Load cell calibration

10/08/21

In our blog series "Technical issues in the world of force measurement" we write blogs to help our customers and visitors.

Earlier we wrote the blogs ‘How to make a load cell selection?’ and ‘How to install load cells?, now we want to inform you about load cell calibration, a subject on which we regularly receive questions.

For a proper use of load cells it is essential to perform a correct calibration of your load cell application. The reason for calibration is to make sure that the sensors are functioning properly and your application is registering the correct values which are in accordance with a calibration standard under a particular set of conditions. In other words; to enable your application to measure as accurately as possible.

Installation

The first step in the load cell calibration process is to make sure the load cells are installed correctly. This means that the load introduction should be according to the described direction and position in relation to the load cell. The quality of the loading surfaces should also be taken into consideration. Both the support surface as the surface on the load cell, both the top and bottom side, where the load cell will be assembled, should be smooth and without any corrosion, dents or cracks. Another point to take into consideration is to check the recommended torque on the load cells fastening bolts. This recommended value can be found on the Zemic Europe datasheets. Please note the advised bolt quality grade of 10.9.

When the mechanical installation is done, it is important to connect the load cell in a correct way. The end of the load cells cable usually shows either a connector or open ends with different colour conductor jackets. The meaning of these individual wires can be found on our load cell datasheets and on the calibration certificate which is delivered with all individual Zemic Europe load cells. These conductor wires should be connected to either your electronics directly or through a junction box.

When installation and wiring is complete, the whole weighing system should be loaded to at least 75% of the capacity for at least 3 times. The reason this should be done is to make sure the mechanics of the application will set itself and will not move during the real load cell calibration. The first few cycles of a new installed application usually show much deviation. If this would happen during a calibration it could cause measuring inaccuracies.

Conditions

Please note there are several conditions to take into account when calibrating weighing applications. The first thing to consider when calibrating is the environmental temperature. It is important that the temperature stays stable during the calibration process. Other factors are humidity and air pressure. It is advised to perform the calibration when all these conditions remain the same during the process.

We would also advise you to keep the load cell in the same temperature conditions for a while before installing it into the assembly. This way the load cell will be at the same temperature as the application during calibration and therefore not have any inaccuracies due to temperature change. The advised time to keep the load cell in the same temperature conditions is at least 8 hours.

Before the calibration of the strain gauge load cells, the display and all other electronics should be switched on for the preheating of the system. This means that, due to electricity flowing through the system, the parts can heat up. Particularly with strain gauge load cells this can have an influence on the stability of the load cell. It is advised to do a pre-heating for a period of 30 minutes to 1 hour.

Two methods of load cell calibration

In general there are two different ways to calibrate a system. Which one of the two can and/or should be used is dependent on the application and what options are available on your electronics. On most of Zemic Europe’s electronics, both options are available.

The two methods of calibration in a weighing system are theoretical calibration and real calibration with sample weights.

The first way of calibrating is theoretical calibration, this can only be done if your electronics allow it. You should check this in the manual or with the supplier of the electronics. The advantage of the theoretical calibration is that no sample weights are necessary to perform the calibration. In addition, the theoretical calibration can usually be performed faster than a real calibration with sample weights. The disadvantage of a theoretical calibration is that in general this way of calibrating is less accurate than a real calibration.

The second way of calibrating is a real calibration with sample weights. Please be advised that, when you are choosing for this calibration method, you need at least 50% of the applications maximum capacity in sample weights. Preferably not in one weight but in multiple different capacities, so you can check whether the correct value is indicated for different weight values. The disadvantage of this way of calibrating is that it takes more time than theoretical calibration and you need to have the sample weights available. However, the accuracy of the system will be higher when a real calibration is performed.

Theoretical calibration

In general, a theoretical calibration will require the installer to put all theoretical values in the electronics of the weighing application. This means that the full scale capacity of the application should be put in the system, just like the sensitivity. The sensitivity can be easily found on the calibration certificate which is provided with all individual load cells from Zemic Europe. When multiple load cells are used in the system, the average of the sensitivities should be used for the full scale output. It is also possible for most electronics to set the resolution/divisions of the electronics. This is the minimum weight increment value which can be shown by your electronics. This is usually 1, 2, 5, 10, 20 etc.

When you are finished with performing the theoretical calibration as explained above, the next step is to calibrate the zero value of the application. This is done by removing all weight from the application. Then the zero value should be set in the electronics. Please note that this is an option in the calibration menu, not a tare function.

You have now completed the theoretical calibration.

Real calibration

For a real calibration of a strain gauge load cell application you need load cell calibration equipment. It requires you to use weights on the application and to register the corresponding sensitivity in the electronics. When this is done, the electronics usually calculates a straight line from zero to that measuring point and on. This is the same as it does during theoretical calibration. However, with real calibration you have the option to do this on more points. This is advised because every load cell has a certain amount of non-linearity and hysteresis error. This can be compensated by measuring multiple points and with this reconstruct the loading curve.

When the load cell calibration is finished as described above it is very important to calibrate the zero value, similar to the theoretical calibration. All load should be removed from the load cell application. When this is done the value should be set to zero. This can usually be done in the calibration menu of the electronics.

You have now completed the real calibration.

Load cell calibration equipment: availability of electronics

Besides the load cells it is also important to have the right electronics for the calibration of your weighing application. The electronics that we, at Zemic Europe, have to offer are able to be calibrated for both theoretical and real calibration. All information necessary for a proper calibration can be found in the manuals which are available on our website. A few examples of transmitters we have to offer are:

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