Heating Oil Tank Calibration - How much have you really got left?

rogerb

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Not wishing to run out of oil this winter (again!) I wanted to calibrate my 2,500 litre Titan heating oil tank to tell with a reasonable degree of certainty just from the inspection level how many litres are left/how many litres to order. I have a Watchman-type ultrasonic oil tank meter (which claims to show how many litres are left) but it is inaccurate, varies depending in temperature and humidity, and consequently us used mainly as a low level alarm!

My 2,500 litre unbunded Titan tank is the horizontal ribbed-cylinder type, one of the more common plastic domestic tanks, so I hope this information is of use to fellow 'oilers'. I've done this largely for an academic challenge, but as a result have learned a few things about my oil consumption, how much the tank expands on a warm day, how much oil you can't get out of the tank (about 100 litres), my daily consumption rate and its correlation to outside temperature.
My tank has a clear plastic inspection tube at one end with a floating ball to show the level of oil inside (some don't and users have a dipstick instead). Here's my tank type: http://www.titanenv.com/2500-litre-single-skin-oil-tank

If the tank had equal cross section, the height of the oil in the inspection-glass would be proportional to the volume in the tank, but it isn't. Neither is it a perfect cylinder shape (also easy to calculate), in fact it's a complex mix of part cylinder, part cuboid and partial sphere (the bulge at each end). What catches me out time and again is that as the tank nears empty, the level of oil still looks deceptively high. In fact the rate at which the inspection glass falls gets faster the nearer it is to empty - you check it one week, thinking it's about a quarter full but the next week it's empty! The resulting graph of oil depth vs. litres remaining (see link below) has been worked out mathematically and empirically. The graph looks deceptively linear but it isn't.

The bottom line is this:
When the tank is near empty (or near full, i.e. minimum cross section), each centimetre of oil depth =15 litres.
When the tank is half full (i.e. fattest part of the tank), each centimetre of oil depth = 24 litres.

In cold weather with my CH and Aga going full tilt, I can burn 45 litres a day, so every centimetre matters;)! In other words, when it looks like the tank is 1/5th full (25cm of oil in the inspection tube) it really only has 320 litres of usable oil left, about 12% of full tank capacity and as little as a week until I run out!
Here's the graph.
https://dl.dropbox.com/u/109185229/Titan%202500%20l.%20Oil%20Tank%20Calibration.gif

Don't forget a tank is not level, it is tilted slightly - this is to ensure condensing water collects in the bottom of the tank furthest away from the spigot. My tank has about 100 litres capacity below the spigot outlet level, and this adds to the illusion you have more oil than you really do.

---

Hopefully I won't run out of oil again, but here we are,10 days to Christmas and I am pretty sure oil prices will drop a few pence after the holidays, so the gamble is whether I can risk my graph's forecast of running out on January 8th and order 2,000 litres then, or buy some now at the higher price..... :eek:

[Deleted User]

45lts a day :eek:

BERBY

having developed a monitoring device for this very purpose by placing a microprocessor on the oil line i am able to live monitor my oil useage. It also tells me instantly if there is a theft or leak or if the boiler malfunctions. This has proved too costly for the manufacturers to take on so i have now developed (from the information gained from the microprocessor) a system that will interlock with the boiler burner and stop it fireing when : 1. the fire valve trips, 2. the filter blocks, 3. the oil tank is empty but the oil line is still full. The third action prevents any need for the engineer to attend and saves wear and tare on the pump. I am presently fitting this system to my customers systems for about £165

rogerb

Berby,
Curious which flow meter you used.
Is it the BioTech FCH-M-POM-LC FLOW SENSOR METER, by any chance

http://www.produktinfo.conrad.com/datenblaetter/150000-174999/150391-da-01-ml-DURCHFL_FCH_M_POM_LC_G1_8_2XM5_de_en.pdf

or have you found a better one?

Thanks
Rog

BERBY

This is a very low pressure sensing system linked to a microprocessor developed inconjunction with Plymouth University. Only one in existance which is fitted to my boiler.

reeac

rogerb wrote: »

Not wishing to run out of oil this winter (again!) I wanted to calibrate my 2,500 litre Titan heating oil tank to tell with a reasonable degree of certainty just from the inspection level how many litres are left/how many litres to order. I have a Watchman-type ultrasonic oil tank meter (which claims to show how many litres are left) but it is inaccurate, varies depending in temperature and humidity, and consequently us used mainly as a low level alarm!

