1. Outline
Each country has its own standards governing the performance and quality of load cells.
In Japan, there is no standard specifically stipulated for load cells, however the Japan Industrial Standards (JIS) (Load cell – Test method JIS B7602) and the Japan Measuring Instruments Federation (JMIF) have prescribed methods for performance tests.
At the same time, the International Organization of Legal Metrology (OIML) has established international recommendations titled “Metrological Regulations for Load Cells,” and each member country of OIML is striving to ensure that their local regulations are consistent with these regulations. These recommendations largely concern load cells used for scales.
Summarized succinctly, OIML’s recommendations treat non- linearity and hysteresis error together, and have adopted a method by which accuracy is evaluated based on whether those errors are within the limits determined for the prescribed temperature range.
2. General Specifications
2.1. Rated Capacity (RL: Rated Load, RC: Rated Capacity)
Rated capacity is defined as the maximum load that a load cell can measure while meeting its specifications. It is also called the rated load. Weighing instruments should be designed so that the load to be measured will be less than the rated capacity.
2.2. Rated Output (RO: Rated Output)
Rated output is the difference when there is no load and when there is a load of rated capacity. It is generally expressed in output per excitation voltage (mV/V); alternatively called “span.”
2.3. Maximum Deadweight
Maximum deadweight is the maximum tare load that can be applied onto a load cell in addition to the load to be measured.
When designing a scale, the weight of tare loads such as weighing pans should be less than the maximum deadweight.
2.4. Safe Load Limit
Safe load limit is the maximum load that can be applied beyond the rated capacity without causing any permanent damage. The safe load limit is expressed as a percentage of rated capacity.
2.5. Compensated Temperature Range
Compensated temperature range is the temperature range within which the rated output and the zero balance are compensated to meet load cell specifications.
2.6. Temperature Effect On Zero Balance
Drifting of the zero balance caused by changes in the ambient temperature. This value is expressed as a percentage of rated output.
2.7. Temperature Effect On Rated Output
Drifting of the rated output caused by changes in the ambient temperature.
2.8. Nonlinearity
Nonlinearity is the maximum deviation in output from a linear calibration curve linking the zero balance and the rated output, measured only when the load increases. It is expressed as a percentage of the rated output.
2.9. Hysteresis Error
Hysteresis error is the maximum difference in output generated when a load increases and decreases.
2.10. Combined Error
Combined error is the maximum deviation of output from a linear calibration curve linking the zero balance and the rated output, including when a load increases and decreases.
2.11. Recommended/Maximum Excitation Voltage
The recommended/maximum excitation voltage is the voltage applied to the input terminals of a load cell.
2.12. Zero Balance
Zero balance is the electrical output generated when a rated excitation voltage is applied without any load on the cell. It is generally expressed as a percentage of rated output.
2.13. Input Terminal Resistance
Input terminal resistance is measured while the input terminals are open, with no load on the cell.
2.14. Output Terminal Resistance
Output terminal resistance is measured while the input terminals are open, with no load on the cell.
2.15. Insulation Resistance
Insulation resistance is the direct current resistance between a load cell unit and its circuit.
3. Load Characteristics
The following conditions are necessary to conduct load characteristic tests:
- A stable environment
- Good and stable load conditions
- Stable power supply and indicator
After the above conditions have been satisfied, load and unload a weight of rated capacity. Repeat this procedure at least three times. Next, read the initial output voltage. Then, set several load points between zero and the rated load so that the interval between each load point is the same. Weigh the loads in corresponding order at different load points for the same interval of time, making certain to record the rated output after each load becomes stable. When the rated capacity is reached, remove the loads in reverse order and read the output voltages. Repeat this procedure three times.
3.1. Zero Balance
Zero balance is the electrical output when the rated excitation voltage is applied without any load on the cell. It is generally expressed as a percentage of rated output.
3.2. Nonlinearity
(Δθ Ln : The largest difference between the reference line and actual output when a load increases)
3.3. Hysteresis Error
(Δθ μn : The maximum difference in output generated when a load increases and decreases)
3.4. Repeatability
(Δθ R : The maximum difference in output for the same load when a load increases)
According to the OIML, nonlinearity and the hysteresis error are not considered separately, and judgments are made based on whether all values are within the specified error range.
