The desire to ensure the constant operation of scales with a given degree of measurement accuracy is explained by the fact that any inaccurate weighing results in tangible material losses for the company. In the absence of accurate information about the values of the volumes of transported raw materials, it is very difficult to distinguish the degree of production load. The high productivity achieved is quickly reduced to zero if malfunctions of the weighing equipment or inaccurate weighing lead to delays in the shipment of products.
Strain gauges work due to the strain effect - changes in the resistance of semiconductors when they are deformed. When stretched, the resistance increases due to the lengthening of the resistors and the reduction of the cross section. Compression, on the contrary, reduces this figure. The difference in resistance is captured by a voltmeter, after which it is converted into an electrical signal, which is processed into a format that is convenient for perception.
With the invention of strain gauges, a new era in the history of the accuracy of weighing equipment has been ushered in. But it was not without its drawbacks, one of which is a direct dependence on the influence of the environment, and in particular, the temperature and humidity of the air have the greatest influence.
Consider the change in temperature and its effect on the strain gauge
Often, when the ambient temperature changes, problems arise in the stability of the sensors, which affects the final weighing readings. This is due to several factors:
The influence of the TCR (temperature coefficient of resistance) of the resistor on its resistance;
Parasitic strain-resistive effect due to the difference in thermal expansion between the substrate and the resistor material.
With an increase in the ambient temperature, the sensitivity of the load cell decreases due to a decrease in the strength qualities of the equipment and softening of the glue in the parts, which leads to an increase in the measurement error.
A decrease in the ambient temperature causes a change in the range of tension and compression of the material, which leads to an increase in the measurement error.
Operating temperature range for various types of adhesive used in strain gauges:
Cyanoacrylate is used at temperatures from -30 to +800C. There are brands with a wider range of -196 ... + 1200С. The error is not more than 6%.
Polyester. From -30 to +1800С.
Phenol. It is used at temperatures of -296...+2000C. The error is 3-10%.
Epoxy resin. Depending on the brand, the spread is -296 ... + 2000С. The error is about the same as that of formaldehyde formulations.
Of great importance is the basis on which the semiconductor is deposited. For foil resistors, TCR can be only ± 0.05 ppm / C in the range from 0 to + 600C. This is one of the most stable types of sensors.
The largest TCR is observed for sensors based on a carbon conductive film. It can reach 500 ppm / C and the error will be at least 5%, or even all 10%. Therefore, such strain gauges are not used where high measurement accuracy is required.
Protection of load cells from the environment
Currently, there are several ways of temperature compensation:
Doping of semiconductors;
Use of constantan wire;
The connection of two semiconductors with p and n-conductivity;
The use of thermal covers for protection against low and high temperatures.
Strain gauge sensors have several types of performance, high-temperature and low-temperature. Such equipment is designed to operate in extreme environmental conditions.
In their manufacture, special materials are used, such as nichrome, and protective coatings are additionally applied, such as a heated thermal cover, heat-resistant glass and aluminum coating, special cement, and enamels.
But it is not possible to completely isolate the equipment from the influence of the environment, especially if you take work in extreme conditions. The highest accuracy is achieved under stable conditions and with the use of protective devices and coatings.
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