Fluoride should never be measured against which type of head?

Fluoride measurement is typically conducted under conditions where the water sample is at rest, which allows for accurate assessments of its concentration and chemical properties. When measuring fluoride levels, it's important to avoid situations that could introduce variability due to movement or differences in pressure.

Suction head involves situations where a vacuum or suction is applied to draw water up from a source. This can lead to the formation of bubbles or other disturbances in the sample, which can result in inaccurate readings of fluoride concentration. Any type of suction can influence the stability of the sample, potentially affecting the measurement by either volatilizing fluoride or causing inconsistencies in how the fluoride interacts with the testing reagents. Thus, static conditions are preferable for accurate fluoride determination, which is why measuring against suction head is not suitable.