What is the best method to calibrate a weigh module system? In a perfect world, the best way is to hang test weights up to the full capacity of the system. Many times, that isn’t possible. Variables like the system’s capacity or the system’s location mean that alternative methods must be used.
METTLER TOLEDO has a well-written guide titled “Weigh Module Systems Handbook”. While geared toward its weigh module line, the handbook contains a lot of excellent information that can be applied to any manufacturer’s weigh module products.
Below is an excerpt from the handbook on weigh module system calibration. It applies to most load cell systems. If you want a copy of this handbook, click here.
Weigh Module System Calibration
When a weigh module system is installed, it must be calibrated so that the readings on the indicator accurately reflect the amount of weight placed on the scale. METTLER TOLEDO recommends calibrating a scale using test weights equal to the scale’s full capacity. Specific instructions for calibration can be found in the technical manual for the digital indicator that will be used with the weigh modules.
The design or size of a tank scale might make it impossible to hang test weights equal to the scale’s full capacity. For those applications, there are several other calibration options: calibration with test weights and material substitution, calibration with material transfer, and electronic calibration.
Calibration with Test Weights
The most accurate, reliable way to calibrate a scale is with test weights. For this calibration procedure, a tank scale needs to be equipped with some type of mounting lugs for hanging test weights (see Figure 5-11).
- Begin by taking a weight reading for the empty tank. Adjust the indicator so that it reads zero when the tank is empty.
- Check each load cell to make sure it is working properly. Hang a test weight near one weigh module and take a reading. Move the test weight to a second weigh module and take a reading. Repeat for each weigh module to make sure that all load cells indicate the same weight.
- Check for repeatability to make sure there are no mechanical binding or support issues.
- Add test weights to the scale, taking a reading for each new weight that is added up to the full capacity of the scale. At the very least, you should take weight readings at one-quarter of capacity, one-half of capacity, three-quarters of capacity, and full capacity.
- If the tank scale will be used to weigh its contents as they are being discharged, you should also take weight readings as you remove the test weights.
- Use the readings to plot a graph of the scale’s performance from zero to full capacity (and from full capacity back to zero if those readings were taken).
Calibration with Test Weights and Material Substitution
For large tank scales, it is often physically impossible to hang test weights equal to the tank’s full capacity. In those cases, you can use a combination of test weights and a material (such as water) to calibrate the scale.
- For example, after taking a zero reading you might hang 500 lb [225 kg] of test weights and take a reading.
- Then remove the test weights and add water to the tank until the weight reading is the same as that obtained with the test weights.
- With the water still in the tank, hang the same test weights and take a second reading.
- Continue substituting water for the test weights and taking readings until you reach the tank’s full capacity.
- Once you have taken readings from zero to full capacity, use them to plot a graph of the scale’s performance.
Calibration with Material Transfer
When test weights cannot be used, you can calibrate a scale with material transfer. Instead of hanging test weights, weigh a material (such as water) on another scale and transfer it to the tank scale that is being calibrated. This can be done in a single transfer or in stages until you reach the tank’s full capacity. This method yields only a rough indication of the scale’s performance. It depends on the accuracy of the existing scale and the integrity of the transfer process. Even in the best conditions, you will not know if allowable errors are cumulative or compensating.
Load Cell Simulator
A tank scale can be calibrated electronically using a load cell simulator. Attach the load cell simulator directly to the digital indicator in place of the home-run cable from the junction box. The simulator sends out a signal equal to the signal the load cells should produce. Electronic calibration is noted for its speed and simplicity; however, it calibrates only the electronics. Because it assumes that the tank and all mechanical connections are working properly, electronic calibration does not verify the scale’s performance.
- With the simulator adjusted to zero output, set the indicator to zero.
- Adjust the simulator to full output (a signal equal to that which all the load cells should produce at their rated capacity).
- Adjust the indicator to show the total capacity of all load cells in the system.
- Attach the load cell input to the indicator.
- Set the indicator to read zero for the empty weight of the tank.
CalFREE Electronic Calibration
The CalFREE™ program is another option for calibrating a scale without using test weights. This proprietary METTLER TOLEDO feature is built into the latest line of METTLER TOLEDO industrial indicators and is compatible with systems that use analog load cells rated at 2 mV/V or 3 mV/V.
The CalFREE program calculates the full-scale system output in millivolts to calibrate the scale electronically. For most analog scales, this value is nominally 2 or 3 millivolts output per volt of excitation at rated capacity. Due to manufacturing tolerances, the output or sensitivity of an individual load cell can vary slightly from these nominal values. The CalFREE program uses the summed average of the individual load cell sensitivities to determine the expected system output at rated capacity. The calibration certificate for each load cell lists the load cell’s specific sensitivity at rated capacity.
A printed calibration certificate is supplied with each load cell that is shipped. Electronic copies of the calibration certificates can be downloaded from the following website:
To locate the calibration certificates for a scale, you will need to know the serial number of each load cell in the scale system. Calibration certificates are stored in PDF format by serial number. For example, the file for load cell serial number 6011154-6LH is 6011154-6LH.pdf.
Like a load cell simulator, the CalFREE procedure only calibrates the scale electronically. It does not compensate for mechanical influences such as piping attachments, movement in structural supports, vibration, etc.
