Level Measurement Using A Pressure Transmitter

MarkNightingale

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Hi Guys,

I am trying to get a level reading of a tank using a pressure transmitter, I have nearly got the formula that I wont but I'm struggling to get it into my PLC code.

I am using RSLogix 5000.
The pressure transmitter is 4-20mA range of 0-500mBar.
The tank is 3.8 meters tall and 1.7 meters wide. At full capacity it will hold 23,000 litres of water. There is a conical bottom on the tank which is the bit that I am finding it hard with.

I know that in a linear tank 27.71 inches of water exerts 1 P.S.I on the pressure transmitter.
This means that 0.70 metres = 1 P.S.I
So 0.70 metres = 0.069 Bar = 69mBar.

Therefore 3.8 metres = 374mBar.

So in my PLC code I am scaling the raw analogue input and scaling it between 0 and 500.

I am then taking that value and scaling it again between 0 and 23,000.

Now all this looks fine but when I meter water into the tank the values don't add up, so this is because of the conical bottom. I have scoured the internet, but without much luck so was hoping someone could point me in the right direction.

I have got all the tank dimensions so I am thinking of working back and using the volume of the tank.

Any ideas or suggestions would be grateful.

Thanks

Mark
 
Hydrostatic pressure does not care about shape of the wessel. If there is 10m of water above pressure transmitter, there is 1bar of pressure, regardles of wessel being s shaped, with cone, straight or any form you might imagine. Hydrostatic pressure is only dependant on depth.

Check your calibration, check your scaling values. Only if you need to know volume you need to take cone into account.
 
TurpoUrpo is correct, the pressure measurement will be directly related to the level of fluid above it regardless of the shape of the vessel. Where is the pressure transmitter mounted? Is it in the piping below the tank, or somewhere else? Be sure to take the location of the transmitter into consideration. It will only measure how much fluid is above it. If it's in the piping below the tank, you will have to offset the level measurement so that zero level in the tank will be more than zero pressure from the transmitter.

The volume though, is another story entirely. You'll have to take the pressure input and convert it directly to level - just as you described (the location of the transmitter into consideration). Once you have the level you'll need to use a LES and GEQ statements to determine which equation to use to find the volume of the tank:

H = overall height of conical section (just the cone, not the cylinder above it)
R = overall radius of tank
h = measured height of fluid (measured from bottom of cone)

if h < H:
Volume = (pi * R^2 * h^3) / (3 * H^2)

if h > H:
Volume = [(pi * R^2 * H) / 3] + [(pi * R^2) * (h - H)]

Basically, in the first equation you're concerned with the cone only. In the second the cone is full so you have the complete volume of the cone added to the additional volume associated with the cylindrical part of the tank. I would fill in most of the above equations with the constants you know based on the actual measurements of the tank and reduce them down as much as possible. If you supplied the measurements of the tank it would not be difficult to do. I think the only dimension missing from your description is the Height of the cone section.

Hope this helps.
 
Personally, many times I just use a triangle shape in place of the rounded conical section. Makes calculations easier.

Or I just take the volume of the bottom head, divide by its height and use that gallons per inch number along with the straight side gallons per inch factor to calculate total volume. If it's not too critical to measure the amount when only the bottom head has fluid.

Also, in 99% of all installations, there is what's called a 'dead leg' which is the measuring point of the pressure transducer to the bottom of the tank. That needs to be subtracted.

Hopefully the transmitter's measuring port is connected to its own flange on the tank. If it's sharing the outlet pipe/valve flange, you are likely to get a lower reading when the outlet valve is open and fluid wooosh's by.
 
Thanks for all your help and advice.

I thought that it didnt matter what shape the tank was but I have a brand new out of the box Endress and Hauser flow meter which is monitoring the amount of water going into the tank and the level and the amount dosed never add up.

I need to have a look at the scaling I am using and make sure the instruments are giving me what I think ie:4mA = 0mBar and 20mA = 500mBar.

If I dont have any success tomorrow I will post my scaling code and see if I am missing something.

Thanks again

Mark
 
Mark, instead of scaling to mBAR, scale directly to mm. If the fluid is water, or the specific gravity of the fluid is 1.0, then 500mBAR = 5080 mm, ie, 4mA = 0mm, 20mA = 5080mm (or 500mBAR = 200 inches).
 
Last edited:
Well, i would just scale to percentage of max. But that is me. It really does not matter, u just need to check the calibration, if its ok, then look at the code.
 
I thought that it didnt matter what shape the tank was but I have a brand new out of the box Endress and Hauser flow meter which is monitoring the amount of water going into the tank and the level and the amount dosed never add up.

The shape of the tank doesn't matter if all you're concerned with is measuring the level of the tank. But if you're going to use a flow meter to compare with the level, you're going to need to convert that level to a volume... see the equations above and the links in Alaric's post.

I would also agree with Alaric that it would make more sense to convert the input directly into the level measurement instead of the pressure.
 
Thanks for all your advice.

I think I am going to have to convert the level into volume. The customer wants to know how many litres of water are in the tank.

But as you have all said if I convert the 4-20mA signal into mm or metres it should be simple enough to use a calculation to display the litres in the tank.

I am just off to work now so will have a little play.

Thanks Again

Mark
 

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