Bulk LPG Tanker (in)accuracy

Flintman

Since 2012, I have closely monitored my bulk LPG tank gauge, particularly before and after a tank fill. I have used this data to predict how much LPG should be delivered. Until 2020, the predicted figure was within 1% of the actual delivered volume, sometimes the deviation was positive, others, it was negative. However, since 2020, there have been some large differences between the prediction and claimed delivery: all bar one, in favour of the supplier.
My last 2 deliveries illustrate this. In September last year, I predicted 809 litres would be needed when only 802 litres was delivered. This is excellent agreement and the only time in the last 5 years where there has been a negative deviation. Contrast this with my latest delivery where 761 litres was predicted, but 931 delivered, according to the tanker meter. This is a 26% deviation which could cost me £116. There is no evidence of any malfunction with my tank gauge, so the only plausible explanation for this variation that I can think of is inaccuracy of the tanker meter. 
It was a surprise to me to learn from my local trading standards office that unlike heating oil tankers, LPG tanker meters are NOT subject to independent testing. I have experienced this deviation with 2 different suppliers, and this suggests the problem could be widespread. 
I fear that it is unlikely that many bulk LPG users will have monitored deliveries as I have, so there is no way of knowing for certain whether we are being ripped off big time by tanker inaccuracies or not. 
I would love to hear from any member who has similar data or who would be interested in starting their own checking. Only then can a case be built to tackle this possible deception

On-the-coast

You wrote
 “There is no evidence of any malfunction with my tank gauge, so the only plausible explanation for this variation that I can think of is inaccuracy of the tanker meter.”
but surely the fact the two separate suppliers are at odds with your meter leads to the more likely explanation / evidence that your meter is inaccurate?
The alternative is that the LPG industry as a whole has become systemically dishonest in the past 2 years - which is what you are looking for evidence of.  Let’s hope you can get to the bottom of this quickly. 

Flintman

Just before my tank was filled, I took (and photograhed) 2 readings, the night before and about 12 houres later. The photo's show a small, but measurable difference of less than 1% between the readings. This shows that the tank gauge shows no sign of sticking. The latter reading, 3 hours before the tank was filled was 31% full. After filling it was 85% full and for a 1,400 litre tank, 761 litres should be required. I have been billed for 931 litres. To get that amount into my tank to fill it to 85%, it should have been only 19% full before filling. Such a deviation between 19 or 31% would require an astronomical failure of the tank gauge causing it to stick on 31% as a further 12% was consumed. As I pointed out above, since the gauge meter did respond to gas use over a 12 hour period, the chance that it was sticking is negligible. Incidentally, my supplier has told me that the gauges are subject to a 5% variance, whatever that means. In any case that is way below the 26% deviation on my last bill.
I have been careful to avoid making the claim that the industry has been systematically dishonest over that last few years. It is one possible explanation but I do not have enough data to prove it. If that did prove to be the case, we would be talking millions of pounds being paid for gas that has not been supplied. We shall never know the truth unless other users run the same checks, thus giving data from many customers. 

Woodstok2000

If your tank gauge hasn't been calibrated recently, you can't use it for this kind of calculation accurately. Guages may be subject to a 5% variance when new, but that will worsen over time. Even assuming 5%, if 31% is actually 26%, and 85% is actually 90%, that equates to a 64% fill, or around 900 litres.

Flintman

These are simple mechanical gauges using a float, much like a car fuel tank gauge. The difference is that after 20 years, the car float will have been subject to a lot of vibration and been subjected to hundreds of up and down cycles whereas an LPG float in that time will have only experienced 50 or so cycles. That is not going to cause much wear or decalibration. In any case, if such wear were to be the problem, it would be expected to be a gradual process, getting progressively worse with each tank filling. What I observed was a sudden increase in the deviation after 7 years of consistency. But it was not progessive, the error varied wildly from one filling to the next. That is not the symptom of some progressive wear.
Your calculation assumes 5% means 5% absolute and that it opposite extremes were real for each reading. Even then, you do not reach 931 litres. An alternative interpretation of 5% tolerance is 5% of the displayed reading. That would give maximum extremes of 29.5 to 89% which would equate to 830 litres. 
I am not claiming that the tanker readings are inaccurate, merely that the evidence I have from one installation indicates that this is a distinct possibility. With more data from as many LPG customers, we would know whether there is a systemic problem.   

Woodstok2000

I'm not discounting your theory, only showing that it is equally plausible that the issue is with your meter and not the trucks.  The LPG codes of practice are not binding, but do recommend annual calibration of supplier flow monitors with certificates available for inspection if required. Have you asked your supplier if they have a calibration procedure and if their trucks are certified routinely?

Flintman

If I were getting a constant deviation, I would agree that my tank gauge wopuld be the cause the cause. But it varies, sometimes dramatically, with every different tanker, that surely points the the tanker being the problem, rather than my meter. I can not get to the bottom of this without help from other bulk LPG customers checking their gauges. It takes on a few seconds. I want to know whether my suspicions are justified because these deviations have potentially cost me several hundred pounds since they started. Is nobody else concerned that they may be being similarly fleeced?
The procedure is simple.  First, if you do not know it, check the volume of your tank on the stainless steel plate on one of the ends. Then as close to a delivery as possible, take a reading. Gently tap the side of the gauge with something like the handle of a screwdriver or chisel to ensure that the needle is where it should be, then photograph the gauge from 10 to 15 cm using a macro setting on the camera. After the tank has been filled, take a 2nd photo. If possible, record the registration number of the tanker.
Crop your photo's to give a good sized image.The bigger the image, the more accurately you can estimate the reading. Subtract the predelivery figure from the post delivery figure and this will show the percentage fill. So, my last filling went from 31 to 85%, a 54% increase. My tank capacity is 1410 litres, so each percent of LPG occupies 14.1 litres. Therefore, 54% occupies 54x14.1 = 761 litres.
In answer to "Woodstok 2000", I did raise this question with my supplier last year and they informed me that their tanker meters are checked every 6 months. However, I was given neither evidence or details on how the tests were performed. The first delivery after that was almost exactly as predicted which gave me hope that they were in fact checking their meters. But the last delivery of 170 litres (26%) more than predicted is not encouraging.

Woodstok2000

Regularly calibrated flow meters across multiple different tankers, or a single old uncalibrated float gauge...I wonder which is more likely to be inaccurate.

Two other things to check on your end, is your tank perfectly level, and are you accounting for potential volume differences due to temperature change?

That said, I can see why you want a definitive answer, so will wish you yhe best of luck!