Mass flow measurement of 58% Nitric Acid

We have recently installed a E&H Promass 83 coriolis mass flowmeter on nitric acid. The specific gravity reads about 1.17 kg/litre when the
acid is flowing through the measurement tube, and about 1.34 kg/litre when the tube is full, but the flow is stationary. This is the same strength acid, so these values should be the same. 1.34 kg/litre is about right for acid at around 60% concentration and at around 50 deg C (which is what we have - confirmed by independent lab tests). It seems like the nitric acid is losing some density when flowing, possibly due to it experiencing some two phasing - some of the liquid flashing off to form a gas entrained in the liquid. Anyone else experienced this ? How did you fix it ?
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