tank capacity

[NetNathan]

Here is what I was talking about with the log scale.
This a vacuum tube based on Pirani/Convectron principle.
Look at the pressure scale per decade.

 

[NetNathan]

Compare the pic above with the readings here, very close...

0m - 0 m3
0.5m - 5.8 m3
1.5m - 21.7 m3
2.5m - 38.6 m3
3.5m - 56.1 m3
4.5m - 72.3 m3
5.5m - 83.0 m3
6.5m - 93.0 m3
7.5m - 102.9 m3

 

[NetNathan]

Remember when comparing the 2. the first one shows pressure across 3 decades of Torr.
The OP is comparing 1 decade of pressure (approx 10-1000 Torr).
760 Torr = ATM = 14.7 psia
750 Torr = 1 Bar

 

[Peter Nachtwey]

You guys aren't listening

Here is a plot of the volume vs level. Does this look like a logarithmic function? The way this thread is going it would have been better for the OP to do 9 compares and 9 SCP blocks to interpolate in between.

 

[NetNathan]

Hmm.....that does show a different result.
I assumed that he since was talking about the response from a pressure sensor, that the signal followed this common log curve for pressure.

 

[Peter Nachtwey]

I told you so. Few listened.

Hmm.....that does show a different result.

What is different from what I told you above? I said there were too linear segments. I also said in my first post that is wrong to blindly start applying polynomials to data.

I assumed that he since was talking about the response from a pressure sensor, that the signal followed this common log curve for pressure.

I didn't assume anything. I plotted the data first which is what all of you should have done. Then I can tell how to best fit an equation(s) to the data.

 

[NetNathan]

I fully understand you and stand corrected, thanks for the lesson.

Not to carry this too far...... I think everyone has assumed a lot since we have not heard back from the OP.

Since he is using pressure to measure amount of liquid in the tank, the sensor could be a diaphragm style. Which means it could be measuring the weight of the liquid in the tank, by pressure on the diaphragm. This means it doesn't really matter how the tank is shaped.

..

 

[maxketcham]

the thing that is confusing me is why? he has a pressure gauge reading whatever scale it is, call it inches of water. it doesn't matter what shape the tank is, the reading should be linear an measure the actual height of the column of fluid above the sensor, nothing more... The conversion to gallons/liters/volume may be a function of tank shape, but the pressure seen by the gauge will only be affected by a change in height (not volume) of the measured fluid (not a capillary system, that is logarithmic)so if you have a rough idea of the cross sectional area of that section of the tank, all you need to do is calculate for what is up to that level. then either use the geometry of the to do the calculations or a look up table to give an approximate value. It's not really hard to do either. OH and by the way if you have a look up table you have a way to do a crude "reverse engineering" the shape of the tank, by knowing the previous pressure/volume readings.

 

[Lancie1]

The thing that is confusing me is why? He has a pressure gauge reading whatever scale it is, call it inches of water.

I think I might contribute something here. I have used many pressure gages to measure the height of a liquid varuous tanks. There are several manufacturers that provide these tank level gages that are calibrated in height of liquid, not pressure. In other words, you specify the height range (feet, inches, meters, or whatever) to be measured, and the company sends you a gage with a pressure transducer and a digital readout that reads in whatever height range you specified. The actual pressure does not show up anywhere.

 

[maxketcham]

The pressure(no matter what the scale reads) on a column (tank)of fluid is dependent only on the height of the fluid, the fluid density (1 for water) and the acceleration of gravity (32 ft per sec per sec (at sea level)) so the only variable is essentially the height of the fluid. the shape of the tank, mass of the fluid nor volume of the liquid have any bearing on the pressure of the column.(See link)

http://hyperphysics.phy-astr.gsu.edu/hbase/pflu.html

 

[Lancie1]

Yes, but the gage in question only reads "height in meters", not pressure. The relationship between the height and the volume of this particular tank is not linear or logrithmic.

 

[maxketcham]

Yes, but the gage in question only reads "height in meters", not pressure. The relationship between the height and the volume of this particular tank is not linear or logrithmic.

exactly, so it should be linear, if it is not then he is doing a conversion somewhere or the gauge is reading incorrectly. (height in meters would be a conversion from pressure and should be linear as pressure is)

 

[Lancie1]

(height in meters would be a conversion from pressure and should be linear as pressure is)

No, the conversion from pressure to height is done internally by the gage (if it is the type I think it is). Those type of gages are calibrated at the factory to read height, so the user never sees, adjusts, or calibrates the pressure at all.

So you have height versus volume, and the volume is not linear due to the fact that the tank is irregularly shaped. We do not know how it is shaped , but because there is a greater change from 0 to 0.5 meters than from 0.5 to 1.0 meters, I think the tank starts with a smaller diameter, bulges out at 0.5 meters, then tapers in diameter up to the small neck. However, the tank could be a rectangular prism instead of round. Whatever the shape, it is such that the plotted curve of height versus volume is not linear. Instead of being linear, it looks more like posiitive one-half lobe of a parabola. The equation for a parabola is y = a(x-h)^2. Because this curve is only the part where x >= 0, then x must be 0 to 7.5 meters.