The minimum weight, or even change in weight, that a load cell can sense is important information when selecting the right load cell for a force measurement application. This article explains how to uncover this critical piece of knowledge.
What Is the Minimum Weight a Load Cell Can Sense?
To answer this question, one must study the load cell data sheet. Every load cell manufacturer provides a data sheet that gives its products’ specifications. On this spec sheet is a division range which can be used to determine the minimum weight that a load cell can measure. Division ranges vary from load cell to load cell, and usually fall between 500-10,000 divisions.
Another item of importance on this data sheet is the sensitivity rating. The sensitivity of a system indicates the rate of change of its output as the system input varies. Systems that have low sensitivity ranges or are of lower quality will not be able to detect small changes that occur when a load cell experiences a force from a minimal weight.
The final section of this paper discusses other important data sheet specifications affecting the choice of load cell for the application. The ability to read the minimum weight required by an application depends on careful consideration of these specifications.
Furthermore, the individual components of a load cell system itself can introduce errors that change the ability of the system to measure small loads accurately. Figure 1 shows a block diagram of a typical load cell system.
The analog portion of load cell system consists of several strain gauge sensors and a Wheatstone bridge (both built into the load cell), an amplifier and offset stage, an ADC converter, and a power supply. Each of these stages can affect the reliability and accuracy of a load cell system.
How a Load Cell System Works
The sensing portion of a load cell is a strain gauge. A strain gauge is a passive transducer that changes its resistance proportionally to the stress placed on it. Therefore load cells made with this sensor respond to different weights by a change in resistance. These resistance changes are usually very small, and measured with a circuit known as a Wheatstone Bridge. A Wheatstone bridge has two voltage dividers. Figure 2 shows an example of a Wheatstone Bridge circuit.
Full Scale Output vs Maximum Measurable Output
Figure 2 shows an unbalanced Wheatstone bridge with a measurable voltage difference between the two output terminals. This load cell requires an excitation voltage across the cell power rails, typically in the range of 5-15V. As previously stated, a force applied on the load cell creates a voltage difference between the output terminals from the Wheatstone bridge, usually in the µV range. There is an upper limit to this measurable change, known as the full scale output. This full scale output of a load cell appears on the manufacturer data sheet. It should be noted however that the maximum voltage difference between the terminals is relative to the excitation voltage. The maximum output equals the full scale output multiplied by the excitation voltage. A full scale output of 3 mV/V and an excitation voltage of 10 V will result in a maximum measurable difference of 30 mV.
The smallest measurable difference between the terminals is the maximum output divided by the number of Divisions from the load cell. If the load cell in the above example has a division range of 10,000, the smallest detectable voltage change would be 30mv/10,000 = 3 µV. Since this is such a small voltage difference, the system required to measure this change must be very sensitive.
Typically this output signal undergoes signal amplification. This process boosts the output signal to a detectable level. An instrumentation amplifier is a common type; this amplifier uses two input buffers for high input impedance, and a difference amplifier between the Wheatstone bridge output and the analog-to-digital (ADC) converter. The ADC takes an analog signal and converts it into a discrete signal usable by a digital display.
Figure 3 shows a standard instrumentation amplifier circuit.
The instrumentation amplifier stage needs to have high differential gain, high power supply rejection ratio, low drift, low offset and low input bias current . If the instrumentation amplifier has all of these qualifications, the signal-to-noise ratio will be large enough for accurate conversion by the ADC. A direct correlation exists between the number of divisions of an ADC from an indicator and the number of divisions of a load cell. An ADC must have enough high enough resolution, i.e. enough bits, to detect small changes from a high division load cell. Manufacturers of load cell indicators will provide the number of bits of the ADC, and sometimes even provide details on which load cells should be used with the system.
Load Cell Specifications and Definitions
As mentioned above, load cells have many important specifications describing their suitability for a particular application and the demands placed on it throughout its lifetime. Figure 4 displays typical specifications found on a load cell data sheet.
