer is a metal capsule, the so called diaphragm. With increasing air pressure this diaphragm is pressed together, with decreasing air pressure it is expanding. This up and down movement of the diaphragm is transformed into a rotation movement of the spindle with the help of a lever system. An indicating hand placed on top of the spindle allows accurate readings of the air pressure measured by this system.
    Barometers used at home are meant to indicate the air pressure in relation to sea level conditions. To be able to compare the air pressure readings between locations of different local altitude it is essential to eliminate the pressure differences which are caused by the different local altitudes. On sea level we have higher air pressure than in the mountains, as the weight of the air above the instrument is much higher.
    For weather forecast we would like to be able to compare the weather-caused air pressure differences of locations all over the country with each other. This is only possible if we compensate and eliminate the air pressure differences which are caused by the different local altitudes. How this is done is described in our next chapter.
  2. How may I adjust my barometer to my local altitude ?
    If the barometer is meant to show the correct relative air pressure for each and every location it has to be adjusted once to adapt to local altitude. On the back of each instrument (or at the bottom of some table instruments) you will find an opening giving access to the adjusting screw which moves the indicating hand. On some models you will have to remove the fixing back plate with its centre screw first to have access to the adjusting screw. To receive the current relative air pressure value for your location please check with your local newspaper, your local radio station or airport at that day. Turn the adjusting screw to set the hand to the correct reading. After having done so once the barometer will reliably react to any changes and will help you to obtain a good weather forecast for your home.

You probably know, that you could give the barometer on the glass a little tick before reading. That will release any latent action of the hand. The mechanism would move also without a tick anyhow to its right position according to changing pressures, but it would do so at any time, and not necessarily when you want to see a tendency. You can set the maker hand whenever you noticed a change.

  1. How does a hygrometer work ?
    A hygrometer measures and indicates the change of the length of a measuring element which expands with increasing humidity and which contracts with decreasing humidity. This change in the length is transmitted into a rotation of a spindle with the help of levers and axles. The rotation of the spindle is shown by an indicating hand.
    The measuring element itself can be human hair, artificial threads or paper layering on a metal spiral spring. For standard hygrometers paper layering are used, for even more accurate models hairs or artificial threads are preferred.
    4. How do I readjust my hygrometer ?
    If a hygrometer is exposed to very dry air over a longer period of time and if you would like to maintain a correct reading, we recommend to re-adjust the instrument every 3-4 months.
    It is very important to get the right screwdriver before you continue as below. The blade must be properly sharpened to fit snugly into the slot of the screw. In some models this is not a real screw, but a U-shaped thin piece of metal with a slit. Take care not to apply undue pressure.
    Put a damp cloth on the back of the hygrometer for about an hour to allow humidity to get into the instrument and circulate within the case of the hygrometer. After that period of time the scale should read approximately 95 %. If it reads a lower number or is off scale completely, merely turn screw on back until the pointer is on 95. The humidity indicator will then start to operate again. To maintain accuracy of the instrument, it is advisable to repeat this procedure every 3-4 months.
  2. How does a thermometer work ?

Liquidity type thermometers contain alcohol (in former times: quicksilver) in a vacuum tube. The liquid expands or contracts with change of temperature. With the help of a scale the change of the length of the alcohol column can be related to correct temperature readings.
Dial type thermometers contain bimetal spiral springs which consist of two different metals. (see Model 820 on the right) These metals contract and expand with change of temperature, but in a different way and to a different extent. One being less reactive than the other, the spiral spring makes a defined rotating move when contracting or expanding.
This defined move is reflected by the turn of a spindle which holds the indicating hand.
The dial type thermometer is of very solid construction and will need adjustment only in exceptional cases. Should need occur however, the indicator hand can be set to the correct reading by turning the screw on the back of the instrument carefully.

  1. How do I best take care of my brass ship’s instrument cases? ( Baro – Thermo – Hygro )
    These instruments are of solid brass, polished and lacquered. This treatment is sufficient for normal use onboard a ship within the cabin or at home.
    On contact with salt water or with air containing salt it is strictly recommended to immediately clean the instrument with the use of a damp cloth to remove the salt. This is to prevent the salt to destroy the protecting lacquering and to start attacking The surface of the brass itself. This procedure will guarantee a long lasting superb finish of your instrument.




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