Measuring Fluctuating Temperature Water Level Using Density

A thermal power plant needed to measure the liquid level in a water storage tank, but due to mounting restrictions and space limitations, they are unable to use traditional liquid level sensing products.

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Another approach for indirectly measuring level of water in a storage tank is by using a differential pressure transmitter to measure pressure, and then calculating liquid level through calculating the density. However, the density of water changes with temperature; when temperature increases the water expands in volume and vice versa. This causes the water level to rise without varying the amount of pressure being applied to the transmitter’s sensors. If only a pressure measuring device is used, the system will lack the temperature data required for correctly adjusting the density calculations. Temperature compensation is needed to accurately determine the level of water within the process.

SOLUTION

An SOR 815DT Smart Differential Transmitter combined with an SOR 1400 Series Temperature Transmitter Assembly gave the plant personnel all the necessary information they required to calculate the density, and therefore, indirectly measure the water level. With the SOR 815DT’s compact design, it was able to be installed into confined areas where other instruments would be too large.

Both transmitters constantly measure the temperature and pressure and the process data is relayed to a digital control system. The temperature transmitter data is used to compensate the level readings accordingly. The combination of transmitters provides the digital control system with the process parameters needed for precisely calculating the density and thus, it is able to accurately determine the water level within the storage tank.

For more information regarding any level, pressure, temperature, or flow requirement, contact Instrument Specialties, Inc. by calling 407-324-7800 or visiting http://isi.group.

What is Inferential Measurement? An Understanding Through Differential Pressure Level Control

Level control using delta-P transmitter.*

Differential pressure transmitters are utilized in the process control industry to represent the difference between two pressure measurements. One of the ways in which differential pressure (DP) transmitters accomplish this goal of evaluating and communicating differential pressure is by a process called inferential measurement. Inferential measurement calculates the value of a particular process variable through measurement of other variables which may be easier to evaluate. Pressure itself is technically measured inferentially. Thanks to the fact numerous variables are relatable to pressure measurements, there are multiple ways for DP transmitters to be useful in processes not solely related to pressure and vacuum.

An example of inferential measurement via DP transmitter is the way in which the height of a vertical liquid column will be proportional to the pressure generated by gravitational force on the vertical column. The differential pressure transmitter measures the pressure exerted by the contained liquid. That pressure is related to the height of the liquid in the vessel and can be used to calculate the liquid depth, mass, and volume. The gravitational constant allows the pressure transmitter to serve as a liquid level sensor for liquids with a known density. A true differential pressure transmitter also enables liquid level calculations in vessels that may be pressurized.

Gas and liquid flow are two common elements maintained and measured in process control. Fluid flow rate through a pipe can be measured with a differential pressure transmitter and the inclusion of a restricting device that creates a change in fluid static pressure. In this case, the pressure in the pipe is directly related to the flow rate when fluid density is constant. A carefully machined metal plate called an orifice plate serves as the restricting device in the pipe. The fluid in the pipe flows through the opening in the orifice plate and experiences an increase in velocity and decrease in pressure. The two input ports of the DP transmitter measure static pressure upstream and downstream of the orifice plate. The change in pressure across the orifice plate, combined with other fluid characteristics, can be used to calculate the flow rate.

Process environments use pressure measurement to inferentially determine level, volume, mass, and flow rate. Using one measurable element as a surrogate for another is a useful application, so long as the relationship between the measured property (differential pressure) and the inferred measurement (flow rate, liquid level) is not disrupted by changes in process conditions or by unmeasured disturbances. Industries with suitably stable processes - food and beverage, chemical, water treatment - are able to apply inferential measurement related to pressure and a variable such as flow rate with no detectable impact on the ability to measure important process variables.

* Image courtesy of T. Kuphaldt and his book "Lessons In Industrial Instrumentation"