Fluctuating Readings from Spot Weldable Strain Gauges

Figure 1. A Spot Weldable Strain Gauge and Pickup

1. Introduction
The Spot Weldable Strain Gauge employs the Vibrating Wire principle. A high tensile steel wire when held under tension between two points and “plucked” will vibrate at its natural resonant frequency. The frequency of vibration is a function of the tension of the wire. As two end blocks gripping the ends of the wire are fixed to a steel structure, the tension in the wire will change as the distance between two blocks varies as a result of strain in the steel member.

2. Possible causes for fluctuating readings are as follows
If after installation of a Spot Weldable Strain Gauge you are receiving fluctuating readings from them, the following are possible causes for these fluctuations:

The pickups (Figure 1.) are not correctly placed over the gauges. Obviously this is difficult to check and confirm once installed but should be discussed with the engineer that installed them to ensure they have confidence this was done correctly. The embedment pickups should have been firmly slotted over the gauge and held with the jubilee clip.

The readout unit is being used incorrectly. The user should ensure the readout unit is set to the correct sweep range, if previously stable readings have been recorded, the user should know what the correct range for the gauge. The user should ensure that the crocodile clips are connected to the conductors and they have a good clean connection. Ideally, a terminal box with a terminal fly lead will be used. Ensure that the batteries in the readout have good charge and are not depleting.

There is vibration or electrical interferences. The vibrating wires are sensitive instruments, If there is high electrical cable running near the instrument signal cable this could effect the readings. As they are vibrating wire technology the sensor can also be effected by site works causing vibration. Ensure these interferences are removed or kept to a minimum.