The Omega’s hygrometer

When you order a grand piano from Sauter you can ask for a hygrometer to be fitted inside the case as an optional extra. The picture below shows the natural hair hygrometer in my Omega.

Hygrometer in Sauter Omega

Hygrometer in Sauter Omega

To the left of frame are the remote sensor units of two digital temperature/RH meters that I have. I use two to average out the readings. Their master control units are outside the piano to measure ambient room temperature and RH. What you can’t see are the RH levels showing on the meters. The digital meters are showing 58%, while the Sauter hygrometer is showing a tad above 48%. Inscribed along the top arc of the scale is a double-headed arrow indicating that a ‘normal’ RH reading should be between 40% and 70%.

The hygrometer’s case is set very snugly into a cavity in a block of wood attached to the rim. The vents that let air into the hygrometer are three large holes in the rear of the case, so that there is almost no mixing of the air in the hygrometer and cavity with the air around the wood block. Even when the RH of the room was at 70% for a couple of hours, the Sauter hygrometer crept up to around 50% over the same period of time. At first I thought that there was something wrong with the Sauter hygrometer. But when I popped it out of the cavity it eventually showed the same RH as the digital units.

What is the Sauter hygrometer measuring? I think that it is indirectly measuring the Equivalent Moisture Content (EMC) of the wood in the piano. From Guus van den Braak’s website in Australia I got the following:

  • The ideal EMC for wood within a piano is determined by most manufacturers to be 8%.
  • To maintain an 8% EMC, an RH of 42% is required. [That’s where that magic 42% RH comes from.]
  • The safest range of RH for wood is between 40% and 60%. [Sauter says 70%.] RH swings within this range reduce the risk of destruction of the cell structure. If RH swings are beyond this range, the cell structure of wood becomes fatigued and starts to break down.
  • Wood that has too high an EMC is soft and structurally weak because the cells are full of water. The acoustic properties are dampened, swelling causes stresses where it is glued together (like at the rim or by the ribs) resulting in fractured glue joints and compression ridges commonly seen in soundboards (which will split when they dry out too much).
  • Too low an EMC creates the possibility of a brittle cell structure. This may be stronger than a too high an EMC, but will result in splitting or cracking if pushed too far.

So, once again, what is the Sauter hygrometer measuring? If the EMC of the wood block in which the hygrometer sits is 8%, then the air in the hygrometer’s cavity should be stable around 42% because the moisture exchange between air and wood is in equilibrium. If the hygrometer is reading more (less) than 42% then the EMC is higher (lower) than 8%. As long as the RH reading is within the safe range then all is well. Now I understand why Ulrich Sauter told me that short term swings in RH are nothing to worry about as long as on average the RH in the room is within the safe limits.

This is an ingenious and accurate way of tracking the room’s RH and it’s direct effect on the piano. All that you need to do is to calibrate the Sauter hygrometer once a year.

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