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Which is the best sensor (cheapest if posible smiley-grin) to measure distances of low range 2/5cm to 20cm with interval of 0.1cm. Or is better measure distances with any method ? I'm needed precision of measure 0,1cm as minimum. Thanks for your answers.
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Hi!

Im doing a project where i'm measuring the water level in my espresso machine. I have tried a couple of different things:

1) Float sensor - On/off a high/low liquid level
2) Capacitive sensing - http://www.embedds.com/arduino-based-capacitive-liquid-sensor/

Finally i tried with a Ultrasonic sensor and it worked great.

http://iteadstudio.com/store/index.php?main_page=product_info&cPath=9&products_id=52

You will have a almost unlimited resolution 1/0,3cm.

The only problem there are is that you will have too keep it dry. So if you have unsteady liquid, it is probably a bad method... 
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I'm needed precision of measure 0,1cm as minimum
Can you tell more about it ? how your reservoir looks like?

0.1cm means that adding liquid / removing must be quite easy otherwise you get disturbances/noise ...

Have you considder putting your reservoir on a digital scale and measure its weight?

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Rob Tillaart

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I'm needed precision of measure 0,1cm as minimum

Wuups! Sorry!  smiley-red

Maybe it could be done with a very accurate pressure sensor and a tube...

What kind of liquid are we talking about? Consistency? Chemical?
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if you change the form of your watertank the volume difference will be centimeters instead of millimeters.

example if you have a tank of 50 cm x 20 cm   a raise of 0.1 cm height  equals  0.1 x 50 x 20 == 100cm3

if you have a tank that is 25 cm x 10 cm  a volume raise of 100 cm3  = 100 /10 /25 =  0.4 cm raise so a less accurate sensor is needed ..

just a thought...
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I did some experiments on capacitive liquid level sensing, see http://arduino.cc/forum/index.php/topic,71628.msg536307.html#msg536307 for my results. The biggest problem I had was with water clinging to the sensor, making the "empty" reading unreliable. But I was using a very crude sensor.
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http://www.adafruit.com/products/464
Looks like some new stuff they just started carrying. Want to try?
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Interesting sensor !

As it works by the pressure of the fluid, it should work for all fluids (Let my Arduino decide when I'm out of beer smiley
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I have been using that eTape liquid level sensor for a half year (not with Arduino), works very well. It is also supposed to work with powder.
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Interesting sensor !

As it works by the pressure of the fluid, it should work for all fluids (Let my Arduino decide when I'm out of beer smiley


Yes, but keep in mind that a fluids density has a direct bearing on what hydrostatic pressure it will develop for a given hight. Water (at standard conditions) is said to have a specific gravity (SG) of one, and all other pure liquid materials state their SG as a ratio to water. Gasoline might have a SG or .5 and an acid might have a SG of 2.

 So the calculation used to measure the sensor that converts pressure to hight will have to include the SG of the liquid being measured. And worst if the liquid isn't composed of one known material of known SG, but rather a mixture of several different and possibly variable ratios of liquids having different SG values then the liquid hight is impossible to measure with a single pressure measurement.

 Measure liquid hight via hydrostatic pressure can be an inexpensive, reliable, and simple method but one must understand the variables involved and the limitations of that method.

Lefty
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That is a wrong assumption Lefty, as far as I know they don't convert pressure to liquid level like in other type of sensors.

In fact, and in layman words, it works by having a long resistor vertically mounted inside the 'tape'. And if anything (liquid, powder, your finger) presses against it, it will change the resistance and hence you know the top level of the liquid.

In fact you don't have to do any material specific calculations with the sensor, it returns the same results no matter it's water, gasoline or sand. I am in fact using it with something that has 1.5 times the density of water but I used water to cross check before using it for real.

http://www.milonetech.com/uploads/eTape_Datasheet_12110215TC-12.pdf

Quote
The eTape sensor's envelope is compressed by hydrostatic pressure of the fluid in which it is immersed resulting in a change in resistance which corresponds to the distance from the top of the sensor to the fluid surface. The eTape sensor provides a resistive output that is inversely proportional to the level of the liquid: the lower the liquid level, the higher the output resistance; the higher the liquid level, the lower the output resistance.

Here is a video on YouTube:
http://www.youtube.com/watch?feature=player_embedded&v=c3VJPi1TeSY
« Last Edit: December 19, 2012, 07:27:02 am by alexacl » Logged

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That is a wrong assumption Lefty, as far as I know they don't convert pressure to liquid level like in other type of sensors.

In fact, and in layman words, it works by having a long resistor vertically mounted inside the 'tape'. And if anything (liquid, powder, your finger) presses against it, it will change the resistance and hence you know the top level of the liquid.

In fact you don't have to do any material specific calculations with the sensor, it returns the same results no matter it's water, gasoline or sand. I am in fact using it with something that has 1.5 times the density of water but I used water to cross check before using it for real.

http://www.milonetech.com/uploads/eTape_Datasheet_12110215TC-12.pdf

Quote
The eTape sensor's envelope is compressed by hydrostatic pressure of the fluid in which it is immersed resulting in a change in resistance which corresponds to the distance from the top of the sensor to the fluid surface. The eTape sensor provides a resistive output that is inversely proportional to the level of the liquid: the lower the liquid level, the higher the output resistance; the higher the liquid level, the lower the output resistance.

