Ways to measure GAS LP liters inside a tank - Page 1
Forum section: electronics / projects

author: atwoz
author_rank: Regular Contributor
timestamp: September 20, 2014, 06:56:27 pm
content: Hello everyone,Is there any reliable way to measure gas LP level inside a tank? In some tanks they use a floating system that turns a gauge as the level rises and falls. This is for some 4000 liter tanks.Could a flow meter work? I need a resolution of 1%. The problem we are trying to fix is to detect exactly what amount of gas LP goes in and out to avoid people stealing gas and this has to be tamper proof.Any ideas?

author: SeanB
author_rank: Super Contributor
timestamp: September 20, 2014, 07:03:30 pm
content: Simplest is to use a capacitive level sensor inside the tank, and use an EEX rated driver to get the liquid level. Otherwise you will have to have a meter on input and output, enclosed in a tamper proof housing, and ensure the tank is also behind a tamper evident enclosure. Then you simply record each meter's readings and do some maths to get the consumption and fill, and get an idea if there is loss and roughly how much.as you are working with gas whatever you use has to be both type approved and designed to the local standards, so you likely will be stuck with using commercial gas meters on the tank, and enclosing them in a large box with a vision panel to read them, or to have remote monitoring meters.

author: IanB
author_rank: Super Contributor
timestamp: September 20, 2014, 07:15:42 pm
content: I'm not sure what you mean by "gas LP"? Do you mean LPG (i.e. butane/propane), or gasoline?If it's gasoline, then in commercial environments flow meters would be used for accurate inventory tracking. You have a meter to measure how much goes in and another meter to measure how much goes out. The meters accumulate totals like a domestic utility meter so if you compare the totals from day to day you know exactly how much liquid has passed through. But as Sean says, you will need security and control to detect/prevent tampering.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 20, 2014, 08:23:12 pm
content: Would weighing the tank be possible?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 20, 2014, 10:40:05 pm
content: If this thing is already installed and has to be kept filled (because, honestly, 4000L is big enough for a person to walk in, if not for the awful smell..), you might be better off with acoustic methods than anything...Which isn't so terrible, but you'd probably need a special purpose sonar module stuck to the bottom. Special because the acoustic reflections between sensor, tank wall (it's not thin sheet metal) and liquid surface will all be different from, say, a water level sensor (if such a device is even available to begin with).Tim

author: poorchava
author_rank: Super Contributor
timestamp: September 21, 2014, 08:35:50 am
content: Perhaps an automotive mass air flow sensor would work? I'd preferably not be a heated wire type, as this doesn't seem explosion-safe.

author: G7PSK
author_rank: Super Contributor
timestamp: September 21, 2014, 09:01:32 am
content: You can get ultrasonic level meters for oil tanks, just installed one on my tank after half the contents were siphoned off. Unit called Watchman, the only problem is that it runs on a coin cell and transmits data via 433 MHz link. Had trouble with the signal dropping out due to mist or someone walking by with a ladder, fixed that with a quad beam antennae fitted to it.If you are talking about LPG it wont work you either need flow meters or other specialist equipment check with the tank makers as they usually make the tanks to take specific equipment.

author: tautech
author_rank: Super Contributor
timestamp: September 21, 2014, 09:26:55 am
content: Quote from: NiHaoMike on September 20, 2014, 08:23:12 pmShould be IMO.All the LPG tanks of that size I have seen have been above ground so why not weigh it with 4 load cells?The tank will need to be empty to get a tare weight to start with of course, or maybe the tank manufacturer has a tare weight for them?

author: amyk
author_rank: Super Contributor
timestamp: September 21, 2014, 10:55:06 am
content: A 4000L tank with LPG in it is likely going to be outside (especially as OP mentioned "stealing gas"), and its weight will change depending on what happens to accumulate on it (rain, snow, etc.)

author: atwoz
author_rank: Regular Contributor
timestamp: September 21, 2014, 02:43:33 pm
content: Thanks everyone for the replies.Yes, I was referring to LPG (sorry for the confusion). The tanks are mounted on trucks.

author: saturation
author_rank: Super Contributor
timestamp: September 21, 2014, 03:45:12 pm
content: LPG tanks? Are weighed the tare difference is the volume of LPG.http://www.elgas.com.au/blog/353-how-much-gas-is-left-in-the-gas-bottle

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 21, 2014, 04:06:24 pm
content: Maybe add a sensor to detect the compression of the wheel springs?

