Liquid Density Sensors

The density of a liquid can be measured as it flows through a piping system. ill. 1 (below) shows a picture and ill. 2 shows a diagram of typical liquid-density sensors. From the diagram (ill. 2) notice that unlike a flow meter, the density sensor doesn't need to sample the entire flow. Rather it can be mounted in the piping to take a sample of the flow. A small portion of the flow will continually flow through a sensing chamber of the density sensor. The sensing chamber has a totally submerged plummet attached to a set of calibrating chains at several fixed reference points. The plummet is an object that will float in the liquid at the equilibrium which is calibrated to be near the midpoint of the density range for the liquid being measured. When the density of the liquid increases, it's thicker and the plummet will be raised slightly. When the density decreases, the liquid is thinner and the plummet will fall slightly. The change of distance the plummet moves is measured by a position sensor called a linear variable differential transformer (LVDT). The operation of the LVDT will be explained later in this chapter. An RTD is used to constantly measure the temperature of the product to adjust the calculation of the density.

The second part of the density sensor is the signal conditioner. The signal conditioner uses the position signal from the LVDT and the temperature signal from the RTD and converts them to 4-20 mA, 0-5 volts, or -5 volt to +5 volt signal that represents the density of the liquid being measured.

The density meter is calibrated and monitored continually. During the time the density meter is operating, a quality-control expert will sample the product at the same point in the piping system as the density meter. The sample will be taken to a laboratory where a small amount is placed on a special testing paper that has been previously weighed. When the sample is placed on the paper, the paper and the liquid sample are weighed together. The sample is then heated in an oven for several minutes until most of the moisture is removed from the paper. At this point the sample is weighed again. The QC expert now has determined the weight of the product by subtracting the original weight of the sample paper from the weight of the original sample before it was heated and after it was heated. The difference between these weights is the amount of water that was in the product versus the amount of solids in the sample.

Above: ill. 1: Typical density sensors and control units/signal conditioners (these are made by Dynatrol). The sensor is mounted in the piping system so that it can sample a portion of the flow.

Above: ill. 2: Diagram of a density sensor. This type of sensor takes a sample of the product as it flows past this sensor.

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