Hi, I am looking at developing a sensor (end node) to calculate/visualize the volume of lpg/propane gas tanks. I see sensors in the market using various methods from load cells using weight, to ultrasonic sensor which are attached externally to gas cylinders ( found it difficult to understand maybe someone can elaborate if familiar to this principle). Also other sensors use the remote capability of mechanical gauges. Would like to hear from you about your thoughts and input.
Please provide more information and more specific questions for your case.
Do provide any relevant references.
I am not an expert in this but ultrasonic level sensors are usually mounted inside the tank and measure distance between fluid level and sensor.
In case of liquid gas tanks this would mean ultrasonic sensor(s) inside a pressurized tank filled with inflamable gas which is probably not an option.
As these tanks in most cases will be made of metal (steel, aluminum aloys etc.) placing ultrasonic sensors outside the tank won’t help (much) in measuring the fluid level inside.
As for the title: the volume of the gas cylinder will be (relatively) constant. 
Actually some are designed for attaching directly to the outside of a (usually metalic or rigid vs e.g. fiblreglass) tank. Often ‘looking up’ to see reflection of liguid-air/gas interface or side monitoring to look at simple ‘shift’ in signal as level drops going past the sensor…
God I hope so, I’ve two Hydrogen & one Helium on the other side of the wall from my desk - don’t want them expanding on their own. 
Interesting. So these can be placed outside of, under the bottom (‘looking up’) of a metal tank?
Assuming that ‘volume of the tank’ here means ‘volume of liquid gas in the tank’:
‘Past the sensor’ would be similar to capacitive/inductive ‘on/off’ fluid sensors but will not be suitable to measure the exact level variably, only the level(s) where such sensor(s) is/are placed.
Indeed side view good for a crude approx level or trigger signal - say for min and max content thresholds with a few inbetween - some of the Semtech capacitive touch sensors used to be used in such use cases 
but also ultrasonic sensors see a shift in relection and pattern if used in similar way (side view).