Load cells are typically given error ranges, which is a sum total of the errors a load cell can experience. The term for this is the maximum possible error. The maximum possible error is important when considering accurate division measurements . If a load cell experiences its maximum possible error, its minimum weight divisions be less reliable. For this reason, load cells are assigned accuracy classes. A load cell with a 0.1 % accuracy class has minimum divisions accurate with a maximum error of ±0.1%. The total error is determined by a number of varying factors, from temperature deviations to magnetic error. The following definitions are important determinants of the accuracy class of a load cell. Understanding the source of error can help increase accuracy when testing a load cell system. These definitions are provided from the Handbook of Electronic Weighting.
The following definitions are important determinants of the accuracy class of a load cell. Understanding the source of error can help increase accuracy when testing a load cell system. These definitions are provided from the Handbook of Electronic Weighting.
- Zero Balance: This is the electrical output signal of the load cell when no weight or load is placed on it.
- Non-Linearity: This expresses the maximum deviation of the calibration curve. This curve is obtained by gradually increasing the applied weight from the zero balance level to the rated output of the load cell, and plotting the output against this weight. The smaller the non-linearity, the more accurate measurement we obtain.
- Hysteresis: This is the maximum deviation of the output signal for the same applied load. The first set of values are obtained by increasing the applied weight from zero balance to the rated output, while the second readings are obtained by decreasing the rated output to the zero balance level. Assume the x-axis is the applied weight and the y-axis is the output on a plot. The smaller this numerical difference is at the same weight (the smaller the difference in the y values at the same x value), the more accurate measurement we obtain.
- Non-Repeatability: This is the maximum difference between the electrical output signal of the load cell for repeated loads under identical environmental and loading conditions. A small value depicts a high system accuracy and reliability.
- Creep: This specification becomes very important when a load is constant over a long time, such as for monitoring purposes. Creep is the change in the load cell output signal level with respect to time under a constant load, with all environmental conditions being constant.
- Temperature Effect on Output: This is the effect of temperature shifts on the output of the load cell as it tends to introduce errors that affect system accuracy.
- Temperature Effects on Zero Balance: Temperature shifts also affect the output signal of the load cell under no-load. To cater for both types of temperature shifts, ensure the load cell design you are using incorporates a temperature compensation technique.
Regulations and Standards on Load Cell Specifications
Multiple organizations maintain industry-accepted standards for measurement applications. These include organizations such as the International Organization of Legal Metrology (OIML) and the National Type Evaluation Committee (NTEP). These organizations have their own standards and tolerances for different load cell classes. Figure 5 gives an example chart of some tolerances given by OIML.
Highlighted in Figure 5 are the minimum and maximum load cell divisions for class IIIL3 load cells. The IIIL3 class includes weights over 5 lb. and over 2 kg. As can be seen, the minimum and maximum number of divisions for class IIIL3 load cells are 2000 and 10000 respectively. To find the minimum detectable weight of a load cell, simply divide its maximum rated weight by the number of divisions. For example, if a load cell is said to have 10,000 divisions with a capacity of 50,000 lb., the minimum weight that is measurable by the cell will be 5 lb. Load cells are typically given a classification indicating the number of divisions of the load cell. An example of this can be seen in Figure 6.
Figure 6 shows the characteristics of the 102BH class of load cells, manufactured by Anyload. The maximum number of divisions (intervals) for this load cell class is 3000, per the figure. The maximum capacity range for this load cell class is between 11 t and 55 t. Similar data sheets for different load cell classes appear on manufacturers websites.
As this article explains, there are several important specifications that give clues to the minimum weight, and the accuracy of that weight, measurable by any given load cell. The number of divisions and range of weight determine a load cell’s minimum weight detectable . However, the sensitivity ratings and the quality of the system being used to detect the changes from a load cell are also critical factors in displaying accurate weight measurements.
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- K. Elis Nordon, “Handbook of Electronic Weighting”, Wiley-VCH, pp. 24-29, Jul. 1998.
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- “Load Cell and Weight Module Handbook”, Rice Lake Weighing Systems, pp. 9-10, 2010
- “OIML Certificate of Conformity”, Number R60/2000-NL1-10.27 , Dec. 2010
- “R 60 OIML-CS rev.2”, NIST Handbook 44, pp. 1-4, Jan 5 2018.