Finding specific weight of unknown fluid in a manometer

In summary, the original poster was having trouble finding the correct answer for a problem involving pressure and distances h1, h2, and h3. They assumed the pressure was the same at levels A and B and established expressions for both, but ended up with the wrong answer. After trying a different approach and repositioning levels A and B, they were able to get the correct answer of 13.0 kN/m3 for the specific weight. There may have been some rounding errors in the original calculation, and it's important to note the correct units of specific weight, which are kN/m3.
  • #1
Bolter
262
31
Homework Statement
See below
Relevant Equations
N/A
Hey everyone!
Here is the problem I have been tackling but did not end up with the correct answer

Screenshot 2020-09-30 at 15.28.43.png

The extra red arrows I have labelled in myself and called these distances h1,h2 and h3. I assumed the pressure to be the same at levels A & B, hence i have established expressions for both of these and got the following

IMG_5301.JPG


However this is not right and the right answer is actually 13.0 kN/m3. What did I do wrong exactly?

Thanks
 
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  • #2
Nevermind I think I got the right answer now by doing something slightly different. I repositioned A & B to to be at the bottom of the manometer on both sides like this

Screenshot 2020-09-30 at 15.31.27.png

I redid the method I used before in my first post. I got specific weight to come out as 12.95320... kN/m3 which is approx 13.0 kN/m3 to 3 sig figs
 
  • #3
Bolter said:
Nevermind I think I got the right answer now by doing something slightly different. I repositioned A & B to to be at the bottom of the manometer on both sides like this

View attachment 270242
I redid the method I used before in my first post. I got specific weight to come out as 12.95320... kN/m3 which is approx 13.0 kN/m3 to 3 sig figs
Probably round off errors. My calculation according to the method in the original post gave, to 3 sig. figs., 1.32×103 kg/m3. Note the correct units of density; the factor of ##g## cancels out.
 
  • #4
kuruman said:
Probably round off errors. My calculation according to the method in the original post gave, to 3 sig. figs., 1.32×103 kg/m3. Note the correct units of density; the factor of ##g## cancels out.

Its not asking for density, its asking for specific weight I believe which has units kN/m3
 
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Likes kuruman
  • #5
You are absolutely correct.
 

Related to Finding specific weight of unknown fluid in a manometer

1. How does a manometer work to find the specific weight of an unknown fluid?

A manometer is a device used to measure the pressure of a fluid. It works by comparing the pressure of the fluid being measured to a known reference pressure. In the case of finding the specific weight of an unknown fluid, the manometer is filled with the unknown fluid and the pressure is compared to a known reference fluid, such as water.

2. What is the formula for finding the specific weight of an unknown fluid using a manometer?

The formula for finding the specific weight of an unknown fluid using a manometer is: specific weight = (difference in pressure between the two fluids) / (density of the reference fluid x height of the manometer).

3. Can a manometer be used to find the specific weight of any type of fluid?

Yes, a manometer can be used to find the specific weight of any fluid as long as the reference fluid is known and the manometer is calibrated correctly.

4. What are the units of measurement for specific weight?

The units of measurement for specific weight are typically pounds per cubic foot (lb/ft3) or newtons per cubic meter (N/m3).

5. Are there any limitations to using a manometer to find the specific weight of an unknown fluid?

One limitation of using a manometer is that it can only measure the difference in pressure between two fluids, so it may not be accurate for fluids with very low or very high specific weights. Additionally, the accuracy of the measurement may be affected by factors such as temperature and air bubbles in the manometer.

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