How to perform liquid level sensing through the bottom of a metal tank
Part Number: TDC1000-C2000EVM
I am attempting to read the level of LPG in a steel container. I have sourced the TDC1000-C2000EVM and using the SMD15T21R111WL transducer. My first test was with a glass jar of water. I was able to connect the transducer to the bottom and got reliable and accurate readings.
I then moved to a 9kg gas cylinder. I get readings from the EVM but they do not correlate to anything. I have tried to repeat with a smaller disposable 500g LPG cartridge. My thinking was that this has a lot thinner bottom. The readings are always almost the same for both gas cylinders. This indicates to me that the EVM is not measuring liquid gas but something else.
I have played around with increasing the number of TXs, increasing the gain and the timeout values. What is strange is that the TOF reading varies quite drastically with these changes. Shouldn't they be more or less the same? The readings are almost always the same for both cylinders.
What I need to understand is whether the EVM should be able to read LPG? Or is the fact that it is pressurised affecting the readings? If I should be able to then what configuration should I be using?
There are a few variables that could be affecting this. I assume that the transducer is fixed properly on the cases and that setup is correct.
The transducer may require higher power to drive a more strong pulses through the cylinder wall. Also the pressurized cyclinder could have some gaseous components which could affect with wrong results. I suggest you start with a known volume/level liquid and check if the EVM can read its output before trying it on a LPG tank.
Sensor Signal Conditioning Applications
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In reply to Bharat Aravamudhan:
Thankx for such a quick response.
I have initially tested the EVM with a glass container and water. It works correctly for that.
For my metal container tests, I am using 2. The one is the standard 9kg steel container and the other is a 500g gas cartridge...
The steel may be thick for the 9kg container but the 500g cartridge is very thin. The readings between the 2 is almost identical. This should not be the case as one is much smaller in diameter and much smaller in height. Both containers are close to full.
I also note that I get very different readings if I increase the TX number and gain. Should the TOF not be roughly the same value??
I also thought of the bubbles scenario but LPG boils when there is a change of pressure. In this scenario, the regulator has been switched off. I would expect the liquid and gas in the cylinder to have come to a state of equilibrium. In such a state, I do not expect bubbling. I also note that the values read are consistent. If it was bubbles the readings would be more erratic.
Would pressure affect readings of the EVM? That is the only thing that I can see as being different to the water scenario besides the actual container.
In reply to Buff Thokoa:
Have you got any more info to help me with this issue?
I am unable to progress any further with the EVM. I need clarity on whether this is actually technically possible with the EVM. There is customer need for my use-case and we are under pressure to bring a product quickly to market to fulfil this need. Any assistance from TI will be of great help.
For the transducer to have power to penetrate the metal walls we may need to drive the transducer at a high voltage and that is not feasible with this TDC1000-C200EVM. Unfortunately we do not have any prior data to measure LPG/GAS in containers.
I don't think the problem is the wall size. The 500g cartridge that I am using is very thin and I see the same behaviour.
I will take it from your responses so far that the TDC1000 device does not support the use-case of liquid level measurement of LPG.
The images below are of the cartridge I am referring to.
The readings from it are almost the same as that of a much larger 9kg cylinder.
I don't think the echo is coming from the air/liquid interface but I don't know what I could change to get an improved reading.
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