3 Ways to Verify and Perform a Digital Weighing Scale Calibration

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Digital weighing scale calibration

An analog or digital scale for weighing is a very popular measuring 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:

  1. If you go to a market or groceries, most of the things you bought are priced based on their weight. ..
  2. When you drink a medicine, like in a capsule form, it is measured in weight, usually in the unit of grams..
  3. When I was a cook (yes, one of my previous career), we always use a scale for weighing food tp pre-portion the ingredients to be cooked.
  4. 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 the scale accuracy? —through calibration.

In this article, I will share with you how to calibrate a digital weighing scale to determine its accuracy in three simple methods; we will concentrate on the calibration of a scale in the digital or electronic type. 

A digital weighing scale for food, also used for determining the price based on its weight.

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, wherein we will just compare a reference standard in one set up or position then finished (compared to verification only).

You will discover here a scale calibration 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 aging will cause a digital weighing scale to encounter a drift. 

Another reason is, 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.

Moreover, this is special to balances, wherein transferring the balance from one place to another has an effect on its accuracy, unless it is verified that transferring to different locations has no effect, the balance should be calibrated.

Additional reasons why we perform a balance calibration includes:

  1. Change in environmental condition like temperature and humidity
  2. After a repair or adjustment.
  3. Harsh usage
  4. Change in range of usage.
  5. Undesirable reading

Digital Weighing Scale 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. The calibration of scales with weights is performed by comparing the display of the balance to the value of the reference standard weights.

Things to consider:

  1. Weighing scale calibration should be performed preferably on its exact location, the location where it is being used. This is to prevent environmental effects such as the local gravity, temperature and strong vibrations that affect its physical condition during the transfer.
    ..
  2. Allow proper stabilization time, power up the electronic balance at least 30 minutes before performing the calibration.
    ..
  3. Perform on a leveled surface or level the balance using the bubbles installed in it (if available).

Objective

To calibrate a digital weighing scale to ensure the accuracy of its reading by using a calibrated reference standard weights.

Requirements

  • 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

  1. Set of Standard weights –It should cover the full range or capacity of the weighing scale
Set of Reference Standard Weights
Set of Reference Standard Weights
  • Thermo-hygrometer – For Temperature and Humidity Monitoring
  • Other Materials includes gloves or tongs for handling weights, cloth for cleaning

     Scale Calibration Procedure

    1. Check the digital weighing scale for any visual defects that can affect its accuracy. Discontinue calibration if any defect is noted.
      ..
    2. 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.
      ..
    3. Level the balance, most balances have a stand that is adjustable to meet the proper level, use the bubble as a guide. See image.
    Position the bubble in the middle for proper leveling
    • Ensure that the area has no vibrations and strong air movement. (use a good cover if available)

    4. Prepare the measurement data sheet (MDS) and record all necessary details or information (Brand, Model, serial #, etc). See this link to learn about MDS

    5. Zero the scale. ( push the zero button)

    6. After zeroing, load a standard weight that is 80% of the capacity of the balance. Record this as the “as-found” in your datasheet. If an error is observed, perform an adjustment if needed.

    Now, we will perform the calibration of scales with weights to verify its accuracy on different conditions or methods through the following test:


    1. Repeatability Test

    This method will test how consistently is the balance in displaying the reading.

    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.

    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 Remove the weights, Re-zero and place the next weight.

    2.6 Repeat procedure 2.1 to 2.5 until the full range is reached.


    3.  Off-center loading

    This kind of test is also known as the Eccentricity Test.

    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).

    weights position or placement for off-center loading test
    weights position or placement for off-center loading test

         Follow the 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)

    position 1

                 3.2 Once readout has stabilized, record the result on the MDS.

    standard weight position 3 and position 5

               3.3 Remove weights, re-zero and transfer to another 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.

    2 Types of Adjustment in a Digital Weighing Scale (DWS)

    weighing scale adjustment
    weighing scale adjustment

    For me, the most full filling instruments to calibrate are the digital weighing scale. Why? As per experience, you can see the actual adjustments and removal of the error. You can return back the instrument in its most accurate range to almost no error at all (visually).

    There are 2 ways where we can perform an adjustment, either by

    1. internal adjustment
    2. external adjustment

    This type of adjustment in balance calibration is also known as ‘internal calibration’ and ‘external calibration’. But in a real sense, it is a method of a weighing scale adjustment.

    Remember that during the execution of any adjustment (internal or external), the adjustment factor that is stored in the system will be changed once you pushed the accept button.

    Internal Calibration Vs. External Calibration in a Digital Weighing Scale

    Internal Calibration

    example of internal adjustment for a Mettler Toledo digital balance

    Internal calibration also known as the internal adjustment is an adjustment using the internal weights installed inside the weighing scale. There is no need to load standard weights.

    It is an automated adjustment where the DWS is adjusted just by a press of a button.

    The digital weighing scale will detect the zero load of the pan then it will use the internal weights installed to perform an adjustment or calibration. Once executed, there are no other steps to be done, but to wait for the final settings to be saved.

    When to use internal calibration?

