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Sensor Technology

precision resistor as a standard feature. Of course, regular physical calibration is recommended as well to ensure the accuracy, stability, reliability, and linearity of the instrument itself.

Strain gage installations in special projects such as naval vessels or other

industrial applications usually will require long cable lengths between the strain gages’ location and the strain control and measuring instrumentations, due to the size of the body that contains the strain gage. The resistance of the signal cable will create an attenuation shift during the

acquisition process of strain data. This error can be compensated for by performing a shunt calibration directly on each active strain gage. But since strain gages are often unreachable after their installation, the shunt calibration process may be performed at another part of the Wheatstone bridge or at the instrumentation. The long cable can also cause errors in the calibration if it isn’t undertaken across the strain gages.

Strain Gauge applications • Bridge Completion

Figure 2. Shunt calibration resistors are chosen to accurately simulate resistance change in a strain gage subjected to specified levels of compressive strain. Strain indicators generally will produce a linear output with a fully active half-bridge or full-bridge input circuit, and will be slightly in error when a single active arm is used. The same nonlinearity occurs whether the gage is actually strained in compression or simulated by shunting the gage with the corresponding calibration resistor

• Common values include 120 , 350 , 1 K, and 11.2 K • While shunt calibration is recommended for low value strain gages, in the case of 5 K gages a low-value precision resistor connected in series can be used for calibration • To maintain long-term precision of the measurements, the ratio arms of the bridge should also consist of high- precision foil resistors

• Signal Conditioning

• Classical amplifier “normal values”: 1 k, 5 k, 10 k, 100 k • Calibration Shunts • High values: 75 k and up • Low power, standard conditions A high-value Bulk Metal Foil resistor (typically 50 K to 200 K) may be placed in parallel across a strain gage or one leg of the Wheatstone bridge to produce a predictable mV offset shift, which can be measured and used to calibrate a load cell instead of a more inconvenient weighing standard. This resistor should be stable over a wide variety of temperatures, a long period of time, and various humidity conditions.

Vishay Precision Group |

Yuval Hernik is Director Application Engineering, Vishay Precision Group (VPG)

Obsolete Chip?

E-tec offers a range of competitively priced package adaptors to solve obsolete package problems. These high quality adaptors are typically produced in 1.6mm thickness material or where the upper footprint interferes with the lower footprint then 2.4mm material is used. Here the pins are fitted into blind holes from below and the combination of a press fit and a solder fillet around the pin shoulder provides a reliable and robust connection.

Daughter boards can be created to customer requirements and raised pins are available to raise the complete assemble off the pcb to the required level.

E-tec welcomes special adaptor enquiries, send the chip package details for the top and bottom parts and we will respond promptly with a quotation.

t: 01892 530260 f: 01892 515560 e: Components in Electronics December 2013/January 2014 41

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