Showing posts with label transmitter. Show all posts
Showing posts with label transmitter. Show all posts

Monday, June 26, 2017

Hydrostatic Pressure Measurement for Determining Liquid Level

pressure transmitter for mounting to flange on a tank
Pressure transmitters can be used to provide liquid level in
pressurized or open vessels.
Photo courtesy Azbil North America
Pressure measurement is an inferential way to determine the height of a column of liquid in a vessel in process control. The vertical height of the fluid is directly proportional to the pressure at the bottom of the column, meaning the amount of pressure at the bottom of the column, due to gravity, relies on a constant to indicate a measurement. Regardless of whether the vessel is shaped like a funnel, a tube, a rectangle, or a concave polygon, the relationship between the height of the column and the accumulated fluid pressure is constant. Weight density depends on the liquid being measured, but the same method is used to determine the pressure. 

A common method for measuring hydrostatic pressure is a simple gauge. The gauge is installed at the bottom of a vessel containing a column of liquid and returns a measurement in force per unit area units, such as PSI. Gauges can also be calibrated to return measurement in units representing the height of liquid since the linear relationship between the liquid height and the pressure. The particular density of a liquid allows for a calculation of specific gravity, which expresses how dense the liquid is when compared to water. Calculating the level or depth of a column of milk in a food and beverage industry storage vessel requires the hydrostatic pressure and the density of the milk. With these values, along with some constants, the depth of the liquid can be calculated.

The liquid depth measurement can be combined with known dimensions of the holding vessel to calculate the volume of liquid in the container. One measurement is made and combined with a host of constants to determine liquid volume. The density of the liquid must be constant in order for this method to be effective. Density variation would render the hydrostatic pressure measurement unreliable, so the method is best applied to operations where the liquid density is known and constant.

Interestingly, changes in liquid density will have no effect on measurement of liquid mass as opposed to volume as long as the area of the vessel being used to store the liquid remains constant. If a liquid inside a vessel that’s partially full were to experience a temperature increase, resulting in an expansion of volume with correspondingly lower density, the transmitter will be able to still calculate the exact mass of the liquid since the increase in the physical amount of liquid is proportional to a decrease in the liquid’s density. The intersecting relationships between the process variables in hydrostatic pressure measurement demonstrate both the flexibility of process instrumentation and how consistently reliable measurements depend on a number of process related factors. 

Share your process measurement and instrumentation requirements and challenges with professionals that specialize in their proper selection and application. Combining your own process knowledge and experience with their product application expertise will help to develop effective solutions.

Monday, July 11, 2016

Level Measurement: Comparing Displacer Transmitters and Differential Pressure Transmitters

diagram of displacer level transmitter for process measurement and control
Electronic Displacer Transmitter
Courtesy Magnetrol
Liquid level measurement is ubiquitous throughout industrial fluid processing operations. Whether via direct or inferential means, the fluid level data point is an integral part of operational and safety plans for an installation.

Magnetrol, a globally recognized leader in the design and manufacture of level measurement instrumentation, has produced a comparison of displacer transmitters and differential pressure transmitters applied to liquid level measurement.

What is a displacer transmitter?

Displacer transmitters are considered a direct means of level measurement.

Quoted from Magnetrol website page on displacer transmitters...
Electronic displacer level transmitter technology operates by detecting changes in buoyancy force caused by liquid level change. These forces act upon the spring supported displacer causing vertical motion of the core within a linear variable differential transformer.
As the core position changes with liquid level, voltages are induced across the secondary windings of the LVDT. These signals are processed in the electronic circuitry and used to control the current in the 4-20 mA current loop. The enclosing tube acts as a static isolation barrier between the LVDT and the process media.

 How does a differential pressure transmitter measure liquid level?

Liquid level can be inferred through the measure of the pressure generated by the column of liquid in a tank or vessel. The measurement device must be calibrated for the specific gravity of the measured media in order to attain reasonably accurate results. There are a number of installation arrangements of differential pressure transmitters that will accommodate tanks open to atmosphere, closed pressurized tanks, and more.

The comparison is included below and provides some interesting points to consider. Share your level measurement application challenges with a product application specialist. Combining your process experience and knowledge with their product application expertise will produce the best solutions for your process measurement needs.