4. Creep Characteristics
4.1. Creep
(θ fa : Output for a fixed period of time (5 sec – 60 sec) after the load has reached the rated load)
(θ fb : Output for a fixed period of time (15 min – 6 h) after the load has reached the rated load)
The fixed period of time after the load has reached the rated load is dependent on respective standards. In general, the time is short in Japan and long in other countries.
4.2. Creep Recoverability
(θ oa : Output for a fixed period of time after removing the load)
(θ fb :Output for a fixed period of time after removing the load)
The creep recoverability is not normally mentioned in catalogs, etc. This is because the creep and the creep recoverability are almost proportional to each other. Thus, only the creep characteristics are written and the other is omitted. Moreover, there are no regulations from the OIML governing creep recoverability. Instead, there are regulations concerning zero return. This means that the difference between θ 0 (output before loading) and θ oa is specified.
5. Temperature Characteristics
5.1. Compensated Temperature Range
Compensated temperature range is the temperature range within which the rated output and the zero balance are compensated to meet load cell specifications.
5.2. Safe Temperature Range
Safe temperature range is the temperature range within which a load cell can be used without permanent damage
5.3. Temperature Effect On Zero Balance
(θ oa θ ob , : Output at each temperature when there is no load)
(Ta Tb , : Testing temperatures)
5.4. Temperature Effect On Rated Output
(θ fa θ fb , : Output at each temperature when there is a load of rated capacity)
(Ta Tb , : Testing temperatures)
Both the temperature effect on zero balance and the temperature effect on rated output indicate errors in load cell characteristics caused by temperature changes. Although there are no OIML regulations governing the effect of temperature on the rated output, the effect has to be within the error range shown in Figure 3.5 between –10 °C and +40 °C.
6. Electrical Characteristics
Electrical characteristics include the recommended excitation voltage, maximum excitation voltage, input terminal resistance, output terminal resistance and insulation resistance, etc.
7. Four-Corner Error
A load approximately 1/4 the rated capacity is applied on the points (○) on the weighing pan shown in the figure on the right. For 1/3000 scales, adjustments are made to ensure that the difference in weight between each point (○) and the center is within 1 count. The size of the loading surface differs depending on the type of the load cell.
8. Other Characteristics
In “Chapter 3,” the characteristics previously mentioned in sections 1 – 7 are described as typical load cell specifications mentioned in catalogs. However, depending on the intended use of the load cell, specifications such as those described here in this section may be necessary.
8.1. Fatigue Test
When a load cell is used for a checker scale, fatigue lifetime is important because there will be a large number of loadings. Figure 3.8 shows fatigue test data for the LC4103-K100.
Single point load cell / platform scale type Medium-sized (400 × 600 mm) platforms can be used (already adjusted to correct four corners).
Temperature compensation for both zero and span. A load can be applied from above or below by pulling downward.
Test conditions: On a load cell with a 100kg rated capacity, a load of 200kg was applied in the cycle once every two seconds, and the zero output was recorded each time.
Below is the data when the rated capacity was loaded in the cycle 2 times /sec.
8.2. Natural Frequency Test
A load cell that has a high natural frequency is suitable when measuring dynamic force or the load cell is installed on a vibrating surface. A load cell whose natural frequency is low will be significantly deformed by loading and is therefore more suited for instruments with stoppers, such as platform scales. Natural frequencies vary depending on the structure of spring materials and the size of loadings such as tare and the amount measured. In general, the Roberval-type spring material has a low natural frequency, whereas the column-type spring material has a high natural frequency. Also, the larger the rated load, the lower the natural frequency becomes. Natural frequencies range widely from several 10Hz to several Hz.
8.3. Power-On Zero Drift
The load cell output immediately after turning on the power will drift due to rising gauge temperature.
8.4. Immersion Test
An immersion test is necessary when there is a possibility that a load cell will be immersed into water or another liquid. It is possible that zero output will vary greatly because of the immersion of the spring material. In these circumstances, it will be necessary to coat the surface of the spring material.
8.5. Effect of Temperature
Strain gauges are susceptible to temperature changes and change the zero output of the load cell. Most load cells are compensated for temperature changes.
