Tank geometry and sender arm travel in an actual fuel-gauge chain

System boundary

Fuel surface and tank geometry through float, arm, pivot, wiper, resistor card, harness and gauge input.

Integration interfaces

Integration risks

Start at the tank, not at the resistance value

The controlled variable may be fuel height, usable volume or a displayed fraction, and those are not interchangeable in an irregular tank. Freeze the installed tank cross-section, vehicle attitude convention, unusable volume and reference plane before defining sender travel. The component boundary begins where the float mechanism applies motion to the wiper and ends at the specified card terminals. A bench resistance reading therefore cannot establish displayed fuel volume by itself.

Name each physical handoff: fuel surface to float, float to arm, arm to pivot, pivot to wiper, wiper to track, track to terminals, terminals to harness and harness to gauge electronics. Record the owner, datum and permissible load at every handoff. This prevents a linkage error, connector drop or software conversion error from being reported as a resistor-card defect without a discriminating test.

Convert tank geometry into required sender motion

Use the real internal tank profile, including slopes, baffles, sumps, pump modules and regions the float cannot enter. At selected liquid heights, calculate or measure the corresponding contained volume. The resulting height-to-volume curve may have changing slope. Choose the height interval that must be sensed and show where the float center sits relative to walls and obstacles throughout that interval.

Check the sender in the installed orientation and at the integrator-defined attitudes. A level surface in a tilted tank can move the float toward a baffle or alter the height seen near the pivot. Preserve clearance envelopes at empty, full and intermediate positions. Mechanical stops should protect the card from overtravel without truncating the required measurable volume range.

Map float-arm rotation to wiper travel

Define pivot coordinates, effective arm length, float-center offset, shaft direction and the relationship between shaft angle and wiper position. For a simple planar arm, vertical float displacement can be written as h(theta)=h_p+L sin(theta), subject to the actual installed angle convention. Linkage bends, lost motion and nonplanar motion require the integrator model rather than an assumed sine curve.

Differentiate the kinematic relation to find sensitivity: dh/dtheta=L cos(theta). Near angles where cosine becomes small, substantial rotation may create little height change, or a small height uncertainty may create a large inferred angular uncertainty. Place resistance-curve breakpoints using the real kinematic and tank-volume slopes instead of distributing them evenly only because the printed arc is uniform.

Calculate the installed height-to-resistance mapping

At each controlled height, follow the mapping h to theta, theta to wiper coordinate s, and s to resistance R. The local installed gain is G_h=dR/dh=(dR/ds)(ds/dtheta)(dtheta/dh). Keeping the three factors visible identifies whether a steep gauge response comes from track design, linkage ratio or arm geometry. It also shows where one uniform track slope cannot create a uniform volume indication.

For an illustrative average, a 180 ohm resistance span over 300 mm of useful height gives 0.60 ohm per millimetre. That arithmetic ignores tank shape and changing linkage sensitivity. It is not a product tolerance or a company capability. A project worksheet evaluates the gain at multiple heights and carries measured bounds for pivot location, arm length, contact position and resistance.

Define excitation, harness and gauge loading

State whether the card is read as resistance, divider voltage, current, ratio or a filtered controller value. Provide excitation level and tolerance, pull-up or reference resistance, input impedance, connector and harness resistance, ground arrangement, ADC range, sampling interval and conversion equation. Measure at the nodes used by the gauge rather than at convenient card pads that bypass installed wiring.

Separate four observations when diagnosing the chain: independent mechanical height, shaft or wiper position, card-terminal resistance and gauge-input value. A stable card resistance with a changing gauge input indicates wiring or electronics. Correct gauge input with an incorrect height relationship points upstream to tank geometry or linkage. Preserve raw readings before software damping or display quantization.

Discriminate tank, mechanism, contact and readout faults

Arm obstruction often appears as a plateau in wiper position while liquid height continues to change. Pivot backlash creates direction-dependent position around reversals. Contact dropout produces a short electrical discontinuity without equivalent mechanical motion. Harness intermittency changes the terminal-to-gauge relationship. An incorrect tank conversion can produce repeatable electrical signals and still display the wrong volume.

Diagnose from the first interface where the reference and observed value diverge. Do not adjust the printed curve to compensate for an unidentified float collision or a software scaling mistake. Retain the installed state before bending an arm, moving a stop, cleaning a contact or changing filtering, because those actions can erase the evidence needed to assign ownership.

Validate the complete installed transfer chain

Use a representative tank or a justified fixture that reproduces pivot, obstacles, sender orientation and electrical interfaces. Exercise filling and draining directions because buoyancy, friction and backlash may be direction-dependent. Sample endpoints, reversal regions, tank-shape transitions and positions near internal obstacles. Record fuel substitute properties if a substitute fluid is used and state which buoyancy or compatibility effects it does not reproduce.

Freeze valid-run criteria before viewing confirmation results. Synchronize liquid-height reference, arm angle or wiper coordinate, terminal resistance and gauge input. Independent assemblies estimate installation variation; repeated readings on one sender estimate observation repeatability. Validation acceptance comes from the integrator-controlled requirement. This page claims no vehicle approval, regulatory compliance or functional-safety status.

Allocate system risks and diagnostic response

The risk review includes false empty, false full, stuck indication, intermittent dropout and an apparently plausible but biased reading. For each event, the integrator records operational consequence, detectability, diagnostic timing and safe or degraded response. Dual sampling or filtering may improve observation but cannot remove a mechanical common cause such as an obstructed float.

The resistor card can be evaluated for its drawing-defined transfer and contact behaviour at specified interfaces. Tank crashworthiness, fuel compatibility of the complete sender, vehicle diagnostics, warning strategy and remaining-range calculation belong outside that component evidence. Unknown requirements remain open in the project record rather than being converted into implied capability.

Control the installed state through change

Link tank drawing, baffle arrangement, float, arm, pivot, stop, card, wiper, connector, harness, gauge electronics and conversion software by revision. Store the synchronized transfer data with assembly genealogy and installed orientation. Identify which tank regions and attitudes were represented in validation. A card with the same nominal resistance span is not automatically equivalent if its travel, contact geometry or terminal reference differs.

Revalidate the affected mapping after changes to tank shape, sender mounting, pivot position, arm length, float, stop, track curve, wiper, harness, excitation, input impedance, filtering or software conversion. State which earlier measurements remain comparable. A mechanical correction should not be credited to the card, and an electrical correction should not conceal unresolved interference.

RFQ inputs for a fuel-level sender resistor card

Provide the tank section or controlled height-to-volume data, sender mounting coordinates, useful height range, pivot and arm geometry, float envelope, direction of motion, mechanical stops and allowed contact travel. Identify the resistance or voltage curve required at named reference positions. Include terminal drawing, wiper definition, excitation, gauge input, harness assumptions, environment, duty history, quantity and failure consequence.

Send the integrator-owned acceptance criteria and indicate how height or volume will be referenced during validation. List known risks such as baffle contact, tilt sensitivity, reversal hysteresis or diagnostic thresholds. If tank geometry, mechanism loading, electrical input or safe response is not defined, mark it for application review. Missing system values are not guessed during quotation.