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 21, 2014, 04:27:44 pm
content: Aren't there already pressure gauges/transducers on the tanks?Wouldn't it be possible to monitor this by tracking the tank pressure and temperature (because the pressure varies with ambient temp).Assuming that you are concerned with theft while the trucks are parked "at home", you could use wireless Arduino modules that would transmit the pressure and temperature every 10 minutes (or whatever) to a central system in the office. Or you could monitor it more frequently (every 30 seconds, etc.) and even program it to send an alarm in case of suspected theft/vandalism activity.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 21, 2014, 04:46:50 pm
content: As long as there is liquified gas present, the P(T) curve is constant.Weight is by far the best, if you can tare the container and arrange to weigh it somehow.Tim

author: Marco
author_rank: Super Contributor
timestamp: September 21, 2014, 04:56:22 pm
content: Through wall ultrasonic, weight or flow ... through wall ultrasonic seems the easiest retrofit.Plenty of commercial systems out there.As for tamper proofing, combine the sensor and the logging electronics in one box and fix it to the tank (I guess you're not allowed to weld something to a LPG tank though).

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 21, 2014, 04:59:23 pm
content: "Arrange to weigh" 4000 liter tanks mounted on trucks doesn't seem very practical for an ongoing detection scheme.And something that could withstand the extreme movement from the truck driving around may not be sensitive enough to detect theft of a few kilograms.What about capacitive proximity sensor scheme that would detect if someone approaches or touches the tank or plumbing?Pesumably there should be nobody messing with anything when the trucks are parked "at home", especially overnight.

author: Marco
author_rank: Super Contributor
timestamp: September 21, 2014, 05:02:08 pm
content: Quote from: Richard Crowley on September 21, 2014, 04:59:23 pmIf the truck driver is going to siphon gas while driving at a low enough flow rate and in low enough quantities that it doesn't show up ... I guess it's just time to throw your hands up and chalk that up to business risk.

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 21, 2014, 05:12:13 pm
content: A 4000 liter tank, even empty, is pretty heavy. It is probably mounted on springs or something compliant vs. hard-bolted to the truck frame. It will jostle around while out on the road, and with a LOT of force, even if the tank is empty. A sensor that will withstand that kind of "abuse" while still able to reliably sense small changes seems rather unlikely, unless you are willing to throw a lot of money at the problem.Assuming these trucks are out making their rounds, dispensing product, how is the product measured? Aren't there some kind of meters that show how much product is dispensed? How are the customers billed? Aren't there accounting processes in place for recording how much goes into the tank, and how much is dispensed to various customers?Is the problem accounting for "stock shrinkage", or is the issue detecting after-hours tampering?

author: SeanB
author_rank: Super Contributor
timestamp: September 21, 2014, 06:10:20 pm
content: If the trailer has gas suspension getting a weight measure is simple, just measure the pressure in the gas springs and you have a somewhat linear relation. With springs you will need to measure the compression of the springs, most commonly done with a LVDT as this is the most reliable. otherwise a pneumatic cylinder with a rolling seal and a fixed volume will give a pressure that can be read easily, and will not lose pressure past seals easily.But as it is a tank, use a meter on input and output, and if the gas is being taken during stops simply have a flowmeter that gives pulse outputs, can be integrated into a regular gas meter simply by opening it and adding a hall sensor and magnet to get a pulse out of the flowmeter if it is not magnetically coupled in the first place. Then you can use any off the shelf GSM modem and do some conditioning to alert you to gas use, so you can see when gas is delivered to the customers and when it is drawn off while the vehicle is not supposed to be delivering.

