What is Creep and how do I select the right creep code?
Creep characteristics exist due to the elasticity of a spring element. This is a material characteristic. Due to this characteristic, a transducers output increases with the passing of time (Positive creep).
This characteristic is depending on several variables such as the spring element material, structure, strain field, span, heat treatment and test temperature. The backing material of the strain gauges
and the bonding adhesive have a very high viscoelasticity which results in an output decrease over time. On the other hand the grid material of the strain gauge has anelastic properties which results in a positive output change over time. The accumulation of these two make that a strain gauge can have either a positive or negative creep under fixed load. The direction and value of this compensation can be adjusted by modifying the design of the grid structure, backing material ratio and key technology parameters. For example, by changing the dimensions of the end grid and
fixing the other parameters, a curve as seen in figure 3 can be created. After selecting the material of a spring element, a strain gauge can be selected with the same creep as the element but in the opposite direction. This way the creep can be compensated to a value close to 0. In the same way, during the production of transducers, the creep error which is caused by other factors can be
compensated. In this way the creep value could be brought to a minimum and within specifications of the transducer. ZEMIC offers a wide variety of strain gauges for which the creep factor should be
decided by the transducers manufacturers. The N* and T* codes in the strain gauge naming system are designated referrals to the creep code. Different codes refer to different creep values. For ZEMIC strain gauges the rule is as follows: the creep difference between two neighbouring codes is 0.01-0.015%FS/30min.
Figure 1 : Choice in Self-Creep compensation
Figure 2: different creep codes
It is advised when using strain gauges for the first time to select one or two models with a great difference in creep values and bond them to the spring element. The actual creep code will be
determined according to the actual value of the creep and the difference with the applied strain gauges. When selecting a strain gauge for transducers with the same spring materials and structure, the smaller the capacity the more positive creep will occur. Therefore the lower the capacity of the transducer, the bigger negative creep code should be chosen.
Different element materials show different creep characteristics. Therefore, different creep codes should be selected for the steel and aluminium transducers with the same capacity and structure.
The creep value of transducers is depending on many variables such as spring elements, strain gauge type, adhesive used as well as the way of sealing, the protective coating etc. The direction
and magnitude of the creep however can be predicted to a certain amount and this should therefore be taken into account when selecting a strain gauge creep code.