The Piano Life Saver System (PLSS) made by Dampp-Chaser contains three basic components: one or more humidifiers, one or more dehumidifiers, and a humidistat control box. In a vertical piano, all three are mounted inside the piano case typically in the area in front of the strings and below the action. In a grand, the components are mounted below the soundboard between the beams. An undercover is recommended in a grand to create a more easily controlled microenvironment below the soundboard. The humidifier is composed of a tray of water with wicks drapped over a heating rod which controls the amount of water vapor released from the unit. The dehumidifer is simply a low wattage 24" to 36" heating rod that heats the surrounding air, with the amount of heat controlled by the humidistat.
Since the relative humidity (RH) decreases by a factor of 2 for every 20° F increase in temperature, the humidity near the soundboard is controlled by heating the air inside the piano. To be more precise, the RH inside the piano is given by the difference in temperature between the inside and outside of the piano multiplied by 2 raised to the power 1/20. As an example, the plot below shows the dependence of the RH inside the piano on internal temperature when the surrounding room is at 70 °F with a RH of 60%. In this example, the temperature inside the piano only needs to be regulated around 80.3 °F to establish a relative humidity of 42% next to the soundboard.
To demonstrate how a Dampp-Chaser PLSS is installed in a grand, we show here some photos of a system as it was installed in a Steinway D. The positions of the components were dictated by a template supplied by Dampp-Chaser.
The humidifiers are rectangular plastic boxes hanging from rods placed between the beams. Baffels above them disperse the water vapor as it evaporates from wicks suspended from heating rods inside each box (not shown here). The Steinway D requires two humidifiers which are connected via a rubber tube so that filling one simultaneously results in the filling of the other. The filling tube is held in place with a clip on the inside of the case on the left. It can be easily released and pulled up to the level of the top of the piano to allow filling of the humidifiers from a supplied water can. Three dehumidifier heating rods required for this piano can be seen in the photo below: a 36" rod next to the belly rail in the foreground, one to the left of the closest humidifier, and the third near the foot of the piano beyond the second humidifier.
The humidistat is mounted between the beams within 6" of one of the humidifiers. All the components are plugged into the humidistat. The power cord hanging down on the right will ultimately exit near the front of the piano to allow it to be hidden (see below).
An undercover made of very strong speaker cloth can be installed to hide all the components. It serves to effectively create a microenvironment below the soundboard to permit efficient control of the humidity without affecting the sound of the piano. The power cord for the entire system can be run under the cover to exit near the front leg. During concerts when the appearance of the cord might be considered distracting, if not unsightly, it can be tucked away behind the front right leg. The grey cord shown in the photo here coming from the center of the piano is a temporary thermocouple used to monitor temperature inside the piano for a few days after installation.
The undercover is held in place by velcro and can be removed to service the units. In this installation I also added a separate analog hygrometer to allow for monitoring of the humidity near the soundboard if desired. It is seen here as the brass circular gauge to the right of the humidistat. The accuracy of the gauge was determined by calibration using saturated salt solutions. This gauge was found to read about 7 % high (i.e. 53% was actually 46 %).
The temperature of the air near the soundboard was monitored using a thermocouple temporarily installed near the humidistat (the grey line above the hygrometer in the photo above). The plot below shows the temperature increasing the first day from 70.5 °F to around 79 °F. Over the next few days the temperature was remarkably stable, with fluctuations usually only about 1° except for a drop to 78.2 °F around 8:00 pm (2055 min) on the second day. A fluctuation in temperautre from 79 to 78 °F results in a change in RH from 43.9% to 45.5%. Given the dampening effect of the laquer coating on the soundboard, short term fluctutions of 1 - 2% RH should cause minimal fluctuations in the moisture content of the soundboard.