My experience is that the strings return to their original tension just as quickly and that it will not effect the overall tuning unless the harp has had enough time to contract. I would imagine that it would take several hours for that to happen. I would always walk away from a tuning in the situation you described and return to finish my work when the temperature had stabilized. That's about all you can do. If it doesn't work out that you have a window of opportunity like that then its their tough luck. The expert on this subject is Jim Ellis. I has done controlled scientific experiments using a rather elaborate monochord. I have seen his class twice and in my opinion it is one of the better classes. Michael Wathen Southwest Ohio
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