Influence of Insulation Resistance

Insulation resistances of strain gages including lead wires do not affect measured values if they are higher than 100 MΩ. However, if they change drastically during measurements, errors may occur in measured values.

Bridge Circult Designed with Insulation Resistance

If the insulation resistance descends from r 1 to r 2 in thefigure above, error strain ε is :

For example,
R g = 120Ω (Resistance of strain gage)
K s = 2.00 (Gage factor of strain gage)
r 1 = 1000 M (Original insulation resistance)
r 2 = 10 MΩ (Changed insulation resistance)

Then, the error strain is approximately -6 μ­m/m.
Such error is no matter in general strain measurement. In practice, however, if insulation resistance is lowered, r 2 is not constant and be drastically changed due to environment change, such as temperature and humidity. In addition, it is impossible to specify insertion place of insulation resistance, r, in circuit. Thus, be careful about influence of insulation resistance.