Fuel level sensors are very popular tools of fuel consumption monitoring it GPS tracking industry. Very important stages of its installation – sensor calibration and tank calibration. Every installer wants to make installation process as simple as possible. This article could help you to cut installation time or avoid calibration errors.

Let’s start from difference between sensor calibration and tank calibration.

All existing fuel level sensors measure level, not volume. However, your customer most likely wants to see fuel volume in reports. To calculate fuel volume we have to know tank shape. In fact – fuel tank calibration is calculation of shape and formula to convert fuel level into fuel volume. We made it with filling tank by small portions of fuel with recording sensor output. Tank calibration by fillings is a time-consuming procedure and someone may try to look for sensors, which does not require tank calibration. Nevertheless, it is not possible, because tank calibration procedure refers to tank and not to sensor.

Sensor calibration – procedure of sensor learning to its new length after cutting or extending.

What is the reason to do that? Most popular fuel level sensors for GPS tracking have RS232/RS485 interface. Sensor comes from factory already pre-calibrated to default length and sensor output signal could change from “0” (empty) to “1023 (or 4095)” (full). After cutting this sensor its output range shifts down and while decreased maximum output signal is not a problem, the empty tank output value cannot decrease below zero. As a result, blind zone appears, proportional to the length of sensor cut. Same problem with sensors with analog voltage output, because after cutting and output range shifting sensor should send negative voltage for empty tank and obviously can’t do that.

Thus, after cutting we should teach sensor to its new length by immersion in fuel by 0 and 100% (“empty” and “full” calibration).

Therefore, we have to calibrate sensor after cut and then make a tank calibration. Now let’s think how to simplify these procedures.

Someone can notice that during tank calibration we fill the tank from empty to full condition. Sensor calibration “empty”-“full” is not needed if sensor configuration software can calibrate sensor itself based on tank calibration data.

For example, we calibrate tank using Eurosens Dominator sensor configurator and first and last points of tank calibration would be sensor calibration points at the same time (Fig. 1). Therefore, we can save about 5 minutes of sensor calibration procedure.

Fig. 1. Tank calibration using Eurosens Dominator configurator

Next step is tank calibration. How can we avoid this procedure?

If your customer will be satisfied with fuel height in mm or percentage, you can skip the tank calibration. However, he doesn’t, everybody wants a liters. Only question – how to do the tank calibration.

1. How does the shape of the tank affect the calibration?

For tanks in which the cross-sectional width is constant in height, two points are sufficient – full and empty. Examples of such shapes: a perfectly rectangular tank (without roundings) or a vertical cylinder (not horizontal!).

Fig. 2. Ideal tank shapes, which don’t require the tank calibration

 The tank calibration results can be plotted on a graph. For the tanks shown in fig. 2, the graph will represent a straight line.

Fig. 3. It makes no sense to calibrate the entire tank, since all points fall on one straight line.

However, such tanks cannot be found in practice. Real tanks have rounding, dents (Fig. 4).

Fig. 4. Most of the “rectangular” tanks have roundings

It affects the fact that the real calibration curve is not a straight line (see Fig. 5).

Fig. 5. Graphs of the calibration tables for rectangular tanks with roundings and irregularly shaped tanks

The graph shows that in the part of the tank where the walls are vertical and not curved the points of the calibration curve still on the straight line, and the graph is curved in the places where the tank is rounded. Thus, we can say that to calibrate rectangular tanks with roundings, you can use a different step of filling portions – fill in smaller portions in the area of the curvatures and larger ones in the middle of the tank, where the walls are vertical. This will save you time while maintaining the accuracy of the calibration.

Hence the answer to the question – what will happen if the tank is calibrated only at 2 points – full and empty? The error will increase depending on the size of the roundings, with the error being higher when the fuel is inside the rounding zone.

But for tanks of complex shape (Fig. 6) calibration with a constant step is required, since there are no straight sections of the graph, as can be seen from Fig. 5. The same applies to the shape of the “horizontal cylinder” tank.

Please note that plotting calibration tables allows you to verify the calibration. Calibration defects are clearly visible on the graph even before the customer complaints about the inaccurate operation of the fuel consumption control system. On the calibration graph vertical or horizontal lines states about calibration errors (Fig. 6).

Fig. 6. Tank calibration error

To test ourselves and our contractors, we’ve developed the simple software that compares calibration files with each other by building plots in Excel.

2. There is absolutely no way to calibrate by filling in portions. What can I do?

It happens, especially during installation on large storage tanks. Filling tens of cubic meters of fuel in portions is a very complicated procedure. In this case, there are several solutions.

2.1 Calculate the calibration table based on the tank dimensions. For horizontal cylinder tanks, you can use our online calculator, which will generate a calibration fule for the Eurosens Dominator level sensor.

Fig. 7. Tank calibration calculator

This method does not take into account the existing shape defects and elements of the tank, which can affect the volume and calibration.

2.2 Build a calibration based on a three-dimensional model. It is used when the accuracy is required higher than in the previous method, but the tank cannot be calibrated with fuel. Then we build a three-dimensional model of the tank based on the dimensions you sent and add the inclination of the if needed. The model is used to calculate the tank calibration.

3. I have 10 identical cars. Is it possible to calibrate the tank only once, and then copy it everywhere?

That’s quite possible but we have to check some things. Individuality is possessed by both outwardly similar-looking fuel tanks and sensors, each of which is slightly different in its measuring electrodes (not to mention the fact that they can be cut in different ways). We advise you to selectively check the “uniformity” of the tanks by filling several tanks to full and thus measuring their total volume.

After that, you will have to carry out a full-empty calibration for all sensors so that the sensor output signal during calibration does not depend on the individuality of its electrode. After that, calibrate one tank and use it for the remaining tanks of equal volume and shape.

4. If the sensors are different, what should I do when replacing the sensor?

The individuality of the sensors mainly lies in its electrodes. If we replace the Eurosens Dominator modular sensor, then the electrodes from the replaced sensor can be used. In this case, it is enough to check the internal value of the sensor’s detector after assembly with old electrodes – if it matches in the “empty” state with the value in the calibration table from the old sensor, then there are no obstacles to using the old calibration.