How does a wheatstone bridge

This section provides information for the quarter-bridge strain-gauge configuration type II. The quarter-bridge type II measures either axial or bending strain. This section provides information for the half-bridge strain-gauge configuration type I. The half-bridge type I measures either axial or bending strain. This section provides information for the half-bridge strain-gauge configuration type II.

The half-bridge type II only measures bending strain. This section provides information for the full-bridge strain-gauge configuration type I. The full-bridge type I only measures bending strain. This section provides information for the full-bridge type II strain-gauge configuration.

The full-bridge type II only measures bending strain. Hence, it is important that R 3 is a variable resistor, which we call R V.

But how do we detect the Balanced Condition? This is where a Galvanometer an old school Ammeter can be used. At this point, note down the value of R V. By using the following formula, we can calculate the unknown resistor R X. Usually, the Unbalanced Wheatstone Bridge is often used for measurement of different physical quantities like Pressure, Temperature, Strain etc.

For this to work, the Transducer must be of resistive type i. In place of unknown resistor in the previous resistance calculation example, we can connect the transducer. Let us now see how we can measure temperature using an unbalanced Wheatstone Bridge.

The transducer which we are going to use here is called a Thermistor, which is a temperature dependent resistor. Depending on the temperature co-efficient of the thermistor, changes in temperature will either increase or decrease the resistance of the thermistor.

This means that the output voltage V OUT is proportional to the temperature. By calibrating the voltmeter, we can display the temperature in terms of the output voltage. One of the most commonly used applications of Wheatstone Bridge is in the Strain Measurement. Strain Gauge is a device whose electrical resistance varies in proportion to the mechanical factors like Pressure, Force or Strain.

For a given strain, the resistance change may be only a fraction of the full range. Therefore, to accurately measure the fractional changes of resistance, a Wheatstone Bridge configuration is used. This circuit is built with two known resistors , one unknown resistor and one variable resistor connected in the form of bridge.

When the variable resistor is adjusted, then the current in the galvanometer becomes zero, the ratio of two two unknown resistors is equal to the ratio of value of unknown resistance and adjusted value of variable resistance.

By using a Wheatstone Bridge the unknown electrical resistance value can easily measure. The circuit arrangement of the Wheatstone bridge is shown below. Among these four resistances, P and Q are known fixed electrical resistances.

The potential at terminal D varies when the value of the variable resistor adjusts. When the resistance value of arm CD varies, then the I2 current will also vary. If we tend to adjust the variable resistance one state of affairs could return once when the voltage drop across the resistor S that is I2.

Most electric interference and noise problems can be solved by shielding and guarding. A shield around the measurement lead wires will intercept interferences and may also reduce any errors caused by insulation degradation. Shielding also will guard the measurement from capacitive coupling.

If the measurement leads are routed near electromagnetic interference sources such as transformers, twisting the leads will minimize signal degradation due to magnetic induction. By twisting the wire, the flux-induced current is inverted and the areas that the flux crosses cancel out.

For industrial process applications, twisted and shielded lead wires are used almost without exception. Guarding the instrumentation itself is just as important as shielding the wires. A guard is a sheet-metal box surrounding the analog circuitry and is connected to the shield. If ground currents flow through the strain-gauge element or its lead wires, a Wheatstone bridge circuit cannot distinguish them from the flow generated by the current source.

Guarding guarantees that terminals of electrical components are at the same potential, which thereby prevents extraneous current flows. Connecting a guard lead between the test specimen and the negative terminal of the power supply provides an additional current path around the measuring circuit. By placing a guard lead path in the path of an error-producing current, all of the elements involved i.

By using twisted and shielded lead wires and integrating DVMs with guarding, common mode noise error can virtually be eliminated.

Strain gauges are sometimes mounted at a distance from the measuring equipment. This increases the possibility of errors due to temperature variations, lead desensitization, and lead-wire resistance changes. In a two-wire installation Figure A , the two leads are in series with the strain-gauge element, and any change in the lead-wire resistance R1 will be indistinguishable from changes in the resistance of the strain gauge Rg. To correct for lead-wire effects, an additional, third lead can be introduced to the top arm of the bridge, as shown in Figure B.

In this configuration, wire C acts as a sense lead with no current flowing in it, and wires A and B are in opposite legs of the bridge. This is the minimum acceptable method of wiring strain gauges to a bridge to cancel at least part of the effect of extension wire errors.

Theoretically, if the lead wires to the sensor have the same nominal resistance, the same temperature coefficient, and are maintained at the same temperature, full compensation is obtained. If further improvement is desired, four-wire and offset-compensated installations Figures C and D should be considered. The lead error is usually not significant if the lead-wire resistance R1 is small in comparison to the gauge resistance Rg , but if the lead-wire resistance exceeds 0.

Therefore, in industrial applications, lead-wire lengths should be minimized or eliminated by locating the transmitter directly at the sensor. Strain-sensing materials, such as copper, change their internal structure at high temperatures.

Temperature can alter not only the properties of a strain gauge element, but also can alter the properties of the base material to which the strain gauge is attached. Differences in expansion coefficients between the gauge and base materials may cause dimensional changes in the sensor element.