Density
What does the term mean?
How much does a given volume of material weigh? Density answers this by dividing mass m by volume V: ρ = m / V. Typical units include kg/m³ and g/ml. Both quantities must refer to the same sample.
Relevance to process measurement
A density trend can reveal changes in a process product, but it cannot identify which ingredient caused them. Converting that trend into concentration requires a relationship checked for the particular mixture and defined temperature conditions.
How should a process density value be assessed?
Consider an illustrative sample of 400 ml weighing 460 g without its container. Its density is 1.15 g/ml, or 1,150 kg/m³. Record sampling time and temperature before comparing it with a process reading.
Entrained air can also affect the comparison. With changing recipes, different combinations of ingredients may produce similar densities. Specify whether the required result is density at process temperature or at a reference temperature. Total solids require a separate reference matched to the drying method.
Considering mass, volume and temperature together
A density value only becomes unambiguous when accompanied by a unit and reference conditions. A process value in kg/m³ at operating temperature and a laboratory value at a specified reference temperature do not immediately yield the same result. Therefore, when comparing samples, document both the measured temperature and the reference conditions used. In the case of a collected product, the mass of the empty vessel must also be subtracted from the total weight. The corresponding volume must relate precisely to this quantity of product.
In the event of a discrepancy, you should first check the sampling procedure and the condition of the product. Trapped air, an uneven distribution of solids or a different measurement time may explain why the laboratory and process results differ. Changing the calibration retrospectively without carrying out this check may mask the cause rather than eliminate it. Repeated, time-stamped comparative samples provide a better basis for evaluation.
If density is used to calculate a concentration, the product composition must also be documented. A relationship verified for a specific mixture is not automatically valid for other formulations. When designing a measuring point, the measuring range, temperature range, potential air ingress and typical product changes should therefore be described together.