28 Oct 2017 By Russell Dupuy

Calibrating ATG Accuracy

We are often asked the question: “Which Automatic Tank Gauge is more accurate?”

With so many ATG brand options available, such as Veeder-Root, OPW, Hectronic, Franklin and other leading brands, it can be difficult to evaluate which offers the best accuracy. Our response is always the same:

There are three factors that contribute to ATG accuracy

  1. Probe resolution
  2. Inherent hysteresis of the probe itself; and
  3. Calibration.

All ATG manufacturers quote a probe resolution with varying forms of full scale deflection error, or similar parameters, that need to be considered in the context they are presented and how the calibration parameters of the particular ATG operate.

Issue 1: Probe resolution

Probe incremental resolution is helpful and referential only, and should not be the single basis for an accuracy assessment when considering calibration.

Issue 2: Hysteresis

Hysteresis is simply the lag time between the physical movement of the float and the corresponding electrical change in either a digital or analogue probe. The electronics and software does account for this [varies brand to brand] to a certain degree through slow and fast buffering however, if a float oscillates, probe resolution reliability becomes compromised.

Issue 3: Calibration

Calibration is typically a set of program parameters that account for the geometry of the tank, such as internal diameter, end shape, and cylinder or barrel length as the basic parameters. Most ATG’s then offer additional parameters that account for probe offset, both vertical and horizontal, and also gradient of the slope or what is commonly called ‘Tilt’. Applied when the probe is not fitted to the fulcrum of the tank.

The later issue is the most problematic, usually guessed when it’s difficult to determine the fulcrum of the tank relative to probe position, further confounded when either volume to height is drawn from the dip stick or a standard tank chart. Rarely is offset, and/or end shape considered, which manifests error in the daily reconciliation, typically highlighted on delivery days.

Often the ‘tilt’ factor is used as a fudge factor that can be counterproductive, especially so for cylindrical tanks on their side, where the height to volume conversion is not linear.

This not only creates fuel reconciliation error, with regard to managing daily fuel loss/gains but further compromises Statistical Inventory Reconciliation [SIR] for the purposes of sound leak detection, if SIR is the method of leak detection used.

If your fuel system is experiencing poor reconciliation, or an average cumulative variance greater than +/- 0.5% of throughput, calibration could be the culprit. For more information contact EMS today.