Water Level Detection

[premkumar9]

Hi,
For a project I need to sense the water level in a tank of 15 feet depth. Level to be sensed at 3 to 5 points. I am studying different possibilities. Can anybody pls give some ideas?

 

[MikeMl]

Mount a pressure sensor in the bottom of the tank

 

[premkumar9]

Mount a pressure sensor in the bottom of the tank

Is it a proven method. In fact I didn't think of this method. I was thinking about other methods.

 

[ericgibbs]

Is it a proven method. In fact I didn't think of this method. I was thinking about other methods.

hi,
For about 15ft water, ie 0.5Bar, 52kPA the attached sensor would work.
There are a range of MPX sensors available.

The problems are sealing the sensor and ideally you would need atmospheric on the other face of the diaphragm.
Check to ensure that they are suitable for liquid insertion.

You can buy ready cabled pressure sensors that have a small airtube within the cable. Outputs in volts or 4 to 20mA loop.

I dont know your budget.? but they cost around £50 to £100.

 

[Mr RB]

You can buy an absolute pressure sensor, that only has one tube (not a differential sensor). Then put the tube to the bottom of the tank (open) and the sensor on the top end of the tube above the liquid (on top of the tank etc).

Provided you don't let any air escape from the tube it will work. You can blow a little compressed air down the tube once it is installed if you need to. Some industrial systems permanently vent a small compressed air bleed into the tube, it bubbles out the bottom and can be used even in tanks with the thickest sludges etc.

 

[Boncuk]

Hi,

for water level detection use a sealed pressure sensor such as MPX2050DP.

It won't get in touch with water since it is completely encapsuled with two pressure ports to connect a PU-hose (PU=polyurethane)

To measure the accurate water height in the tank connect the positive pressure port to a PU hose (inner diameter 4mm) completely submerged to the ground of the tank. Leave the negative pressure port open (closing it with a short piece of PU-hose with a cigarette filter inserted taking care that no insects will block it).

The measured air pressure equals exactly the height of the water column in the tank.

At 50KPa the (linear) output voltage is 40mV while at zero pressure the output voltage has an offset of -2mV. Measuring differential pressure the measuring result will always be correct regardless of the present barometric pressure.

Here is the datasheet.

Boncuk

 

[aljamri]

or you can install external tube higher than your tank, use a small magnetic float, and install 3 to 5 reed switches along the tube.

 

[premkumar9]

Thank you

Thank you all for the good suggestions. I will come back after thinking about these suggestions.

 

[Boncuk]

Hi premkumar9,

I've already designed an Eagle package for the MPX2050DP.

The pin arrangement is such, that the pins will produde through the PCB if bent at a right angle.

Boncuk

 

[premkumar9]

Hi,

for water level detection use a sealed pressure sensor such as MPX2050DP.

It won't get in touch with water since it is completely encapsuled with two pressure ports to connect a PU-hose (PU=polyurethane)

To measure the accurate water height in the tank connect the positive pressure port to a PU hose (inner diameter 4mm) completely submerged to the ground of the tank.

It is necessary to keep the sensor at the ground level of the tank. Right?

 

[MikeMl]

It is necessary to keep the sensor at the ground level of the tank. Right?

Read this link

 

[premkumar9]

Read this link

So the answer to my question is 'YES' (sensor has to be at bottom level). Is it not?

 

[Boncuk]

The answer is definetely NO.

The sensor, if the suggested type with two pressure ports is used it might be located on the moon and still indicate the correct pressure, provided both ports are located near the "target" (Who wants to to install a 380,000km long plastic tube? )

Seriously, the sensor might be located far away from the water tank which is not much practicable because of the need of long PU-hoses. (cable is cheaper)

The end of the hose connected to the positive pressure port of the sensor has to reach down to the bottom of the water tank, because it has to transfer the height of the water colomn in an equivalent value of air pressure. The negative port remains unconnected to sense atmospheric pressure.

Pressure sensors with one port have the disadvantage, that the output voltage is only true on days with barometric pressure equalling the standard atmosphere air pressure of 1,013.2mb, 1,013.2 HPa or 29.92inch/Hg. (One side of the diaphragm is encapsuled under that pressure)

This type of pressure sensor might be used in aviation, since the flight level sytem relies on standard atmoshere settings. For take-offs and landings the altimeter has to be set to local pressure so the altimeter will indicate field elevation on the ground.

Boncuk

 

[premkumar9]

The answer is definetely NO.

The sensor, if the suggested type with two pressure ports is used it might be located on the moon and still indicate the correct pressure, provided both ports are located near the "target" (Who wants to to install a 380,000km long plastic tube? )

Seriously, the sensor might be located far away from the water tank which is not much practicable because of the need of long PU-hoses. (cable is cheaper)

The end of the hose connected to the positive pressure port of the sensor has to reach down to the bottom of the water tank, because it has to transfer the height of the water colomn in an equivalent value of air pressure. The negative port remains unconnected to sense atmospheric pressure.

