Simple water level measurement...

D

Thread Starter

dan_054

I want to use a standard pressure transmitter near the base of a cylindrical tank to gauge water level...plain old ambient temperature, filtered well water. The tank is about 30' - 35'tall. The pressure reading will indicate the height of the water and with known tank geometry, I can calculate gallons, right? Right.

How does the transmitter need to be connected to the tank? Does the size of the fitting(s) matter?

I noted that at least one big-name manufacturer makes a transmitter specifically for this purpose, and the transmitter has a flange and diaphragm for connection to the tank. Is this necessary, or can I just tube up a regular old transmitter to a handy pipe fitting near the bottom?

If the tank is open to atmosphere at the top, do I need a differential pressure transmitter with connections at the top and bottom?

Hair-splitting accuracy isn't necessary for this application...I can easily tolerate errors of maybe a couple or three inches or so.
 
J

JAMES JOHNSON

You may use a small fitting 1/4" or 3/8" without effecting the reading. Run the line to the differential transmitter 'High' Port. Leave the
Low port open to atmosphere. The physical eleveation of the transmitter will be your referance in this configuration. You will need to
include a zero offset representing the difference between the bottom of the tank and the transmitter level. There are other ways to
accomplish this (static lines, bubbler systems) but this seems to be what your looking for. Good luck.

 
If it is an open tank, you just need a 1 input pressure transmitter with calibration referenced to atmosphere.
A flanged transmitter is not necessary. Your solution will work well. Though, dont't forget to install a small valve between the tank
connection and your transmitter. It will be easier to change the transmitter while the tank is full!
Another valve very next to your transmitter process input will be necessary to purge the process pipe from air when the tank is first
filled. You will be sure then that filled water reach the transmitter pressure input and no air will be trap between.

Good luck.
 
W
In this application, you can get away with using darn near any old pressure transmitter.

The reason for the flange and diaphragm arrangement is that water tanks (yes, even drinking water tanks) fill up with sediment, and it is useful to shield the transducer from the sediment. Also, water freezes, and a fluid-filled diaphragm seal with a liquid that freezes well below the freeze point of water will protect the transducer. The tank itself may not freeze,
but small appurtenances may well.

I always include a shutoff valve in the piping, just so I can get the pressure transmitter off the tank for repair or replacement without blowing
the tank down. Sometimes, especially in a drinking water situation, it is nearly impossible to blow down a tank used for head pressure on a
distribution grid.

You may also, depending on your location, want to use an environmental enclosure, with either insulation or heat, to preserve the transmitter from very cold temperatures. O'Brien bags work very well.

Walt Boyes

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C

Curt Wuollet

> I want to use a standard pressure transmitter near the base of a
> cylindrical tank to gauge water level...plain old ambient temperature,
> filtered well water. The tank is about 30' - 35'tall. The pressure
> reading will indicate the height of the water and with known tank
> geometry, I can calculate gallons, right? Right.
>
> How does the transmitter need to be connected to the tank? Does the size
> of the fitting(s) matter?

Same level as bottom of tank and the size of the fittings shouldn't mattere as there is no flow.

> I noted that at least one big-name manufacturer makes a transmitter
> specifically for this purpose, and the transmitter has a flange and
> diaphragm for connection to the tank. Is this necessary, or can I just
> tube up a regular old transmitter to a handy pipe fitting near the
> bottom?

Pressure tap should be at the bottom if dead ended, if open ended it should be at the same height as the bottom.

> If the tank is open to atmosphere at the top, do I need a differential
> pressure transmitter with connections at the top and bottom?

No.

> Hair-splitting accuracy isn't necessary for this application...I can
> easily tolerate errors of maybe a couple or three inches or so.

Should be fine.

Regards

cww
--
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Consultancy: Wide Open Technologies: Moving Business & Automation to Linux.
 
Water height can be converted to psi. 27 inches of water height (also known as inches of water column ("wc)) is equal to 1 psi. It does
not matter what the diameter of your tank is. Any pressure transmitter will work. You would not want to use a differential pressure
transmitter. If your tank is 30 feet high you would see a pressure of about 13.3 psi when it is completly full. e.g. If you read a
pressure of 8.25psi that would be equal to 18.56 feet of water or 62% full.

I want to use a standard pressure transmitter near the base of a cylindrical tank to gauge water level...plain old ambient temperature,
filtered well water. The tank is about 30' - 35'tall. The pressure reading will indicate the height of the water and with known tank
geometry, I can calculate gallons, right? Right.
 
