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A radar dead zone should be checked with a dimensioned calculation, not by looking at a full-scale percentage. The useful result is the distance remaining between the highest surface that must be measured and the installation's confirmed near-measurement limit. That margin can change when the mounting reference changes.
Calculate Radar Dead Zone Margin From One Datum
Define H as the radar measurement reference height above the tank datum, L as the highest surface requiring a valid reading, and dmin as the confirmed near limit for the selected assembly and installation. The surface distance is H minus L. The geometric measuring margin is H minus L minus dmin.
This margin is not the tank's safe filling allowance. Stopping inflow, waves and process protection require their own assessment. Where such effects determine the highest surface that must remain measurable, include them in L rather than silently treating the normal operating setpoint as the maximum.
Example: Same Tank, Two Different Requirements
For an illustrative installation, assume H is 6.000 m and the approved near limit is 0.200 m. These values are examples, not model limits. Compare two proposed highest measurable surfaces:
| Case | Calculation | Meaning |
|---|---|---|
| L = 5.600 m | 6.000 - 5.600 - 0.200 = 0.200 m. | Positive geometric margin; other installation and process checks still apply. |
| L = 5.900 m | 6.000 - 5.900 - 0.200 = -0.100 m. | The required surface extends 0.100 m inside the assumed excluded region. |
| New mount | Recalculate H from the approved as-built reference. | Do not reuse the old margin after adding or removing an adapter. |
The MV-RD12 lists a 0.1-30 m liquid interval. Obtain the applicable installation limit rather than substituting that catalogue near value into every tank calculation. Document the source of dmin on the calculation sheet so a later reviewer can reproduce the decision.
Carry the Result Into the Test Plan
A negative margin identifies a design conflict before commissioning. Changing the percentage scale does not resolve it. Review a suitable installation change, a different measuring arrangement or a reduced operating requirement with the responsible project team.
A positive margin shows geometric room under the stated assumptions; it does not prove that the surface echo remains valid there. State the highest safely demonstrated test level and any untested interval. Keep that evidence separate from the calculation so a design estimate is not reported as an observed result.
Calculation Questions
Q1. Is zero margin acceptable?
It places the required surface exactly at the assumed limit and leaves no allowance for dimensional uncertainty or unaccounted surface movement. The project must assess those effects and the applicable installation requirements rather than accepting zero by arithmetic alone.
Q2. Which dimensions belong in an enquiry?
Provide the reference height, highest required surface, datum, nozzle section and proposed near-limit basis. Identify estimated dimensions and the conditions that can raise the surface above its normal setpoint.
Summary
Use an explicit margin calculation to expose an upper-range conflict early, then verify the usable region in the installation. Send the calculation and its assumptions for an upper measuring-range assessment before releasing the tank's operating range.
