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
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:
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.
See also: Vertical pressure variation
Hydrostatic pressure is the pressure exerted by a fluid at equilibrium due to the force of gravity. 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:
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.
In medicine, hydrostatic pressure in blood vessels is the pressure of the blood against the wall. It is the opposing force to oncotic pressure.