S

Satish Salankimatt

You can measure the level of water tank using any pressure measuring device. You can use flanged pressure transmitter but it does not
contribute for any improvement in the measurement, as the application of flanged transmitters is for viscous fluids level measurement. While
measuring level of a viscous fluid if you use a tube type impulse line, there is a possibility of the impulse line choking. Flanged
transmitters are also used when the fluid is corrosive and the diaphragm material is chosen so as to withstand the corrosive action of the
fluid.

In case of water level measurement you need not worry about size of impulse line as you are measuring the pressure.

If you are using a DP transmitter then connect the HP side to the tank and LP impulse line should be vented to atmosphere(since it is open
tank) with the help of a impulse line facing the ground.

Bye

Satish Salankimatt
 
Yes, you can use a standard pressure transmitter, but it must be a relatively low pressure transmitter (in the range of 0 - 20 psig to
give reasonable accuracy. You can expect to see a maximum pressure with 35 feet of about 15 psig.

The size of the fitting does not matter. The flange mount is used when there is an opening in the vessel that will accept the flange. A small
valve (0.5 to 1.0 inch) with tubing is sufficient. Remember to protect the tubing and transmitter from sub-freezing temperatures.

For a tank open to atmosphere, the standard pressure transmitter may be used subject to the above caveat, however, a dP cell will also work, and will give greater accuracy. Make sure to protect the vent side of the transmitter from wind, rain and insects.

Nearly all the applications I have had experience with have used a dP cell, with excellent accuracy.

John Beck
Control System Engineer - Retired
 
From: [email protected]
To: [email protected]
Subject: Re: APPS: Simple water level measurement...

Dan:

Rather than drilling a hole in the bottom of your tank to mount a fitting, why not just hang the transducer from the top? KPSI and others make
hanging differential pressure transducers, the 4-20 ma wiring and atmospheric reference hose are in the same cable. A bellows on the other end of the cable keeps moisture out.

Joe
 
You can use 1/2" NPT process connection if there is not sedimentation in the bottom tank. If you have this you need to use 2" flange ANSI 150. The
process connection can be through impulse line for 1/2" option. Use cut off valve between tank and transmitter for maintenance purpose. Right you
can calculate the volume.

Also, you can use any gage pressure transmitter because you tank is open. Only in closed tank you need use differential pressure transmitter.

Regards,

Jorge Diaz Z.
GPI Ltda
Chile
 
R

rameshbparmar

Hi,

The principle odf level measurement is

p = hdg

where p= hydrostatic pressur
h= height of the tank
d= densiy of the liquid
g= gravitational force

If density is constant then p=h
as g= g/gc=1

this is principle of measurment.

Ramesh Parmar
 
A

Alan Rimmington

You should just need to connect the pressure sensor near the bottom of the tank for most accuracy. Record pressure when empty, half full, and full just to confirm everything is working as expected. 1 metre of water = 0.1 bar pressure
or roughly 1 foot of water = 0.304 PSI. The metric system is easy to convert so 10 metres of water = 1 bar. You will find this will vary according to the density of the water (mineral content etc) and height above sea level, but your
scaling check/tuning while filling should give you fairly good results.

Good luck

 
Hello

Nice web site.
We have a new pressure sensor product not shown on line yet.

30’ – no problem
Housing – aluminium (option)
Temp - -10 to 80 deg. C
Supply Voltage – 13 to 36 VDC – you supply
Output – 4-20 mA
Wetted material – 304 or 316
Probe – 1 3/8” OD x about 6”
Unit is suspended on a custom cable.

Just connect VDC to the probe – you get 4-20 out - convert that to a scale. Many products are on the market to convert 4-20 to a level in digital. We have one that has 4 programmable relays, a bar graph showing level and a 3-digit % scale.

You would have to email us direct for a price. Each unit is custom.

You may see some other tank level ideas on our site at: www.almegcontrols.com

PS - the pressure unit has a breather tube in the cable.

Thanks Bob Hogg

 
A calibrated accurcy of 2.5% (any transmitter) will give you ± 0.75".
Differential transmitter are for pressurised vessel.
A simple 1/2" connection at the bottom.
Volume OK if the geometry is linear. If the tank is horizontal and cylindrical you need a linearising function. This function depends upon the ratio: measured height versus tank diameter. Does it answer your question(s) ?
 
If you want good resolution in your capacity calculation use a transmitter with a diaphram at least 3" in dia.
Cheers
Mark Ray
 
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