Training Stuff

Selection of Strain Gages

release date:2017-02-07 23:53:21



In practice, we should first follow the experiment or application conditions (i.e. application accuracy, environmental conditions including temperature, humidity, abominable environment, all kinds of interference and tested object dimensions, bonding area, radius of curvature and installation conditions, etc.). Second, we should conform to the materials of tested objects or spring elements (material linear coefficient of expansion, elastic modulus, structure, approximate force load or stress distributing status, etc.). Taking advantages of the above-mentioned principles, we can choose gages with the best performance-price ratio to match above conditions.

I.Steps

1. First, choose strain gage series according to application accuracy and environmental circumstance.
2.Choose grid length according to the tested object dimensions, bonding areas, radius of the curvature, installation conditions and strain grads.
3.Choose grid pattern according to strain grads, stress type, heat dispersing conditions, installation space and gages resistance.
4.Choose standard resistance according to application conditions, power consumption and the maximum voltage allowed.
5. Choose self-temperature compensation or self-elastic modulus compensation codes according to types of tested object material, working temperature range and application accuracy.
6. Choose creep compensation code according to proper creep characteristic of spring element, actual measured accuracy, workmanship and process technology, protection adhesive type and encapsulation form and so on.
7. Choose lead wire according to actual requirement.

Ⅱ. How to choose strain gages

1. Choose grid length:

The output of strain gages under loading come from the average strain of its grid area. In order to get actual measured value, the grid length of strain gages should not be longer than 1/5~1/10 radial of the measuring area. Strain gages with longer grid length are easy for bonding, wiring and thermolysis, which can improve the performance of gages. However, we should choose it according to actual requirement. For common transducers and relative even stress field, gages with 3~6mm grid are recommended. For heterogeneous materials, such as concrete, cast iron and cast steel, gages with grid
length no less than the grain dimension of the heterogeneous materials are recommended, so that the average strain of the inner structure can be measured exactly. For the stress measurement of higher strain-grads, we had better choose gages with short grid length.

2. Choose grid and backing materials:

If environmental temperature is less than 60℃, and the maximum strain is less than 1000μm/m for a long time, Constantan foil or Karma foil strain gages with modified phenolic backing or polyimide backing or thin polyimide flin (BF, ZF, BA,BHB,BAM series and strain gages used for daily used transducers) should be recommended. If environmental temperature goes higher up to 150℃, Constantan or Karma foil gages with polyimide backing (BA series) should be recommended. If used for high precision transducers under environmental temperature less than 60℃, either Constantan or Karma foil gages with modified phenolic backing ( before ZF,BHB,BAM series) could be used.

3. Choose grid pattern

Measuring the principal stress of unknown direction or shear stress, we should choose multi-axis strain gages. The former one can use strain gages of three axis with 45°or 60°or 120°angle(Rosette gages). The latter one can use strain gages of two axis with 90°angle(Shear gages). Single-axis strain gages can measure principal stress of known direction. Pressure transducers can use diaphragm strain gages with multi-axis. Measuring strain distribution, 5 to 10 grids in parallel or chain should be selected.

4. Choose central distance between two grids

The central distance between two grids of double axis gages produced by us are L6= 6.0mm, L68=6.8mm, L7=7.0mm, L8=8.0mm, L0=10.0mm, L2=12.0mm, L4=14.0mm and so on. The customers can choose strain gages with proper central distance according to their requirement.

5.Choose gages resistance:

Usually we choose gages resistance according to heat dispersing areas, the effect of cable wire resistance, signal-noise ratio and power consumption. As for transducers we often recommend strain gages with resistance of 350Ω and 1000Ω. As for test of stress distribution, strain and stress measuring in static status, customer should choose the resistance which are close to the matched instrument. Generally we recommend strain gages with resistance of 120Ω and 350Ω.

6. Choose ultimate temperature.

This temperature means ultimate working temperature. If ultimate working temperature is lower than 60℃, this component ( Temp. Number) will often be omitted in our designation.

7. Choose self-temperature compensation or self-elastic modulus compensation code

Choosing self-temperature compensation or self-elastic modulus compensation code, we should conform to the functions of self-temperature compensation or self-elastic modulus compensation described in above paragraph.

8. Choose creep code

Customers can choose creep code according to the methods described in the function of creep selfcompensation.

9. Choose lead wire form

ZEMIC strain gages have diversiform lead wire forms:
① Standard lead wire form, BQ,BA,BB,ZF,ZCF,ZFF series and KA,BA,CA,BC,CB, CC,FD,AA-W,HA-W pattern strain gages adopt column lead wire. BE,BF,BCF,BFF,RNF, RBF series stain gages adopt belt lead wire. The length of lead wire is 30±3mm except that HA series belt lead wire is 25±2mm.
② Belt lead wire form, when HA series lead wire length is 25mm and other stain gages lead wire length is 30mm, it is no need to show in model name. Otherwise, the lead wire length must be notified.
③ Other lead wire form, like varnish wrapped wire, high temperature wire, etc.