Chapter 8 Tutorial

Measurement Fundamentals

Making Strain Gage Measurements A Wheatstone bridge is commonly used to enable instruments with low-sensitivity measuring capabilities to measure small resistance changes common in strain measurements. Instruments with high-resolution resistance measuring capabilities, like the 34970A internal DMM, can directly measure small resistance changes with high precision and linearity. You should also use the 4-wire resistance method when measuring strain gages to eliminate system cabling errors.

An initial, unstrained, gage resistance measurement is used as the reference (R0) measurement from which strain (∆ R/R0) is measured. For best results, you should perform this reference measurement after the strain gage has been mounted to the test body. The table below shows the resistance changes corresponding to 1 ∝ε strain for common gage factor and unstrained gage resistance values.

Strain

GF

R0

∆ R

DMM Sensitivity

1 ∝ε

2.0

120Ω

0.24 mΩ

0.1 mΩ

(0.4 ∝ε)

1 ∝ε

2.0

350Ω

0.70 mΩ

1.0 mΩ

(1.4 ∝ε)

1 ∝ε

2.0

1000Ω

2.0 mΩ

1.0 mΩ

(0.5 ∝ε)

Using the Mx+B scaling function with the equations shown below will allow you to display results directly in strain on the 34970A front-panel display. You can use a custom measurement label to display readings in “uE” (micro-strain) directly. The instrument will automatically add the micro (“u”) prefix based upon actual calculated values. For more information on scaling, see page 119.

M =

1

 

B = −

1

 

 

GF x R

0

GF

 

 

 

 

 

 

 

 

 

 

Temperature Effects The resistive element of a strain gage will

 

produce a resistance change ∆ R due to measured strain as well as

 

changes in gage temperature. This will create an “apparent” strain

 

change which is undesirable. A second gage of similar type can be used

 

to detect temperature changes and thus remove this error source.

 

 

You should mount the second gage in close proximity and at 90° to the

 

first gage, thus responding to local temperature changes but rejecting

 

strain changes. Subtracting measurements from the second gage will

8

remove any undesirable strain errors.

 

 

 

 

 

 

 

375

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Agilent Technologies 34970A manual Strain, DMM Sensitivity
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34970A specifications

Agilent Technologies 34970A is a versatile data acquisition and measurement system that has gained popularity in various industries, including research and development, manufacturing, and education. The primary purpose of the 34970A is to offer an efficient solution for data collection, monitoring, and analysis, which can significantly enhance productivity and accuracy in testing applications.

One of the main features of the 34970A is its modular design, allowing users to customize the system according to their specific measurement needs. The mainframe can accommodate up to three plug-in measurement modules, which can include various types of measurements such as analog, digital, temperature, and frequency. This modularity provides flexibility for users to tailor the system to their requirements, making it highly adaptable to different applications.

Another notable characteristic of the Agilent 34970A is its impressive channel configuration. The system can support up to 120 measurement channels when fully configured, enabling extensive data acquisition without the need for multiple devices. This capability is essential for applications requiring simultaneous monitoring of multiple parameters or locations.

The 34970A utilizes advanced signal processing technologies to ensure high-accuracy measurements. The internal 6.5-digit resolution multimeter provides precise readings, while the device also supports various input types, such as thermocouples and RTDs for temperature measurements. The ability to perform mathematical functions, such as summation and averaging, on the acquired data further enhances its usability.

For data storage and management, the Agilent 34970A features built-in memory for storing up to 20,000 readings. Users can also easily transfer data to a PC through the RS-232 interface or GPIB (IEEE 488) for further analysis and reporting. Additionally, it offers the capability to program automatic data logging and scheduling of measurements, which streamlines the testing process and reduces manual intervention.

In terms of user interface, the 34970A is equipped with a large graphical display that provides clear visualization of measurement data and easy navigation through settings and options. The menu-driven interface makes it accessible for users of all skill levels, simplifying the setup and operation of the device.

Overall, the Agilent Technologies 34970A stands out for its modularity, high channel capacity, advanced measurement technologies, and user-friendly interface. As a reliable and efficient data acquisition system, it is well-suited for a wide range of applications, making it an essential tool for engineers and technicians looking to enhance measurement precision and efficiency.
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