    Internal calibration is usually used every time a DWS is transferred from one place to another,  a big change in environmental conditions or a suspected deviation in readings.

    But be sure to have standard weights with you every time you perform this to verify that errors are removed or within an acceptable range. If you do not need a set of weights, at least use standard weights that are equal or near to the range that you are using.

    The disadvantages of this method of adjustment are:

    > the adjusted range is limited and therefore the accuracy

    > since the internal weight is not certified, there is no traceability of the performed calibration

    > once you have executed this adjustment in the balance, it will delete the past calibration settings and therefore invalidate the performed past calibration.

    Advantages of Internal Calibration of a Weighing Scale.

    • Easy to execute.
    • No need for a physical or standard weights
    • Can be executed anytime.

    External Calibration

    External adjustment on a Mettler Toledo digital balance

    In External calibration or external adjustment of a weighing scale,  we will need a set of weights, depending on the range that is required or programmed in the DWS.

    We will load the weights manually on the pan of the DWS to perform the adjustment. The DWS will prompt you what range of weights you need to put in its pan (or depending on what range you have programmed).

    External calibration of balance is used during scheduled calibration (full-range or what is required as per user) and to ensure a traceable calibration.

    Advantages of External Calibration or Adjustment

    • More accurate adjustments
    • Multiple ranges can be adjusted.
    • Traceability is ensured as long as the standard weights are calibrated
    • Full range calibration based to an International Standard procedure

    Remember that every time an adjustment is executed, do not forget to perform balance verification.

    Conclusion

    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 also known as eccentricity.

    Each one of this has its own requirements in order to determine how accurate your weighing scale is.

    Furthermore, I have explained the terms used during adjustments, which is the internal and external calibration, also known as internal and external adjustments. I have presented its meaning and some advantages of each use.

    Thank you for visiting my site.

    If you like this article, I appreciate if you could share, leave a comment and subscribe.

    You can also connect with me on my Facebook page.

    Best Regards,

    Edwin

    14 Responses

    1. Kourtney Jensen
      | Reply

      I have had my balance for a while now and haven’t had it calibrated. I wonder if it’s still calibrated correct. This was very informative and made me realize the importance of doing so! You mentioned that so many things are measured in weight now and I never realized that until reading this. Weight being off can make a big difference in the food market and also in the science field with medication and chemicals. If one number is off, it can throw everything off. I’ll be sure to pass this information on to my husband so that he knows the importance of having our balance calibrated!

      • edsponce
        | Reply

        Hi Ms. Kourtney,
        Good Day! I appreciate that you read my post and had taught you something.
        Yes you are correct.This is one of the importance of calibration not just in manufacturing areas but in our daily lives as well. By knowing how accurate the instrument we use gives us the assurance of quality, reliability, and fairness to everyone whenever we use it. If you own a balance and you want to verify it personally if it is reading accurately without using a standard weights, you may visit one of my post here. https://calibrationawareness.com/verifying-a-weighing-scale-by-using-a-bottled-water
        Just let me know if ever you or your husband has a question about balance calibration.

        Thank you and Best Regards,
        Edsponce

    2. Thank sir its very useful to me

      • edsponce
        | Reply

        You’re welcome Sir, I am glad that my post has helped you.

        Best Regards,
        Edwin

    3. John Smith
      | Reply

      Very useful article. Thanks. I actually find that hiring professionals to take care of calibration needs gives me greater peace of mind. And with services like Calibration Program Management, a business can get turnkey solutions that actually free up resources that can then be invested in the core business.

      • edsponce
        | Reply

        Hi Sir,
        Thanks for the comment. I agree with you. Managing a calibration program has a lot of things to consider both technical and in the documentation part.

        Best regards,
        Edwin

    4. M.Saravanan
      | Reply

      Hi sir thank you for a clear informative presentation. It was very useful for our PG students for routine calibratione. Good job.

      • edsponce
        | Reply

        Dear Mr. Saravanan,
        You are welcome Sir, Happy to help. Appreciate your comment.

        Thanks and regards,
        Edwin

    5. Vinod
      | Reply

      Hi Sir, I go through your post and its fantastic.can you help me the calculation or measurements uncertainty for digital pressure gauge, calibration using hydraulic pressure comparator. Please sir

      • edsponce
        | Reply

        Hi Mr. Vinod,
        I am happy to help. Do you have some background on how to calculate measurement uncertainty?

        Thanks and regards,
        Edwin

    6. Dingiswayo
      | Reply

      Goodday,

      Your article is very refreshing and informative. Enjoyed reading through it, good job!

      Thank you very much

      • edsponce
        | Reply

        Hi Sir,
        You are welcome. Thank you for the time spent in reading my post. I appreciate your comment.

        Best regards,
        Edwin

    7. Ansul
      | Reply

      Calibration is important task to do while working with weighing machine. People tend uninformed about this. You write the best article for this problem. Keep writing.

      • edsponce
        | Reply

        Hi Mr. Ansul,
        I agree more with you.

        Thank you for visiting my site.I appreciate your comments.

        Best Regards,
        Edwin

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