Measuring liquid level of a sealed container

What is the liquid?

How does the liquid get in and out of the cylinder? 

How accurately do you need to know the liquid level?

Without knowing more about the system, your best bet is likely to somehow measure the weight.

There might be a way to use acoustic resonance, but it would require lots of experimentation. (I tried it here)

Thank you for your answer.

This is a hydraulic liquid used in shock absorbers. It's exact name I'm not aware.

And measuring weight is not possible as this is tightly fixed on a a big machine which moves back and forth violently when it is in operation.

Liquid is filled in from a opening at the bottom of the cylinder and later it is sealed tightly and fixed on the machine.it can't be open again. Only on repair its open and takes long time to open and close the sealing .

It is ok if there is a 1 to1.5  cm error in the measurement .

<speculation>It might be worth talking to the manufacturer of the shock absorbers for advice.

It's possibly not feasible to retrofit your own sensors, and any proposed sensor might not detect all failure modes anyway, meaning that the shock absorbers may still need to be swapped out at maintenance cycles even if the liquid volume has not changed. It might not even be supported if it involves drilling mounting holes for sensors and so on. Leaving you even more liable for unexpected failures. Achieving (and guaranteeing) 1-1.5% accuracy as you refer to, might be extremely difficult, and perhaps the manufacturer might best know of ideas to help. On the other hand, detecting liquid leaks, or detecting 'anomalies' (through say vibration or sound differences) is a horse of a different color.

I'm sure it's not an unsolvable problem for the manufacturer, but it might not be a viable product if customers won't purchase it, if (say) it's way too expensive to have re-designed shock absorbers with internal sensors, compared to the cost for customers to swap them out periodically. </speculation>

In a hydraulic cylinder, the stroke (h) of the cylinder is proportional to the volume of liquid in the cylinder = 3.14*(D1*D1/4)h

This sounds like a difficult problem - especially as any modification to the shock absorber could be very dangerous.

Why do you want to measure the liquid level? What exactly is the problem you're trying to solve? Perhaps there's an easier way to do this. For instance, if this shock absorber was on a car I'd suggest perhaps measuring the suspensions movement instead. Without knowing your end goal, it's hard to suggest solutions.

Perhaps an unconventional ultrasonic solution. I don't know if this concept will work, but...
The cylinder with the hydraulic oil will be more dense than the cylinder without it. Set the sensor outside the second cylinder and at a level just below the oil. If the oil level falls, the ultrasonic reading should change.  Maybe it's not ultrasonics, but whacking an empty cylinder produces a different sound that whacking one filled with liquids.  Maybe it's a series of vibrational sensors.

Thank you all for your time and effort for trying to help. This is just a routine check. It's like you check oil level by dip stick on car time to time or if you have not used it for a while. You just make sure it for safety. This is very crucial in this machine as if it fails many lives may be lost. So it's a safety measure.

I did some searching, there was a suggestion to check temperature from outside along the cylinder as liquid will quickly absorb heat energy rather than air. At the interface there will be change of temperature. Will this be a practical solution what are the challenges to it?

We still use Gas cylinders for cooking purpose and there is liquid inside(LPG).

For that one idea could be to send some wires inside for level measurement using sensor from the Gas outlet...Typically it would need at max 4 wires if you use some kind of I2c, Uart sensor.?

Hasanga said:

This is very crucial in this machine as if it fails many lives may be lost.

If this is true then it really isn't compatible with asking random strangers on the internet for a solution - especially when they don't have all the background information.

I'll be the first to say that the people replying to you here in E14 are some of the smartest and most helpful people you'll find online. However, this is not enough if you're working on something with the potential to kill.

Speak to the manufacturer of this machine. Get someone who is qualified to devise a solution to do so. Anything else is irresponsible and perhaps even criminal.