Here is a video on YouTube:
http://www.youtube.com/watch?feature=player_embedded&v=c3VJPi1TeSY

Well we can agree to disagree I guess. The fact that the following is stated:

Quote
The eTape sensor's envelope is compressed by hydrostatic pressure of the fluid in which it is immersed resulting in a change in resistance which corresponds to the distance from the top of the sensor to the fluid surface.

 That states the resistance change is the result of the hydrostatic pressure against the resistance tape, not the absolute hight of the liquid level. And hydrostatic pressure is a product of the materials density and hight. Look up the terms 'hydrostatic pressure' or 'head pressure' and see if you might not agree with me in the final outlook. Now if they misstated that it's measuring hydrostatic pressure applied against the tape, but rather the absolute liquid level relative to the tape then they are not using hydrostatic pressure but some other means which wouldn't be obvious to me.


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Hydrostatic pressure
See also: Vertical pressure variation
Hydrostatic pressure is the pressure exerted by a fluid at equilibrium due to the force of gravity.[1] A fluid in this condition is known as a hydrostatic fluid. The hydrostatic pressure can be determined from a control volume analysis of an infinitesimally small cube of fluid. Since pressure is defined as the force exerted on a test area (p = F/A, with p: pressure, F: force normal to area A, A: area), and the only force acting on any such small cube of fluid is the weight of the fluid column above it, hydrostatic pressure can be calculated according to the following formula:
,
where:
p is the hydrostatic pressure (Pa),
ρ is the fluid density (kg/m3),
g is gravitational acceleration (m/s2),
A is the test area (m2),
z is the height (parallel to the direction of gravity) of the test area (m),
z0 is the height of the zero reference point of the pressure (m).
For water and other liquids, this integral can be simplified significantly for many practical applications, based on the following two assumptions: Since many liquids can be considered incompressible, a reasonably good estimation can be made from assuming a constant density throughout the liquid. (The same assumption cannot be made within a gaseous environment.) Also, since the height h of the fluid column between z and z0 is often reasonably small compared to the radius of the Earth, one can neglect the variation of g. Under these circumstances, the integral boils down to the simple formula:

where h is the height z − z0 of the liquid column between the test volume and the zero reference point of the pressure. Note that this reference point should lie at or below the surface of the liquid. Otherwise, one has to split the integral into two (or more) terms with the constant ρliquid and ρ(z')above. For example, the absolute pressure compared to vacuum is:

where H is the total height of the liquid column above the test area the surface, and patm is the atmospheric pressure, i.e., the pressure calculated from the remaining integral over the air column from the liquid surface to infinity.
Hydrostatic pressure has been used in the preservation of foods in a process called pascalization.[2]
In medicine, hydrostatic pressure in blood vessels is the pressure of the blood against the wall. It is the opposing force to oncotic pressure.

Lefty
« Last Edit: December 19, 2012, 09:16:52 am by retrolefty » Logged

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Now if they misstated that it's measuring hydrostatic pressure applied against the tape

As far as I know they did not mention that they are measuring hydrostatic pressure, they simply state that the resistor envelope is compressed by hydrostatic pressure.

The 'amount' of pressure doesn't matter. In fact if you press against the sensor/resistor and slide your finger up and down, you are simulating change in liquid level. Hook up a multimeter and try it yourself (I just did). It's like a slider.
« Last Edit: December 22, 2012, 01:59:14 am by alexacl » Logged

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Now if they misstated that it's measuring hydrostatic pressure applied against the tape, but rather the absolute liquid level relative to the tape then they are not using hydrostatic pressure but some other means which wouldn't be obvious to me.
This is a multi-element sensor that use hydrostatic pressure to add/subtract individual resistive elements from the circuit. Individual element sensitivity is related to pressure, but total restiance is proportional to and determined by liquid level only.
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Now if they misstated that it's measuring hydrostatic pressure applied against the tape, but rather the absolute liquid level relative to the tape then they are not using hydrostatic pressure but some other means which wouldn't be obvious to me.
This is a multi-element sensor that use hydrostatic pressure to add/subtract individual resistive elements from the circuit. Individual element sensitivity is related to pressure, but total restiance is proportional to and determined by liquid level only.

Then I would have called it a pressure sensitive tape rather then use the word hydrostatic which carries a more formal definition and is a widely used method of measuring total liquid 'head pressure'.

 At the refinery I worked at we did have switched resistance ladder type level gauges based on an array of magnetic reed switches and resistors that would engage resistance elements inside a non-magnetic tube, as a round hollow magnetic 'donut' would ride on the surface of the liquid level engaging resistors as it followed the liquid level from the outside of the tube. I seem to recall it had 1/8" resolution steps over a 40ft span. Quite expensive and designed for sever service.
 

Lefty
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