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!

lohr500

Presumably your tank is a horizontal cylinder with rounded ends.

Is your gauge scale linear or is it calibrated to take account of the difference in volume at different points in the tank?

Could it not be the case that the readings will differ depending n what level of LPG is in the tank before and after filling?

Certainly this was the case with our oil tank which had a simple float gauge with a linear scale.
It would probably have worked fine on an old fashioned rectangular steel tank, but was way out on a horizontal cylindrical tank.  

Flintman

The tank is virtually horizontal: it slopes at an angle of 2 degrees and it has been like that since it was installed. Any error that such a slope might cause would be small, but more importantly, constant (we are looking for something that could cause a variable error). I have never been able to detect any effect of temperature on the readings. The gauge scale is linear between 20 and 85% which is what would be expected with a cylindrical tank. This time last year, I monitored (and photographed) the tank level daily from 78% down to 31.5% when it was refilled. Without exception, the indicated level fell slightly every day. This is in line with expectations for a normally functioning gauge. I have not found any correlation between the level in the tank and the error.

There is scepticism in some responses that an old un-calibrated gauge can not be expected to give as accurate a reading as a sophisticated meter on a delivery tanker. It might be helpful to compare the 2 units, starting with the tank gauge.

The gauge is a Rotarex SRG 487 which the manufacturer says will “precisely measure the content of your cylinder”. They claim nothing quantitative so how my supplier got the idea that readings are subject to a 5% variance is unknown. The only moving parts in the gauge unit are a float arm and meter shaft which is attached to the meter needle. The float arm and meter shaft are connected to each other via a crown wheel and pinion so as the float arm rotates due to falling gas levels, the meter shaft, and thus the needle also rotate.The accuracy of the meter is a function of the gear ratio of the crown wheel and pinion and once fixed, can never change. Bearing in mind the light loads and slow movements involved, wear due to the 50 or so refill cycles that the system has been subjected to in the last 21 years is surely negligible. Considering the thousands of refill cycles that it must be capable of, this meter can not be considered to be old.

A correctly calibrated tanker meter is capable of giving a figure to a precision of 0.1 litres compared to about 7 litres for the tank gauge. When I started monitoring tank levels, my previous supplier had fairly new tankers and there was very good agreement between predicted and actual deliveries for 7 years. My last delivery, in January, was from a 12 year old tanker and I was billed for 170 litres more than predicted (26.2% variance). For obvious reasons, I have no information about the tanker meter or its history, but if it is as old as the tanker, it could have metered something of the order of 40 to 50 million litres of LPG, at the rate of over 100 L/m in its lifetime. That gives ample opportunity for wear and de-calibration.The supplier admits that the meter can be calibrated (i.e. adjusted) so by definition, it can also be inaccurate if not calibrated correctly. I never see the same driver so I guess I always get a different tanker. Unbelievable as this may sound, inaccuracies in tanker meters are possible and would explain the variability between predicted and actual deliveries. But how do we get to the truth?

There is virtually no chance that any supplier is going to admit that their tanker readings are inaccurate and we can’t turn to trading standards for help. So, it is down to us tank “owners” to get more evidence and the only way to do this is for as many people as possible to start monitoring their suppliers to find out whether my experiences are common or not. Since 2019, when variations between predicted and actual deliveries started, I calculate that I have paid £450 more than predicted. That could also have happened to YOU. So, let us stop theorising over what might cause these variations and start monitoring our deliveries – PLEASE.

(Removed by Forum Team)

lohr500

I have no idea if you have received a short delivery or not, but I can guarantee that the volume of gas in the tank cannot be linear between ranges of 20% and 85% full.

If your gauge is simply measuring the drop in the tank on a linear scale driven by a rack and pinion, then the actual volume cannot be linear due to the cylindrical nature of your tank.

Take a hypothetical horizontal cylinder tank of 1m x 4m (ignoring the convex ends for simplification).

If filled to 0.5m, the volume would be 1570 litres.

If filled to 0.4m, the volume would be 1173 litres.397 litre reduction.

If filled to 0.3m, the volume would be 793 litres. 380 litre reduction.

If filled to 0.2m , the volume would be 447 litres. 346 litre reduction.

I imagine that the volume/depth curve will be sinusoidal.

Without knowing the actual tank dimensions, and the start and end fill depths, I can't compute if this could lead to the size of discrepancy that you have seen. However I agree that a 26% discrepancy is high.

My point though is that you cannot assume the gauge is linear across the 85% to 20% fill range.