MethodWhat is calculated?
The bridge signal grows in proportion to the excitation, but the heat generated in the gauge grows with its square. In a full bridge each gauge sees half the excitation, so P_DMS = U_B²/(4R). Whether the heat is removed depends on the power density referred to the grid area together with the object's thermal conductivity. Keil's example: 5 V on 120 Ω gives 52 mW and 21 mA per gauge, on 350 Ω only 18 mW and 7 mA.
Equations
P = U²/R (Keil Gl. 2.40)
P_DMS = U_B²/(4R) (Keil Gl. 2.41)
Leistungsdichte = P_DMS / (l_g · b_g)
U_B,max = √(4·R·p_zul·A)
LimitsAssumptions and typical mistake
Guidelines apply to well-bonded foil gauges on flat surfaces; stacked rosettes and quarter bridges with completion resistors inside the instrument need separate consideration.
Applying the full excitation instead of U_B/2 to a single gauge, or confusing grid area with carrier area.
InputsWhat you enter – and where the values come from
- Bridge excitation U_B [V]
- The bridge excitation is the DC or AC voltage with which the amplifier supplies the bridge circuit. The signal is proportional to it – which is why signals are usually stated as a ratio in mV/V and the excitation is only needed for the absolute signal voltage. Source: amplifier setting, usually 2.5 V, 5 V or 10 V; the upper limit is set by gauge self-heating (see the excitation calculator).
- Gauge resistance R [Ω]
- Electrical resistance of one strain gauge, not the complete bridge. At the same excitation, 350 ohms carries less current than 120 ohms and therefore produces less heat. Source: package marking or manufacturer datasheet for the gauge used.
- Grid length [mm]
- Length of the active measuring grid (the meandering conductor, not the carrier foil). Together with the width it gives the area through which heat flows into the part: a large grid tolerates more power. Source: datasheet; common 1.5 / 3 / 6 / 10 mm.
- Grid width [mm]
- Width of the active measuring grid, measured across the measuring direction. Together with the grid length it gives the heat-dissipating area; for rosettes with stacked grids only the area of the top grid counts. Source: dimension drawing in the gauge datasheet.
- Allowable power density [mW/mm²]
- Guideline for how much electrical loss per square millimetre of measuring-grid area the part can dissipate without noticeable heating. Massive, conductive metals tolerate more; thin sheets, plastics, wood and stacked rosettes much less. Source: the gauge manufacturer's loading table for specimen material, grid size and required accuracy. Use the lower value when uncertain.
ContextWhat this calculator is for
Setting the amplifier excitation, choosing between 120 Ω and 350 Ω gauges and assessing warm-up behaviour (zero drift after switch-on).
What comes next
- Enter grid dimensions and resistance from the datasheet.
- Choose the power-density guideline by material and type of measurement.
- Set the excitation so the power density stays below the guideline; reduce it markedly for plastics, wood or stacked rosettes.
SourceTechnical basis
Keil, Dehnungsmessstreifen, 2nd ed. 2017, sec. 2.8 electrical loading capacity, eqs. (2.39)–(2.41) and guidelines per [2.8.3].
The source supports the equation structure and worked examples; this calculator does not replace calibration of the measuring chain.
FAQFrequently asked questions
Why not simply use the highest excitation?
Warm-up error rises with the square of the excitation; creep and hysteresis increase and self-compensation can be disturbed.
Does the formula apply to carrier-frequency excitation?
Yes, using the RMS excitation; Keil found no difference in warm-up between DC and 1 kHz.
What does the adhesive change?
Hot-curing epoxies reduce the warm-up error to about 30 % compared with cold-curing adhesives.
In the same cluster
All strain-gauge calculators