author: ConKbot
author_rank: Super Contributor
timestamp: September 21, 2014, 10:30:02 pm
content: There should already be some sort of tamper resistant method for metering LPG in and out.http://www.liquidcontrols.com/en/Literature/Documents/Overviews/100-10%20PD%20Overview.pdfcame up on the first page of searching for "LPG totalizer"QuoteDont know about the security features, but this seems like a problem that should have been encountered and have a COTS solution available already.

author: atwoz
author_rank: Regular Contributor
timestamp: September 21, 2014, 10:32:48 pm
content: The ultrasonic sensor looks promisinghttp://www.hitechtech.com/Appl_SONOGAS_engl_081126.pdfhttp://www.xact-data.com/process/I wonder how accurate it is.The problem with flow meters, is that they need a minimum flow to operate correctly. The thief can just drain the gas slowly so that the flow meter does not detect the correct flow.Here is a flow meter:http://www.alibaba.com/product-detail/high-accuracy-low-flow-turbine-lpg_1157630331.html

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 21, 2014, 10:43:54 pm
content: One would think you could simply put a lock on the outlet spigot. And add a security camera if you want to see who is doing the deed. I see dozens of examples of those almost every day.

author: IanB
author_rank: Super Contributor
timestamp: September 21, 2014, 11:20:57 pm
content: Quote from: atwoz on September 21, 2014, 10:32:48 pmUnfortunately they do not indicate the rangeability of that meter.Generally, I am still puzzled by the problem. Firstly, there must be some kind of level gauge or high level alarm on the truck? Otherwise, when you are filling the truck how do you know when it's full (and avoid overfilling it)? Secondly, for accounting purposes you must meter how much goes into the truck and how much is delivered out to customers. When a customer receives a delivery they must expect to pay for the precise amount delivered, which means deliveries must be metered. So if you run a balance sheet and measure how much is charged into the truck each day and how much is delivered, any difference is loss. If you charge the truck each day with the exact amount your customers want to receive, then any customer who doesn't get the right amount will soon be complaining, will they not?If you need a second check, how about a weigh station for the trucks? If you weigh the truck you will find out exactly how much inventory it contains.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 22, 2014, 02:38:15 am
content: Does the truck engine run off the propane? If so, that makes it more complex to account for losses.

author: IanB
author_rank: Super Contributor
timestamp: September 22, 2014, 02:53:31 am
content: Quote from: NiHaoMike on September 22, 2014, 02:38:15 amI wouldn't imagine the truck engine gets its fuel from the delivery tank. That would be far too complicated and dangerous. But of course if you use a weigh station to check inventory you still need to account for how much fuel is in the fuel tank when computing the tanker load.

author: Rerouter
author_rank: Super Contributor
timestamp: September 22, 2014, 07:51:02 am
content: A positive displacement flow meter and a totalizing counter sounds like what your after, you cant steal anything through most of them, as the volume of the meter must be filled before it appears on the outlet, so they can catch drips in a bucket and you will know where those drops went,

author: mzzj
author_rank: Super Contributor
timestamp: September 22, 2014, 08:19:00 am
content: Quote from: IanB on September 21, 2014, 11:20:57 pmI am also puzzled with this. I mean LPG is not the easiest product to stole and value per volume sucks.Are the truck drivers moonlighting the gas to their own house heating tanks or to another tanker truck or whatI understand that LPG gas cylinders get stolen in India or something like that...

author: SeanB
author_rank: Super Contributor
timestamp: September 22, 2014, 08:13:28 pm
content: Here too, as they have to be outside. You buy a mesh casing that is bolted to the wall with a lock of your choice, so that the theft can be reported as "Forced Entry" and then you claim for the cylinders from insurance, along with a new cage, regulator and locks. Not like Selebi's sons will actually catch a burglar, or keep him in jail for more than a few days before he gets bail and carries on.