Central Heating Oil Measurement Tips
For the reason outlined in the paragraphs below it is recommended depth readings are taken in the morning on the same day of the week - fortnightly in winter and monthly in summer to avoid unnecessary measurement errors. Record keeping sheets for heating oil use can be found in can be found in the record sheet section.
If you want to convert heating oil used into CO2 generated check out the conversion page.
Depth Measurement Accuracy
The lower the oil consumption per day and the more frequently measurements are taken the more accurate depth measurements have to be to get a realistic measurement of oil consumption.
For example a depth measurement error of 0.25 inch on a 48inch high 2000 litre tank is equivalent to an error of just over 10 litres. If daily usage is 7 litres a day and measurements are taken weekly this is equivalent to an error of around 21%. The two biggest sources of measurement error are those due to the accuracy of the measuring stick and parallax error. Parallax error occurs when the reading os not taken by looking at exactly 90 degrees (straight on) to the measuring stick or ruler.
The oil consumption is likely to vary widely from week to week due to changes in the weather, the time the heating is switched on, how often outside doors and windows are opened, the number of people in the house and other factors. For this reason it is best to keep an eye on the average consumption rather than get excited about high consumption in a particular week.
Variation Of Volume WIth Temperature
The volume of oil increases with temperature, however provided a simple precaution is taken these errors will end up being negligible in practice. For example a 20 degree C change in temperature of a half full 2000 litre 48 inch high tank gives an increase in volume of 20 litres or around half an inch. If daily usage is 7 litres a day and measurements are taken weekly this is equivalent to an error of around 40%. This is obviously a very extreme example but it is worth making sure measurements are done early in the morning each time to avoid the effect of direct sunlight heating the tank and causing errors. This effect also means it is not a good idea to fill tanks absolutely full. A similar error can occur when a tank is filled with oil at a different temperature to that already in the tank for this reason it is best not to take level measurements for several hours after a tank has been filled.
Tank Not Level
If the tank is not level horizontally depending on where the oil level is measure there will be an error in oil due to the tank being at an angle. If the level is measure in the centre of the tank this error will be zero however if the measurement of oil depth is made at one end the measurement of volume of oil left will be affected. If the horizontal level difference is less than 5mm then it can be ignored since the effect will be negligible. Larger differences need compensating for if measurements are taken at the end of the tank.
The simplest way to do this is find the horizontal centre of the tank, measure the distance to the point at which measurements are made and then measure the same distance the other side of the centre point. Measure the horizontal level difference between each of these two points and divide it by two to give the offset to be added to the level reading if the measuring point is higher than the other end of the tank (subtract if it is lower).
Tanks Fitted With Level Gauges
Often tanks are fitted with an external gauge that shows the level in the tank. These usually take the form of a long tube the height of the tanks where the outlet pipe is. These rarely have any depth markings on them and if they do they are often inaccurate. To use these gauges properly you need to know where the bottom of the tank is, not easy if it is a bunded (double skinned) tank. The simplest way to allow for this is measure the depth of the tank and mark the top of the gauge level with the top of inner tank (not the top of the filler or neck of the tank) measure the distance from this point to the base of the outside tank. Subtract the tank depth and you will know how high above the base the bottom of the inner tank is. With this information either the gauge can be marked in inches or centimetres or you will know how much to subtract from the depth measure from the base of the outer tank.
Finding Out The Real Capacity Of A Tank
Sooner or later it will become obvious there is a problem measuring oil consumption in that when oil is added to the tank the consumption per day shoots up or goes down significantly for no apparent reason and then goes back to normal on the subsequent readings. This is due to the tank capacity being a nominal rather than an absolute figure. There can be quite a variation between quoted and actual tank capacities, especially with plastic tanks. Solving this problem is straightforward but does involve a bit of maths.
- Measure the oil level before and after a delivery - the difference (depth 1) is equivalent to the amount of oil added
- Using the tanks quoted capacity work out how much oil depth 1 equals (nominal oil used)
- Divide oil added by nominal oil used and multiply the result by the quoted tank capacity - gives the actual capacity
Done correctly this will reduce this type of error almost to zero. For cylindrical tanks it is important the same depths are used because the change of volume with depth is non linear. It does of course assume that the delivery volume is accurate.
Tank Quoted Capacity = 1760 litres (rectangular tank 48 inch depth)
Oil Added = 500 litres (starting depth 20 inches, fill up depth 30.5)
Volume represented by 20 inches to 30.5 inches using tanks nominal capacity is 486 litres
Actual capacity of tanks is (500 / 486 ) * 1760 = 1811 litres (a difference of 2.9%)
To help monitor oil use JSutils has produced two heating oil calculators called EnCalcOL and EnCalcOU which work based on tank depth or volume readings and copes with different shape tanks. EnCalcOU calculates cost and use, EnCalcOL calculates use. Both can also predict oil use. They are both free for personal non commercial use.