Pressure sensors with one port have the disadvantage, that the output voltage is only true on days with barometric pressure equalling the standard atmosphere air pressure of 1,013.2mb, 1,013.2 HPa or 29.92inch/Hg. (One side of the diaphragm is encapsuled under that pressure)

This type of pressure sensor might be used in aviation, since the flight level sytem relies on standard atmoshere settings. For take-offs and landings the altimeter has to be set to local pressure so the altimeter will indicate field elevation on the ground.

Boncuk

Do you mean that a pressure sensor of suggested type will give correct O/P even if it is located 20 Ms (whatever height) above the tank, provided the tube connected to the port reaches the bottom of the water tank?

 

[Mr RB]

Both types of sensor will. The pressure at all points in a closed system will be the same, so the open end of the tube is at the bottom of the tank, at that pressure and so is the rest of the inside of that tube provided it NEVER leaks. The problem is that tubes do tend to leak, and most plastics are porous so in a proper system they feed a constant bleed of air into the tube, OR you have regular maintenance to blow the tube whenever the pressure readings get funky (low). It sounds like a hassle but in a lot of industrial situations it's best to keep the electronics high and dry well above whatever is down in the tank.

Or you can just put the sensor at the same height as the tank bottom.

 

[kinarfi]

I always liked the bubble tube, mount you switch anywhere above the top of the tank, hook an air supply to a tube going to the bottom of the tank and adjust the air flow to a slow bubble and measure the pressure on the line going into the tank, this works good for mud, sewage, and water, if it plugs up, the pressure will build and blow out the plug and continue working, another method is capacitive proximity detectors.
Kinarfi

 

[Boncuk]

Do you mean that a pressure sensor of suggested type will give correct O/P even if it is located 20 Ms (whatever height) above the tank, provided the tube connected to the port reaches the bottom of the water tank?

Yes!

All the sensor measures is the height of the water column in terms of air pressure. As the air density decreases with increasing height the open port (negative pressure) will compensate for the decreased air pressure.

You'll always get the true water height above tank bottom (provided the PU-hose reaches all the way down) regardless of the sensor location.

You can only get erroneous results if the tubing at the sensor pressure port is not 100% air tight. The smallest leak will lead to water filling the PU-hose up to the water tank level since the air will escape at the leakage, resulting in a measured water height of zero.

Using PU-tubing with inner diameter of 4mm (outer dia 6mm) the PU-tube has to be pushed beyond the collar widening the tube to 4.62 to 4.93mm (as given in the data sheet). Dunk the tube end in 70 to 80 deg/C warm water before slipping it over the sensor port.

It is advisable to leave an extra loop in the tube for maintenance reasons. Once slid over the collar, preventing pulling the hose back because of it's shape (the more you pull the tighter the tubing will become) you'll have to cut a small amout off to separate the tubing from the sensor. This will reduce the useable length. So cut to the absolute minimum means wasted material since you'll have to replace the entire hose to reconnect.

When cutting be careful not to damage the collar.

If you are interested in a PCB containing a TL074N wired as instrumentation amplifier with a gain adjustable from ~50 to ~5000, and and offset compensation circuit please PM me. I'll send you the schematic and PCB layout (single sided) via email. Board size is 2.275X2.1 inches.

Boncuk

 

[Boncuk]

Both types of sensor will. The pressure at all points in a closed system will be the same, so the open end of the tube is at the bottom of the tank, at that pressure and so is the rest of the inside of that tube provided it NEVER leaks.

A dual port sensor guarantees exact measurement with change of atmospheric pressure only. A single port sensor is only accurate if the local air pressure is the same as standard atmoshere of 1,013.2mb which it is charged with.

The problem is that tubes do tend to leak, and most plastics are porous so in a proper system they feed a constant bleed of air into the tube,

PU-tubes (polyurethane) are not porous and they are tested to withstand pressures up to 8bar. They are not sensitive to UV-light other than e.g. PVC, which gets unusable after one year of exposure to sunlight.

I used PU-tubing for electro-pneumatically controlled variable pitch fans with a max diameter of 2.85m with no leakage in the tubing. Since the tubing must be 100% leakage proof not to vary pitch angle inadvertantly, hence preventing fluctuations in air volume and fan pressure. Those fluctuations once became pretty nasty when a fan got out of control and busted the filters the contents of which unloaded into the operation room of a hospital while an operation was being performed.

OR you have regular maintenance to blow the tube whenever the pressure readings get funky (low). It sounds like a hassle but in a lot of industrial situations it's best to keep the electronics high and dry well above whatever is down in the tank.

Or you can just put the sensor at the same height as the tank bottom.

I fully agree with that. Leaving the sensor below water level a damaged (leaking) tube would allow the water to penetrate inside the sensor making it useless.

Boncuk

 

[premkumar9]

Thank you very much for your very informative reply. I never had to learn these things before. I will come back later.

 

[kinarfi]

Yes!
You can only get erroneous results if the tubing at the sensor pressure port is not 100% air tight. The smallest leak will lead to water filling the PU-hose up to the water tank level since the air will escape at the leakage, resulting in a measured water height of zero.
Boncuk

Bubble tube level gauge:
Got to disagree with you about air tight, if you have small leaks in the bubble tube, as long as you have bubbles coming out of the tube from the bottom, you will still have a good reading until the air flow is so great as to create pressure drop from to much air flow. That's one of the other nice things about bubble tubes.
Kinarfi