Horizontal Tank Level Monitoring with AB Control Logix

Hi all,

I need some guidance in how to approach some PLC coding for a tank farm, using RSLogix 5000 with a 1756-L62 processor & 1756-IF61 analog cards.

There are a number of horizontal tanks that I need to monitor the levels of.

They are installing Endress+Hauser FMP40 level sensors.

I'm currently thinking of either doing up some sort of lookup table, but not quite sure how to approach this.

Another thought was to possibly program the level transmitter with the tank type/size/parameters and possibly get a 0-100 signal out of it (4-02mA = 0 to max level in litres).

I am interested in your comments as to which way would be better/effective/more efficient or if there's another way I could approach this.

If MM are in millimeters and L are in liters, you can use a Fgen instruction, where your input is 4-20 mA scaled to your millimeters and you get liters at Fgen output.
Attached Fgen instruction from Rslogix 5k help file.

widelto said:

If MM are in millimeters and L are in liters, you can use a Fgen instruction, where your input is 4-20 mA scaled to your millimeters and you get liters at Fgen output.
Attached Fgen instruction from Rslogix 5k help file.

Click to expand...

Ty. I've seen this function referenced in other Horizontal Tank threads. However I'm currently updating a current PLC project file that uses Ladder Logic. I believe that the FGen function is not available in relay ladder logic. Forgive me if I am wrong.

3 methods
1) Wrap the FGEN routine into an AOI to use it in ladder
2) Roll your own FGEN AOI writing your own code
3) Use the MSCV instruction where the master axis is the height and the slave axis is the Litres

d.dinh said:

I believe that the FGen function is not available in relay ladder logic. Forgive me if I am wrong.

Click to expand...

No. it´s not available in relay ladder logic, that is explained at the beginning of the file I sent.

I seem to remember someone using the CAM instruction for this.

I've never tried it myself, but it looked as if it might be a a good solution.

http://www.plctalk.net/qanda/showthread.php?t=64671&highlight=horizontal+tank&page=2

http://www.plctalk.net/qanda/showthread.php?t=64611&highlight=horizontal

This one is also good:
http://www.engineeringtoolbox.com/content-cylindrical-tank-d_1301.html

E+H FMP40 has horizontal tank linearization built into the transmitter. Let it do the math and give you a clean, linear 4 to 20mA output based on the diameter of your particular vessel. Then you can scale it into whatever engineering units you or your customer want.

d.dinh said:

Hi all,

I need some guidance in how to approach some PLC coding for a tank farm, using RSLogix 5000 with a 1756-L62 processor & 1756-IF61 analog cards.

There are a number of horizontal tanks that I need to monitor the levels of.

They are installing Endress+Hauser FMP40 level sensors.

I'm currently thinking of either doing up some sort of lookup table, but not quite sure how to approach this.

Another thought was to possibly program the level transmitter with the tank type/size/parameters and possibly get a 0-100 signal out of it (4-02mA = 0 to max level in litres).

I am interested in your comments as to which way would be better/effective/more efficient or if there's another way I could approach this.

Click to expand...

If you know the relationship between the analog input counts and the level (i.e. data), you can type the data into Excel, plot it in a CHART, and have Excel do the work, and show you the formula (right click the line while viewing the chart. Easy, quick, .

Then, just a CPT to enter the formula.

Move on.

It all depends on the spec. How accurate do you need the tank volume ?

Getting accurate contents (volume) measurements from any shape (or orientation) of a tank is very difficult if you base your calculations on the tank's design (or even measured) dimensions.

Tanks are never perfectly round (or square), and tank ends are most often not flat !

Also, horizontal tanks are rarely horizontal !

The "lookup" method is undoubtedly the best, but it usually involves a tank "calibration" phase where you progressively fill the tank through a good flowmeter. It can be a lengthy process, but gives the best results.

It all depends on the accuracy you require, and the amount of time and effort you spend on commissioning.

In many cases the relationship between height and volume is difficult to express mathematically (especially if the tank has "inclusions" such as heater coils, mixer paddles, etc.) the brute force "fill and measure" approach really is the only correct way to get a good volume result.