Author: Instrumentation Tools Categories: Level Measurement Basics of DP Level Measurement Level the parameters that exist in virtually every industrial process, there are many ways to measure the level, the simplest is to use the sight glass. By using a sight glass, the height of liquid in a vessel / vessel will be physically visible, so as to make the scale on sight glass, we can immediately determine what percentage of the liquid surface height of a tall vessel / tank / vessel.
In the picture, a tank connected by a transparent hose using a scale of 0-100% of the total height of the tank. This level measurement principle utilizing the properties of the liquid will fill all the space that he passed on the associated vessel. Height of liquid in the tank will be the same as the height of a liquid residing on a transparent hose that serves as a sight glass. We can directly determine height (level) of liquid inside the tank by looking at the height of a liquid residing on a transparent hose (sight glass) is. However, this information can only be served directly in the field, or directly see where the transparent hose attached. Level measurement method is relatively inexpensive. Pressure Hydro static Any liquid that occupies a vessel / vessel / tank, will have a hydrostatic pressure in proportion to the level of the liquid, with assume densities (sg = specific gravity) of his remains. The picture above is an open tank (surface connected to the atmosphere), where there will be work pressure P1 at atmospheric pressure, which then will we ignore because we will measure pressure gauge. Assume melting substance is water, with a density of? = 1000 kg / m³. With the height of the surface from the bottom of the tank where pressure measurement is 10 meters. Then the P2 pressure acting on the pressure gauge is:? = density of water = 1000 kg / m³ g = gravity = 9.8 m / s²
h = height of base water tank = 10 m P2 =? g h P2 = 1000 kg / m³ 9.8 m / s² 10 m P2 = 98000 kg / m³ m / s² m P2 = 98000 kgmm / m³s² P2 = 98000 kgm / s²m² P2 = 98000 N/m2 P2 = 98000 Pascal P2 = 98 kilopascals = 14.2136983 PSI = 0.9993218887 kg / cm² 1 kilopascals = 0.1450377377 PSI (pounds per square inch) 1 kilopascals = 0.01019716213 kg / cm² Consider the following table:
From the table and from the graph, we can see that the level (h) is proportional to the pressure (P), so by measuring the pressure at the point of the tank bottom, we can know the level of water in the tank. For example: Results pressure on the bottom of the tank, we get 4.2641 PSI, then by reversing the above calculation, we would get a level of 3 meters. How to present levels in the DCS, PLC or Controller? Pressure gauge is installed at the bottom of the tank before, can be replaced by using a pressure transmitter calibrated measuring ranges (range) input 0 to 14.2137 PSI, let me easily (not recommended in practice in lapaangan), we round it off to 14PSI, and output, for example, 4-20 ma (milli-amperes). 4-20 ma signal that represents the input signal from the pressure transmitter in this instance 0-14 PSI calibrated transmitter for 4-20mA output, transmitted to a receiver that can be either DCS, PLC or controller, which is connected with the station that serves as MMI (Man -Machine Interface) or HMI (Human-Machine Interfacer), the DCS, PLC or controller, the 4-20mA signal on-scaling longer a form of engineering units (meters) so that the variation of 0-10 meter level in the tank, can be displayed 0-10 meter (engineering unit) on the HMI / MMI. So that the overall representation of the signal to be: 1. 0-10 meter level in the tank 2. 0-14 PSI hydrostatic pressure on the input Trasmitter 3. 4-20mA input signal transmission in the DCS, PLC, controller 4. in DCS, PLC, controller-scaling into engineering units back (0-10 meters), with no attention to the process of analog to digital conversion 5. The display on the MMI / HMI in the form Engineering Unit (meters) Determine Range Differential Pressure Transmitter To Measure Level The level measurement can be done by utilizing the hydrostatic pressure of the liquid inside the tank to be measured its level. To measure the hydrostatic pressure caused by the liquid level in a tank, can be used a differential pressure transmitter with a measuring range (range) the corresponding input.
Consider the example in the image below: A vessel with a working pressure P1 = 200 PSI, has a measuring range for liquid high as 150 inches (3.81 meters). The question is, what is the input range for the level transmitter (differential pressure transmitter) which will be used? Before continuing, we know that on both sides (H and L) Differential pressure transmitter works work pressure vessel of the same (200psi), if dp = H L = (P1 + P hydrostatic) P1 = P hydrostatic, then pressure work on the DP transmitter is P hydro static only. To that end, the next calculation, only P hydro static are included in the count. If the liquid to be measured its level is water, it is easy once we determine the level transmitter range to be used, namely 0-150 H 2 O, so that we can calibrate the transmitter 0-150? H 2 O. Or to obtain the hydrostatic pressure, we specify the formula to get the hydrostatic pressure, as follows:? = 1000 kg / m³ g = gravity = 9.8 m / s² h = 150 inches = 3.81 meters) P =? g h
Powered by TCPDF (www.tcpdf.org) Instrumentation Tools P = 1000 kg / m³ 9.8 m / s² 3.81 m P water = 37338 Pascal P water = 150 H 2 O How the case if the liquid contained in the vessel is a liquid that has a density of (or specific gravity = SG) are different, for example, 800 kg / m³ (0.8 g / cm³) for condensate. Is the level transmitter range 0-150 H 2 O is still valid? Of course not, because of the hydrostatic pressure caused by liquid which has a different SG will be different. Now let us count, what is the hydrostatic pressure caused by condensate at a height of 150 inches levels?? = 800 kg / m³ g = gravity = 9.8 m / s² h = 150 inches = 3.81 meters) P =? g h P = 800 kg / m³ 9.8 m / s² 3.81 m P condensate = 29870.4 Pascal Pascal unit