A weighing scale is a very popular instrument used to measure the mass or weight of a body. Almost everything today is measured in terms of their weight.
Some of the Importance of using a weighing scale involves the following:
- If you go to a market or groceries, most of the things you bought are priced based on their weight.
- When you drink a medicine, like in a capsule form, it is measured in weight, usually in the unit of grams.
- When I was a cook (yes, one of my previous career), we always use a weighing scale to pre-portion the ingredients to be cooked.
- During a check-up in a clinic, doctors are measuring our body weight first before other diagnostics and so on.
We can see how important and critical a weighing scale is. This is why it is important also that the weighing scales we use are accurate. How can we determine its accuracy? —through calibration.
In this article, I will share with you how to calibrate a weighing scale to determine its accuracy in three simple methods; we will concentrate on a digital or electronic weighing scale.
This procedure can be used in your internal calibration laboratory to calibrate most of your digital weighing scale.
This is not just a simple or basic procedure where we will just compare a reference standard in one set up or position then finished (compared to verification only).
You will discover here a procedure where every aspect of weighing will be checked for accuracy.
Why calibrate a weighing scale.
Just like other electronic equipment, it has a component that will be exposed to environmental stress like temperature, humidity, and vibration. This exposure plus the aging will cause a digital weighing scale to encounter a drift.
Moreover, a digital weighing scale has a sensor, which is usually known as a load cell that can also drift because of the compression and contraction of its parts every time it is used.
Digital Weighing Calibration Procedure
This procedure is applicable to all digital weighing scale, analytical balance or a top loading balance which is generally termed as a Non-Automatic Weighing Scale (NAWS) with a maximum capacity up to 50 kilograms. This is performed by comparing the display of the balance to the value of the reference standard weights.
Things to consider:
- Weighing scale calibration should be performed preferably on its exact location, the location where it is being used. This is to prevent the environmental effects such as the local gravity, temperature and strong vibrations that affect its physical condition during the transfer.
- Allow proper stabilization time, power up the electronic balance at least 30 minutes before performing the calibration.
- Perform on a leveled surface or level the balance using the bubbles installed in it (if available).
To calibrate a digital weighing scale to ensure the accuracy of its reading by using a calibrated reference standard weights.
- Warm-up time (UUC): At least 1 hour for proper stabilization
- Temperature: 23 +/- 5 deg C
- Humidity: 50 +/- 30%
- Measurement Data Sheet (MDS)
Reference Standards to be used
- Set of Standard weights –It should cover the full range or capacity of the weighing scale
- Thermo-hygrometer – For Temperature and Humidity Monitoring
Other Materials includes gloves or tongs for handling weights, cloth for cleaning
- Check the digital weighing scale for any visual defects that can affect its accuracy. Discontinue calibration if any defect is noted.
- Carefully clean the weighing scale, most weighing scale pans can be removed for better cleaning. Check for batteries (for battery operated balances) to ensure accurate display and functionality.
- Level the balance, most balances have a stand that is adjustable to meet the proper level, use the bubble as a guide. See image.
- Ensure that the area has no vibrations and strong air movement. (use a good cover if available)
- Prepare the measurement data sheet (MDS) and record all necessary details or information (Brand, Model, serial #, etc). See this link to learn about MDS
- Zero the scale. ( push the zero button)
- Now, we will perform below test to verify its accuracy on different conditions or methods
- .Repeatability Test
This method will test how consistently is the balance in displaying the reading.
- .Repeatability Test
1.1 Load the pan with a standard weight that is 50% of the capacity of the balance.
1.2 Wait for the reading to stabilize ( others have annunciator or a beep)
1.3 Once stabilized, record the reading on the MDS.
1.4 Remove the weights, Re-zero and repeat for at least five (5) times.
2. Accuracy test or Departure from the nominal value
This method will test how accurate the displayed reading in various ranges of weights until the full capacity. Get 10 readings.
2.1 Divide the full range of the balance to at least 5 or use 5 set points depending on the availability of weights until the full range is reached. ( from min to max)
2.2 Load the pan with the first standard weight
2.3 Wait for the reading to stabilize
2.4 Once stabilized, record the reading on the MDS,
2.5 Return to zero, then repeat for at least five (5) times
2.6 Remove the weights, Re-zero and place the next weight.
2.7 Repeat procedure 2.1 to 2.5 until the full range is reached.
3. Off-center loading
This method will test the accuracy of the pan just in case the weights are placed off the center. This test will show us if the corner-load error is negligible or has a high effect on its accuracy.
We will use a single weight that is nearly 1/3 the capacity of the balance. For example, if the capacity is 1200 grams, we will use a 400 grams weight (1200/3).
Follow below position (1.Center, 2. Back, 3. Front, 4. Left and 5.Right):
3.1 After zeroing, load the pan with the required weights in the first position (center)
3.2 Once readout has stabilized, record the result on the MDS.
3.3 Remove weights, re-zero and transfer to other position.
3.4 Repeat until all positions are done.
8. End of procedure.
Most of the electronic or digital weighing scales today are programmable where an auto-calibration can be performed. This means that it can be adjusted. Once the calibration menu is unlocked or open, adjustment can be done by using the standard weights.
The principle is that the unit will require or search for an exact weight as a reference to be programmed internally. This procedure depends on the model of the weighing scale so be sure to have a manual while performing a calibration.
I have presented in this article the procedure for calibrating a digital weighing scale that you can use for your in-house calibration. Weighing scale calibration requires only standard weights but be sure to use the proper weights (class of weights) by considering the specifications of the balance (accuracy and resolution).
We discussed here the different methods to test the accuracy of a weighing scale. These are the repeatability test, accuracy test for defined nominal ranges and the off-center loading. Each one of this has its own requirements in order to determine how accurate your weighing scale is.
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