Float Limitations in Magnetostrictive Measurement
In magnetostrictive level measurement, one or two floats are used inside the tank. Floats have a certain volume and height; they do not move until the fuel or water reaches the level needed to make them float. For this reason, the region from the bottom up to a certain height cannot be measured — this is called the deadzone. If this threshold is not defined on the automation side, the system will show fuel/water equal to the physical position of the float even when the tank is empty. To prevent this, the deadzone and offset values must be correctly configured in the automation.
Definition of the Unmeasurable Region
The deadzone threshold varies according to the float height, number of floats, and distance of the sensor from the tank bottom. Single-float systems and fuel+water combinations produce different deadzone values.
Fuel Float
Measures only the fuel level. The volume from the tank bottom to the point where the float starts to float is considered the deadzone — an unmeasurable region.
Fuel + Water Float
Two separate floats are used: the heavier water float operates near the bottom and measures only water that collects on the floor; the fuel float rides above it at the fuel level. The fuel deadzone equals the sum of the water float's physical length and the fuel float's floating height.
What happens if deadzone is not configured?
When deadzone information is not defined in the automation, even if the tank is completely empty, the system will show as much “ghost” fuel/water as the physical position of the water or fuel float. Measurements in this region are considered unreliable. To prevent this, the deadzone and offset values must be correctly entered into the automation.
How Is Deadzone Information Entered?
- 1
Measurement with Empty Tank
With the tank completely empty, the value reported by each float is recorded. This value is the deadzone threshold for that float.
- 2
Entering the Threshold into Automation
The recorded value (e.g. 17.65 mm → 18 mm for water; 52.73 mm → 53 mm for fuel) is entered into the automation system as deadzone information.
- 3
Below Threshold Is Treated as 0 by the System
All level values below the defined threshold are automatically treated as 0 mm / 0 L by the automation, preventing the unmeasurable region from appearing as a misleading volume.
- 4
Actual Volume Is Shown Above Threshold
Once the level exceeds the threshold, the automation reads the volume value from the calibration table and displays the real quantity in the system.
Float Readings Taken with Empty Tank
The values below belong to a sample installation. Readings taken with an empty tank are rounded up and entered into the automation system as deadzone.
Water Float
17.65 mm → 18 mm
All levels below 18 mm are treated as 0 mm / 0 L.
Fuel Float
52.73 mm → 53 mm
When 53 mm is exceeded, the volume from the calibration table is displayed.
Correcting Float Deviation
The value read by the float may deviate by a few millimetres from the actual value obtained through calibration in the tank. This difference is defined in the automation system as an offset; the system applies this correction to the float reading to improve the displayed value.
Float Does Not Reach the Tank Bottom
When the float is too short, an unmeasurable height remains between the bottom and the lower limit of the float. This height must be entered as the offset.
Density (Specific Gravity) Difference
If the specific gravity of the fuel in the tank differs from the reference fuel used during float calibration, the float may read a few millimetres off. Temperature changes also affect fuel density, so this deviation can be seasonal.
Mechanical / Calibration Deviation
Due to manufacturing tolerances, tank deformation or field conditions, the float may deviate by millimetres from the actual value obtained through calibration. This deviation is entered into the automation as a single offset value.
Example 1 — Density / Calibration Deviation
Actual Value (Calibration)
102.34 mm
Float Reading
102.06 mm
Offset to Enter
+0.28 mm
Example 2 — When the Float Is Too Short
Actual Value in Tank
102.34 mm
Float Reading
92.34 mm
Offset to Enter
+10 mm
Calculation on the Automation Side
The automation first corrects the float reading (applies the offset), then compares this corrected value against the deadzone threshold. Values below the deadzone are processed as 0; once the threshold is exceeded, volume is calculated from the calibration table.
1. Offset Application
Corrected Level = Raw Level + Offset
A fixed correction is applied to the millimetre value from the sensor; this value is entered into the automation.
2. Deadzone Check
Below threshold → 0 mm / 0 L | Above threshold → Corrected Level is shown
Levels below the deadzone are processed as 0; levels exceeding the threshold are converted to volume using the calibration table.
3. Volume Conversion
Volume (L) = Calibration Table( Displayed Level )
The valid level is converted to volume via the calibration table. Alarm thresholds in the automation are linked to this volume.
Quick Reference
Level
The millimetre value of the height at which the float is positioned inside the tank.
Volume
The litre equivalent of the level as converted through the calibration table.
Deadzone
The bottom region where the float physically cannot float and measurements are considered unreliable.
Offset
The fixed millimetre correction between the float reading and the actual level.
Calibration Curve
A measurement table derived from the tank geometry that maps level (mm) to volume (L).
Float
The magnetic level-sensing element that floats on the magnetostrictive measurement probe.