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A liquid-level card receives motion from a float system whose equilibrium depends on liquid density, surface effects and mechanism friction. Its electrical surfaces may be immersed, isolated behind a seal or exposed to vapor. A calibration performed in one reference liquid can shift when density changes, while seal drag can introduce hysteresis that resembles a track problem. Interface review names the liquid states, mechanical conversion and readout circuit so each observed error is assigned to the correct owner.
System boundary
A liquid volume, float, arm or guided element, sealed penetration where used, resistive card, terminals and receiving circuit. Vessel integrity, fluid suitability, process control and system safety remain integrator responsibilities.
System integration decisions
- Specify density and temperature at calibration and service states.
- Describe the seal as both a fluid barrier and a mechanical load.
- Define the electrical measurement boundary and timing.
Describe the liquid as an engineering input
Record composition or controlled designation, density versus temperature, viscosity where motion depends on it, contamination, conductivity, pressure and gas phase. Include cleaning and service fluids. Do not generalize from water to oil, reagent or fuel. The integrator determines chemical suitability and safe handling. The card review uses the named exposure only to bound materials, protective layers and measurement states.
Connect density to available restoring force
A float equilibrium screen compares buoyancy with weight and mechanism loads.
F_available=rho_liquid g V_displaced-W_float-F_seal-F_friction
- rho_liquid and V_displaced determine buoyancy.
- W_float is float and attached moving weight in the motion direction.
- F_seal and F_friction represent bounded opposing loads.
Quasi-static condition with defined orientation; dynamic acceleration, wetting and turbulence require validation.
Illustrate density sensitivity without inventing a product curve
For the same displaced volume, changing density from 1000 to 850 kg/m3 reduces buoyant force by 15 percent. If mechanism reserve was small, equilibrium position or return behavior can shift even though the resistor card is unchanged. This percentage example is not a liquid recommendation. Actual float immersion and linkage geometry determine the level-to-position relationship.
Treat sealing friction as a measured motion interface
A rotary shaft seal, bellows, magnetic coupling or sliding rod contributes different friction, preload and temperature response. Measure breakaway and running behavior in both directions under relevant pressure and fluid exposure. Aging can increase drag or create leakage. Do not compensate unknown seal behavior by reshaping the printed curve. Allocate mechanical hysteresis separately and preserve raw position during calibration.
Map immersed, splash, vapor and dry regions
Show maximum and minimum levels, orientation, venting, seals, drainage and terminal location. Capillary paths along conductors, wires or narrow gaps deserve explicit review. Protective glaze coverage must follow the drawing and its material qualification; appearance alone does not prove barrier performance. If the wiper operates immersed, define debris, current and motion because contact behavior becomes part of the media interface.
Make fluid and electrical assumptions reviewable
A compact matrix links each physical input to its downstream decision.
| Input | Affected decision | Owner |
|---|---|---|
| Density and temperature | Float equilibrium and curve | System calibration |
| Seal friction and pressure | Hysteresis and return | Mechanical design |
| Exposure zone | Material and ingress validation | Housing/material owner |
| Excitation and input load | Electrical transfer | Instrumentation owner |
Define excitation, loading and sampling state
Record topology, source limits, contact current, harness and receiver impedance. State whether sampling occurs during motion, after dwell or under digital filtering. Compare direct card resistance with receiver-boundary voltage when diagnosing error. Any diagnostic bias or power-off state that energizes the card belongs in the state matrix. The passive element does not provide process alarm logic.
Validate rise, fall and disturbed-liquid conditions
Acquire fluid reference, mechanical position and raw electrical output together. Perform controlled rising and falling sequences to reveal seal and contact hysteresis. Include named temperature and density corners plus selected agitation or settling states defined by the integrator. ChipSimple can review drawing-defined card construction; vessel, seal, fluid and complete instrument validation remain customer-controlled.
Release a configuration-specific interface sheet
Identify vessel, fluid, float, seal, card, harness and receiver revisions. Include density range, motion forces, exposure map and measurement sequence. Reopen after formulation, temperature, pressure, float, seal, vent, orientation, card or electronics changes. A generic liquid-compatible statement cannot replace this configuration boundary or the application owner’s qualification evidence.
Installation attitude and acceleration modify the apparent force direction on the float. A vessel mounted on a slope or moving platform can report a different level distribution from the laboratory reference. Define which attitudes require accurate measurement, which permit delayed indication and which are outside operation. Baffles and narrow chambers may slow equilibration. Capture the reference liquid surface or another traceable level measure alongside float position rather than inferring it from commanded fill volume alone. For viscous liquids, dwell criteria may vary with temperature. The interface sheet should state whether calibration uses mass, volume, depth or a geometric reference and how uncertainty propagates to the electrical acceptance points.
Sensor response during rapid level change may be limited by float inertia, seal friction, restricted chamber flow or controller filtering. Separate these contributors with time-aligned liquid reference, mechanical position and raw voltage. Do not assign the slowest response automatically to the resistor card. Establish an approved settling definition for calibration and a separate transient criterion if the application needs one. Preserve both rising and falling records because direction can change the dominant mechanical force.
If the sensing card is isolated from liquid, verify that the isolation mechanism preserves the intended motion range and does not add unallocated magnetic, elastic or friction effects across temperature.
Document gas-pocket or foam conditions if they influence the float. The reference method should distinguish true liquid height from an aerated mixture and assign stabilization responsibility to the application owner.
Provide the fluid, seal, mechanism and readout definitions
Level accuracy depends on all four interfaces.
- Named liquids, density/temperature range, pressure and exposure states.
- Float or moving element, linkage, friction, seals and vessel geometry.
- Card track, wiper, terminals and protective-layer requirements.
- Excitation, receiver, sampling, calibration method and acceptance ownership.
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