My 2,500 litre unbunded Titan tank is the horizontal ribbed-cylinder type, one of the more common plastic domestic tanks, so I hope this information is of use to fellow 'oilers'. I've done this largely for an academic challenge, but as a result have learned a few things about my oil consumption, how much the tank expands on a warm day, how much oil you can't get out of the tank (about 100 litres), my daily consumption rate and its correlation to outside temperature.
My tank has a clear plastic inspection tube at one end with a floating ball to show the level of oil inside (some don't and users have a dipstick instead). Here's my tank type: http://www.titanenv.com/2500-litre-single-skin-oil-tank

If the tank had equal cross section, the height of the oil in the inspection-glass would be proportional to the volume in the tank, but it isn't. Neither is it a perfect cylinder shape (also easy to calculate), in fact it's a complex mix of part cylinder, part cuboid and partial sphere (the bulge at each end). What catches me out time and again is that as the tank nears empty, the level of oil still looks deceptively high. In fact the rate at which the inspection glass falls gets faster the nearer it is to empty - you check it one week, thinking it's about a quarter full but the next week it's empty! The resulting graph of oil depth vs. litres remaining (see link below) has been worked out mathematically and empirically. The graph looks deceptively linear but it isn't.

The bottom line is this:
When the tank is near empty (or near full, i.e. minimum cross section), each centimetre of oil depth =15 litres.
When the tank is half full (i.e. fattest part of the tank), each centimetre of oil depth = 24 litres.

In cold weather with my CH and Aga going full tilt, I can burn 45 litres a day, so every centimetre matters;)! In other words, when it looks like the tank is 1/5th full (25cm of oil in the inspection tube) it really only has 320 litres of usable oil left, about 12% of full tank capacity and as little as a week until I run out!
Here's the graph.
https://dl.dropbox.com/u/109185229/Titan%202500%20l.%20Oil%20Tank%20Calibration.gif

Don't forget a tank is not level, it is tilted slightly - this is to ensure condensing water collects in the bottom of the tank furthest away from the spigot. My tank has about 100 litres capacity below the spigot outlet level, and this adds to the illusion you have more oil than you really do.

---

Hopefully I won't run out of oil again, but here we are,10 days to Christmas and I am pretty sure oil prices will drop a few pence after the holidays, so the gamble is whether I can risk my graph's forecast of running out on January 8th and order 2,000 litres then, or buy some now at the higher price..... :eek:

I have a 2600 l. tank and an annual consumption of 2000l. which means that I only need to order in the Summer. I've calculated the curve of volume versus depth of oil in the tank, treating the tank as a simple, horizontal axis cylinder and the curve departs from a straight line only at 250 l. or less [ignoring the nearly-full end of the scale]. I use a timber roof tile lath marked at 500l. intervals to take measurements. My model is that we use 200l./month for an 8 month heating+hot water season and 50l./month for 4 months. This means that once we get to May the consumption [50l./month] is so low that I can order anytime from then until end-September although in practice June is usually the best month. I use the Boilerjuice site to watch oil price trends.

That's my c.h. oil policy - as for your situation, I would suggest
1. that you install additional tankage if at all possible so that it exceeds 12 months consumption.
2. ignore the niceties of height/volume curves, tilted tanks and any electronic level measuring and get a dipstick - totally reliable and accurate.
3. you know your peak consumption rate so you know how many days oil you have left at any time during the critical Winter months so you know within a week or less when you need to order.

Pansy_Potter

Phew, this is all far too complicated!
I agree that the watchman thingies are useless and very inaccurate at least mine is... I just have a long stick which I put in the tank (which incidentally is a bunded Titan and doesn't hold anything like the capacity the manufacturer claims) and when it gets to last few inches I order some more. We are out way out in the wilds of north northumberland but still get deliveries very quickly. I have back up energy options on everything - if I was to run out of oil I just use my woodburning stove a bit more and heat my water using the electric connection on my hot water storage cylinder.

rogerb

The calibration exercise was done primarily to see if I could derive a tank volume formula, given that the tank is a complex shape and because my oil consumption is quite high. As the tank nears empty, the remaining depth gives a false sense of security whereas it is actually running out rapidly. I got caught out 2 years ago with the December cold snap when oil companies appeared to conspire to hike prices and delay delivery, and I ran out over Christmas.
It isn't practical - or legal - to store a year's worth of fuel (8,000 litres) or buy it in one go (VAT at 20% applies to any purchase over 2300 l.) so it becomes necessary to be able to forecast the rate of oil consumption and buy at the optimum price.
Anyway, each to their own. The main point was to discover a 2500 litre Titan oil tank holds 25 litres per cm depth when half full and 15 l/cm when nearly empty, and that if it's very cold and I'm burning 40 litres a day, I need to reorder PDQ when the tank looks 20% full or I'll run out in a week. Not exactly rocket science , more peace of mind. :cool:

ilikecookies

In case it's helpful to those with branded tanks I simply emailed my tank supplier (Titan in this case) and they kindly emailed me a calibration chart stating how many litres would be in the tank at various depths.

All I had to do then was transfer this onto a long dipping stick and voila.

I'm sure it's not totally accurate but it's much better than the Watchman which came with it which is all over the place and no help at all!

highrisklowreturn

How is it even possible to use that much per day - 900 ltrs does me 14 months, maybe 15.

shumicat

We have just become victims of oil tank theft - discovered when we ran out of oil on Friday. House temp is now down to 8 C in unheated rooms. No delivery until Wed subject to snow.
Anyway - I'm now very anxious to see if anyone can recommend an alarm that is effective?