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Hi @SisterMaryClare - one explanation for a flat line is the watch wasn't able to measure your oxygen level or there was actually very little variation (within it's tolerances)
If there is too much movement or the watch is not on a place where it can measure successfully, there may be no reading. Ideally just in case, it should be placed where there are no tattoos as that interferes with HR measurements and may be oxygen too.
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