math used to determine level of water in a tank

need to know how to determine how much water in a tank by taking the MA output of a transmitter and converting it to feet of H2O.

thanks

You did not state what shape the tank is, look at this for vertical and horizontal cylindrical tanks, nor what type transmitter is being used, is it pressure or level transmitter; the make and model of transmitter and plc may offer more detailed information.

http://www.plctalk.net/qanda/showthread.php?t=25255&highlight=tank

It is a pressure transmitter the tank is a ground storage tank with the pressure line coming off the bottom of the tank it is 45 ft tall with a capacity of 8 MG of water.

What is the range of your transmitter. The most common ma range is 4 to 20 ma. What does 4 ma=? and 20 ma= ? If yours is different what does the high and low equal


You first must know what your milliamps represent in terms of engineering units.

Here is an example:

The most common ma range is 4 to 20 ma, so let’s say this represents 0 to 100” H2O

Where 4 ma = 0”H2O and 20 ma= 100”H2O.

(((Raw ma – 4)/ 16) * 100 ) / 12= 4.167 ft H2O

As Mickey points out, you need to tell us more about both your transmitter and your PLC's analog inputs. I'm assuming you're wiring that 4 - 20 mA signal to a PLC's analog input. You didn't specifically say so, but this is a PLC site.

First, you're measuring pressure to a maximum depth of 45 feet of water. What is the maximum range of your transmitter? Can you calibrate it so that it outputs 4 mA at zero depth and 20 mA at 540 inches (45 feet)? Pressure increases with depth at the rate of 1 PSI for every 27.68 inches. The pressure at the bottom of a 540 inch tall (full) tank will be 19.5 PSI (above atmospheric). If you're using a transmitter rated for 0 - 20 PSI, you can probably calibrate it as I indicated above. If you're using a transmitter rated for 0 - 50 PSI, you will end using less than half of the transmitter's available range.

The next point you need to consider is the PLC's analog input. Are you using a 12-bit or a 16-bit analog input? A 12-bit analog input divides the the 4 - 20 mA range into 4096 steps. A 16-bit analog input gives you 32,768 steps. For a 12-bit AI, each incremental step represents a little over 1/8 of an inch (0.13") or 1953 gallons of water.

Then once you get the height of the liquid in the tank by converting the pressure input as Steve said, then you can use the standard volume of a cylinder formulat to calculate the volume in gallons or whatever, V = 2 x Pi x r^2 x H, where H is the height and r is the radius or the tank.

micro said:

... with the pressure line coming off the bottom of the tank...

Click to expand...

If that is an electrical signal line, no problem.

If that line is actually a tube filled with water, then you need to locate the pressure transmitter at the same altitude as the bottom of the tank.

that is correct the pressure range is 0-20 psi 4= 0 ft. and 20= 45 ft. thanks for you info. micro