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 its weight.
Some of the Importance of using a weighing scale involve 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 scale for weighing food tp 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 the scale accuracy? —through calibration.
In this article, I will share with you how to calibrate a digital scale to determine its accuracy in three simple methods; we will concentrate on the calibration of a weighing scale in the digital or electronic type.
This procedure can be used in your internal calibration laboratory to calibrate most of your digital weighing scales.
This is not just a simple or basic procedure, wherein we will just compare a reference standard in one setup or position and then finished (compared to verification only).
You will discover here a scale calibration procedure where most capabilities, especially a digital weighing scale, 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, that 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 only in the location where it is being used.
Additional reasons why we perform a balance calibration includes:
- Change in environmental condition like temperature and humidity
- After a repair or adjustment.
- Harsh usage
- Change in range of usage.
- Undesirable reading
Digital Weighing Scale Calibration Procedure
This procedure is applicable to all digital weighing scales, analytical balance, or a top loading balance which is generally termed a Non-Automatic Weighing Scale (NAWS) with a maximum capacity of up to 50 kilograms.
How to calibrate a digital scale? The calibration of scales with weights is performed by comparing the display of the balance to the display of the reference standard weights.
The digital scale calibration is performed in different techniques (3 methods) that you can see below, where every possible position and performance is checked for accuracy.
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 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. The standard weights for scale calibration should be appropriate to provide the best results. How to determine the appropriate reference standard weights? Check out my other post HERE.
Other Materials includes gloves or tongs for handling weights, cloth for cleaning
Scale Calibration Procedure
- 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)
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 for you to create your weighing scale calibration format or template for a faster recording of data.
**For a weighing scale calibration format, you can use the 3 methods below as its content under the ‘Calibration results’ section.
5. Zero the scale. (push the zero button)
6. After zeroing, load a standard weight that is 80% of the capacity of the balance. This is the initial test for its accuracy. Record this as the “as-found” in your datasheet. If an error is observed, perform an adjustment immediately if possible (read down below about balance adjustment).
Now, we will perform the calibration of scales with weights to verify their accuracy under different conditions or methods through the following performance 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 is 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).
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)
3.2 Once the readout has stabilized, record the result on the MDS.
3.3 Remove weights, re-zero and transfer to another position.
3.4 Repeat until all positions are done.
8. End of procedure.
Finalize your results and create the calibration report. Check the content of an ISO 17025 calibration certificate in this link >> Accredited calibration certificate
2 Types of Adjustments in Digital Weighing Scale (DWS) Calibration
Scale adjustment is needed once we verify that our DWS is showing a large error or an error that exceeds our tolerance limit.
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 automatically with a push of a button. 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.
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 with almost no error at all (visually).
There are 2 ways where we can perform an adjustment, either by
- internal adjustment
- external adjustment
These types of adjustments in balance calibration are also known as ‘internal calibration’ and ‘external calibration’. It is termed as ‘calibration’, but in a real sense, it is a method of a weighing scale adjustment because it brings back your scale to its most accurate performance.
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.
Before performing balance adjustment, it is important that you know the tolerance of your weighing scale. If you do not know your tolerance limit, visit my other post here for a calculation guide >> determining the tolerance limit of a weighing scale
Internal Calibration Vs. External Calibration in a Digital Weighing Scale
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.
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 on 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) to ensure a traceable calibration.
Advantages of External Calibration or Adjustment
Remember that every time an adjustment is executed, do not forget to perform balance verification.
Take note that not all electronic weighing scales or balances are adjusted successfully even if you performed the above scale adjustment method. One reason is that the load cells are already defective where a scale calibration service by a 3rd party is needed, usually the OEM for repair or replacement of load cells.
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 these 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.
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