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A high-temperature level sensor that disagrees with a cold-tank reference is not necessarily drifting. The liquid can expand, the vessel can move and the radar reference point can shift as temperature changes. Re-zeroing the instrument at each temperature hides these different causes and can leave the next batch with an unexplained offset.
What Changes Between the Cold and Hot Readings?
Record raw distance, calculated level and the independent reference at matching times. Include liquid temperature, heating state and whether filling, agitation or withdrawal is occurring. A comparison taken during an active transfer cannot isolate thermal behaviour unless the changing quantity is independently established.
For a repeatable check, use stable process states that the site can reach safely. Do not assume the same liquid mass must produce the same liquid height at two temperatures. Likewise, a gauge attached to a moving vessel may not share the datum used by a fixed external survey.
| Pattern | First check | What it means |
|---|---|---|
| Raw distance changes | Reference geometry and actual surface position. | A changed distance can be physically correct. |
| PLC alone changes | Scaling, offsets and selected process variable. | The discrepancy may originate after measurement. |
| Echo jumps | Condensation, surface motion and competing reflections. | A thermal cycle can alter the selected target. |
| No return | Cold-state geometry, residue and configuration history. | Investigate a lasting change before another zero adjustment. |
High-Temperature Level Sensor Limits Apply to the Assembly
The METRAVON MV-RD12G has a heat-dissipating process assembly and a published process-temperature interval of -40 to 200 deg C. That figure is not an electronics ambient rating. The pressure, seal and connection must be approved at the concurrent operating temperature; separate maxima cannot be combined automatically.
Inspect the insulation boundary and nearby hot surfaces before blaming the distance measurement. Record housing conditions where the approved procedure permits measurement. A hot process and a cooler outdoor start can challenge different parts of the assembly. Preserve the original settings throughout the comparison unless an identified error requires correction.
Questions During Thermal Testing
Q1. Should cold and hot level readings be identical?
Only if independent evidence establishes the same physical level relative to the same datum. Thermal expansion and process movement can change the actual surface. Compare the instrument with a suitable reference at each state, not with an assumed constant height.
Q2. When is an offset adjustment justified?
After proving a consistent reference error and confirming that the echo tracks the intended surface. Save the before-and-after settings and repeat the heating and cooling comparison. A correction that works at one temperature but fails on return is not a completed repair.
Summary
For an MV-RD12G review, supply the temperature cycle, paired reference readings, raw distances and insulation drawing. This evidence distinguishes a thermal installation problem from a genuine calibration issue without inventing a universal warm-up time.
