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Liquid level creates buoyant force, not a commanded wiper position. At equilibrium, displaced-liquid force and its moment about the pivot balance float weight, arm weight, spring effects, contact load and friction. Begin with the actual float shape and mass properties.
Key design decisions
- Define the float equilibrium before track geometry.
- Calculate immersion from real float geometry.
- Convert liquid height to arm angle without assuming linearity.
Define the float equilibrium before track geometry
Liquid level creates buoyant force, not a commanded wiper position. At equilibrium, displaced-liquid force and its moment about the pivot balance float weight, arm weight, spring effects, contact load and friction. Begin with the actual float shape and mass properties. A hollow molded float may have a waterplane area that changes with immersion, and a pivoted float changes orientation as it rises. State which liquid density and temperature correspond to each calculation. Do not use a single nominal density to represent every fuel, oil or process liquid. The output of this step is a defensible arm-angle-versus-height relation with unresolved friction shown separately, not a resistor curve.
rho g Vsub dB = Wf dW + Warm darm + Mcontact + Mfriction
- rho: liquid density for the evaluated state
- Vsub: displaced volume
- dB: buoyancy moment arm
- Wf and Warm: float and arm weights
- Mcontact: moment from the contact mechanism
- Mfriction: bounded resisting moment
Quasi-static equilibrium; dynamic slosh, acceleration and transient drag require separate system analysis.
Calculate immersion from real float geometry
For a constant-section vertical float, a small height change displaces waterplane area multiplied by the change in immersion. Tapered, spherical or tilted shapes require geometry-specific volume integration. As a numerical screen, 18 cubic centimetres displaced in a liquid of 750 kilograms per cubic metre produces about 0.132 newton of buoyancy. That figure must be reduced by the weight and translated through the pivot moment arm before it can move a contact. Use the example only to check units and order of magnitude. The customer calculation needs measured mass, true submerged volume, orientation and any liquid trapped or retained by the float construction.
Convert liquid height to arm angle without assuming linearity
Locate the pivot above the tank reference, then solve the intersection between the liquid surface and the float centre or attachment geometry. A circular arm path produces a trigonometric height relation; nearby walls, guide slots or a shaped reservoir can constrain that path. Plot height against angle at fine increments and inspect the derivative. Near a top or bottom extremum, a large height change may produce little angular motion, consuming electrical resolution. Elsewhere, small height changes may sweep the contact quickly. These regions should influence track allocation and calibration-knot spacing rather than being hidden by a straight-line fit.
Measure fill and drain equilibrium as separate branches
Static friction and wetting can hold the float away from its ideal force-balance position. Fill the reservoir slowly to defined levels, allow a stated dwell and record height, arm angle and contact coordinate. Continue beyond each station before approaching it from the opposite direction during draining. The branch difference contains contributions from pivot friction, wiper load, float-wall contact and liquid effects. Change one contributor at a time where practical: a low-contact-force surrogate can separate linkage behavior from the production wiper, while an empty dry motion check can expose mechanical interference. Neither surrogate alone proves performance in liquid; their value is cause separation.
Reserve track only for mechanically reachable motion
Overlay the lowest and highest equilibrium angles, dynamic overtravel, assembly tolerances and stop positions on the card. The usable printed span begins only where the complete contact footprint remains on its intended route. Do not allocate resistance to an angle that the float cannot reach under the specified density or installation attitude. Conversely, preserve controlled continuity through any mechanically possible overtravel required by the system design. A sump, baffle or tilted vehicle can alter reachable liquid height without changing sender geometry. Those tank-level cases belong in the integration map and must not be converted into unconditional card capability.
Measure the four quantities needed to test the model
A useful fixture records liquid height, liquid condition, arm angle and wiper coordinate on a common time base. Measuring only electrical output cannot reveal whether an error came from buoyancy, linkage or card transfer. Confirm reservoir verticality and height scale independently. Observe pivot angle at the sender rather than at a remote drive. Where direct wiper imaging is impossible, use a validated geometric surrogate before applying current. Repeat at several temperatures only when the liquid and fixture can be controlled safely. The equipment and safety owner defines suitable handling; this guide does not prescribe a universal fluid test.
Interpret a mismatch by which coordinate stopped agreeing
If liquid height changes while arm angle remains fixed, inspect float contact, friction and force balance. If angle changes but wiper coordinate pauses, examine linkage clearance or the carrier. If mechanical coordinates agree and electrical output moves incorrectly, investigate the printed path, contact and readout. A consistent fill-versus-drain offset points to hysteresis; a sharp local event points to interference or a transition. Retain anomalous points and their direction instead of deleting them from a smooth calibration fit.
| Observation | Mechanical question | Next evidence |
|---|---|---|
| Height changes, arm fixed | Is buoyancy moment overcoming friction and weight? | Float clearance, mass, immersion and pivot-force observations |
| Arm moves, contact fixed | Is motion lost in follower or carrier clearance? | Simultaneous arm and contact coordinates |
| Coordinates agree, output differs | Is the error electrical rather than mechanical? | Loaded circuit and fixed-position contact readings |
| Fill and drain curves separate | Which resisting or wetting effect changes direction? | Bidirectional repeats with controlled dwell |
Join mechanics to the electrical design at one declared coordinate
Deliver the reservoir section, liquid-property envelope, float mass and shape, pivot geometry, force contributors, stops and measured height-angle-contact data. Choose one coordinate at the handoff: arm angle, contact centre or another inspectable mechanical quantity. The resistor-card designer can then map that coordinate to the required electrical function without rebuilding the tank model. Revisit the handoff after changes to liquid family, float, pivot height, arm, friction interface, wiper force or installation attitude. The RFQ should state the intended level range, density states, permitted hysteresis, electrical endpoints and validation owner. Complete-system volume indication and safety behavior remain the responsibility of the equipment program.
Send the buoyancy, immersion and usable track travel inputs
Provide the layout geometry, circuit and validation logs needed to assessment liquid height and density state against wiper displacement available to the resistor track.
- reservoir section, liquid-property envelope, float mass and physical geometry, linkage feature dimensions, observed travel map and specified level interpretation
- Governing permitted variations and raw observations for float waterplane area, arm torque and guide friction.
- Definition of the installed reservoir vertical axis and pivot elevation, motion or exposure history, fixture and receiver load influence.
- Allowed functional error, validation ownership, unresolved assumptions and specified technical basis format.
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