Measuring liquid levels with a capacitance probe in a conductive container - Page 1
Forum section: electronics / projects

author: eerocketman
author_rank: Contributor
timestamp: October 30, 2017, 11:52:25 pm
content: Hi everyone. I am currently working on a rocket for my college organization. This rocket will be powered by RP1 and liquid oxygen. I need to create a system to measure the tank levels of the fuel and oxidizer. I am considering using the TI FDC2212 in a single ended sensor configuration to achieve this. As for probes I want to use these (last page):http://www.americanmagnetics.com/brochures/model175.pdfI was hoping to make a configuration where the system adjusts to the different dielectric constants of the fluids but this seems impossible without using another sensor inside the tank.As we are a college organization we need to keep the manufacturing of the tanks as simple as possible due to budget and time constraints so i would rather not use more invasive sensors.If anyone has experience with cryogenic fluid level measurement (or knows how to do this) any advice would be greatly appreciated. Could I somehow perform this operation by observing the impedance between different points on the tanks?

author: Rerouter
author_rank: Super Contributor
timestamp: November 01, 2017, 09:02:53 pm
content: Could you give a crude sketch of how the tanks are arranged, e.g. there inlets and outlets. And how far things are stuck inside them, What you might be able to do is measure the capacitance between these 2 points, or from 1 of these to the tank wall.If you use backfill gas, you could measure the pressure on those tanks. If you use pre pressurised fuel tanks you can do the same.

author: floobydust
author_rank: Super Contributor
timestamp: November 01, 2017, 10:08:48 pm
content: I would recommend against this route. It's just too dangerous for college-level projects as these are flammable/explosive liquids.Because there is a safety aspect to seal/electronics failure, the tank is after all a certified pressure vessel, there's testing and regulatory approvals and liability here that you may have not considered.If you can accept a liquid oxygen or RP-1 leak (until the tank is empty- as you can't have a shut-off valve with a conductor passing through) as not being dangerous, then there are hurdles. Apollo 13 incident is an example of how dangerous liquid oxygen tanks are.The mechanical engineering required is advanced, as you are making a seal that is electrically-isolated to permit your feed-through electrode signals. The wide temperature swings from cryogenic to a hot tank, the vibration, the pressure swings, and safety aspect make this a huge piece of engineering. There are several patents on such a gland. I've used Teflon and O-rings. The explosion hazard from a fire/high heat melting the plastic seal prevented use in many applications. Part of a tank's regulatory approvals is withstanding a fire for X hours with no leakage.Capacitance-wise, you have the narrow feed-through adding a lot of capacitance and saturated-vapour above the liquid line. You can calculate the min/max capacitance with the geometry or using liquid nitrogen for test. Your electrode at the free end will whip about and intermittently short, as you can't anchor the electrode. It is near impossible to put anything at that end deep in the tank, such as a welded bracket - unless you make a custom tank, which is big bucks getting that safety tested.My experience is with LNG, propane tanks, crude oil and aviation fuel, using floats, capacitive, free-space and guided-wave radar technology to measure liquid level.I think a pressure-sensor or weighing the tanks is the only alternative.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: November 02, 2017, 12:38:30 am
content: Quote from: floobydust on November 01, 2017, 10:08:48 pmI second the method of weighing the tanks, this is how they are managed in most places that handle these types of vessels.