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Piano Pages
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How does temperature and humidity affect the piano?
Pianos are affected by changes in both temperature and humidity. Temperature changes affect both wood and metal parts, whilst humidity changes affect wooden parts, but can eventually cause corrosion on metal parts under certain circumstances. Traditionally it is said that humidity rises will raise the pitch of a piano, but humidity must be considered in combination with temperature.
The rate at which a piano will loose moisture content will depend on the temperature of the room, and
the amount of moisture in the air relative to the amount it could hold, at that temperature and pressure. The air is 'like a sponge', but the amount of moisture it can hold depends on its temperature and pressure. As the temperature and/or barometric pressure rises, the amount of moisture it can hold increases. Therefore, rises in either temperature or pressure, without adding more water vapor, make the air's relative humidity 'dryer'. It can then absorb more moisture from anything that is in it, including a piano.
Humidifying the room puts more moisture in the air, which raises its relative humidity again, i.e. the amount of moisture in the air compared to the amount it can hold. This reduces its 'drying' or 'sponge-like' effect. Humidifying just the piano, with a humidifier inside (or under) the instrument, does not do this. The instrument will be being 'dried out' on the surfaces open to the air, whilst being re-humidified on the other. We are attempting to put the moisture in the piano, rather than into the air.
The air can absorb moisture much more quickly than wood. If we humidified the room, the piano would not dry out in the first instance. By humidifying the piano, the result depends on the ratio of the rates at which moisture is lost and gained by the piano, which are not necessarily the same. As we said, the instrument will be gaining moisture on one surface, and losing it from the other.
If we humidify a room or a piano to obtain normal humidity at that temperature, what happens when the heating goes off, or the ambient temperature falls? Now the amount of moisture that the air can hold decreases, so the relative humidity increases. The piano now finds itself surrounded by relatively 'humid' air. Unless a complete humidity control system is in place, including a dehumidifier to remove moisture from the air when the temperature drops, the piano will still be subject to ambient humidity change.
The best strategy is to use a hygrometer in the location of the piano to keep a check on what the humidity is doing. When the humidity is outside the 'normal' range, appropriate measures can be taken to correct the problem. If dampness is a problem, it is probably better to dehumidify, rather than heat the room, if the heating cannot be on all the time (including night time) that dampness is a problem. Excess damp can often be tackled by fitting a "Dampp Chaser" inside the piano, but it is important to ensure air circulation is possible around the soundboard.
Humidifiers
Humidifiers must be kept full with water. A typical unit has a heating element of some kind to evaporate the water. This may be on/off controlled using a hygrostat, which is like a thermostat that detects humidity rather than temperature. This is fine as long as the hygrostat is in the right place and working properly.
Many of these units work with relatively low technology - usually a strip of cellophane that expands or contracts with changes of humidity, without a water level detector. Consequently, if you inadvertently let the water container run dry, the heated evaporator bar will stay on, now effectively drying the piano, which makes the hygrometer keep the system switched on.
Dampp-chaser system
This is the standard humidity regulation system. May include both humidifier and anti-humidity heater bar.
