# Determine volume of liquid in a fluid bladder

I am trying to determine if it is possible to "physically" determine the volume of liquid (such as water) in a non-solid container like a fluid bladder like this or this?

Assume that it is not possible to keep accurate track of the difference between how much fluid is placed in the bladder and how much is taken out, and that a "physical" measurement must be made (where "physical" can include using a sensor, or using some sort of actual measurement of the dimensions of the bag to compute the volume).

• You could do a indirect measurement and measure weight. If you now the substance that is. Aug 5, 2016 at 20:03
• How would this work on the bigger bladders? I can see it working on smaller bladders, but don't know how to achieve measuring the weight on one of the much bigger bladders. Aug 5, 2016 at 22:48
• You should be able to get a reasonable estimate by measuring the pressure (not the total force) which the bladder exerts on the foundation. You only need to measure the pressure at a small number of locations - maybe only one. You may have to do some empirical calibration to account for the changes in the geometry of the bladder as it fills and empties, and the area in contact with the foundation changes. Aug 6, 2016 at 14:26
• Put strain gauges on the structure that holds the bladder. Ive seen that done for grain silos and storage of liquid raw materials in factories. For volume you probably also want to measure temperature as volume is quite dependent on this. You can also measure fluid pressure at the bottom of the bladder. Aug 6, 2016 at 20:51
• @Carl Witthoft Curiosity. Aug 8, 2016 at 18:28

Here are some options:

1. Weight- Reliable if the density is known. Bladder can be hung from a load cell, or placed on a surface supported by load cells, and volume can be derived continuously.

2. Bag in tank - One easy way to calculate volume is to place an awkward shape in a tank of water and measure the increase in volume. Simply inflate the bladder submersed in water in a straight wall tank and an ultrasonic (or other) level transmitter will be able to accurately give you an continuous volume measurement. Archimedes' principle

3. Open Loop Calibration Curve - Reliable if the bladder is designed such that it always grows in a predictable fashion (not one side then the other). A proximity sensor or distance sensor (possibly ultrasonic or laser) would be used to target a location on the bladder that changes the most during fill and empty (center of a whoopie cushion for example). You would then take an empty measurement, record the distance, fill with 10ml, record the distance, fill with 10 more ml, record the distance, and so on... The cure you create can then be used in a computer or PLC to continuously derive your volume from the distance sensor input.

4. 3D Scanning - I would only recommend this option if the others were not feasible, the geometry was unpredictable, and a continuous measurement was not needed. Multi-photograph reverse rendering, laser scanning, and LIDAR(as Inquisitive mentioned) are all possible options. The computer scans the object over a period of seconds or minutes then analytically computes the volume.

You may be familiar with LIDAR. This is how land topography is put into digital format to be used in various 3D models.

Is there some kind of "point cloud" device that could penetrate the bladder material but reflect off of the fluid contained within? If you could find such a device, you could create a point cloud of the fluid mass and calculate the fluid volume by importing the point cloud into volume/topography calculation software.

It would be like an x-ray machine that penetrates the bladder, but bounces off the water. The whole bladder would have to be scanned to create the point cloud of the fluid surface. You could then do a volume difference calculation between the 3D fluid surface and the bottom, flat, supporting surface.

• That's a pretty far stretch (and expensive). Aug 8, 2016 at 12:57
• I think you're on a pretty good track but theres an assumption that there is volume within the bladders which is not filled by the material. From my experience with these types of bladders they are fairly free form and take the smallest shape possible to contain the fluid they are filled with. Using the point cloud systems you mentioned OP could determine the volume of the bladder itself at that current shape then verify against physical measurements of inflow/outflow to determine accuracy. Aug 9, 2016 at 3:22