A hydrometer is a weight with a vertical scale attached. When placed into a liquid, the hydrometer bobs upright, and sinks to a certain level. The specific gravity or solution composition can be read from the liquid level on the vertical scale. The smallest detectable concentration an analytical instrument can determine at a given confidence level. Mass is a measure of the tendency of an object to resist acceleration.
Measurement is the collection of quantitative data.
Measurement involves comparison of the quantity of interest with a standard called a unit. The comparison is never perfect. As a result, measurements always include error. You must consider the reliability of the measurement when using it to make decisions or estimate other quantities. For example, 3.2 MJ is 3200000 J. meter.