You’ve invested in a high-quality balance for your lab, and you want to do everything you can to ensure consistent, accurate results. An important part of maintenance is regular balance testing. What are the best methods to test your lab balance? Let’s find out.
Getting Started
Before you begin testing your balance, it’s important to consider the environment the balance operates in, as well as the calibration weights you’re using.
Environmental factors – including temperature, air drafts, static electricity and vibrations – can have a dramatic effect on balance readings and must be consistent to ensure accurate results. While these factors can be mitigated with thermostat settings (regulated to within one degree Fahrenheit), draft shields, ionizers and anti-vibration tables, it’s best to test your device under the same conditions that will be present during normal operation.
Before handling calibration weights, make sure you’re wearing clean gloves. Do not handle the weights directly, as the heat, oils and dirt from your fingers can compromise their CMV (conventional mass value). Also, it’s important to remember to handle calibration weights carefully and avoid sliding them across the balance’s weighing pan.
Things to Test For
The factors that should be tested for include repeatability, cornerload, linearity and span. We’ll start with defining those terms.
Repeatability, which is also called reproducibility, is the capability to show the same weighing result when a weight is placed on a scale more than once.
Cornerload Errors refer to errors associated with placement of the object on the weighing pan. The displayed result should be the same, regardless of where the object to be weighed is placed on the pan. Cornerloading is sometimes referred to as off-center loading.
Linearity is the ability of a weighing device to show the correct value throughout the entire weighing range. It may be tested by placing calibration weights – which have a known value – on the balance from near zero to full capacity.
Span refers to the difference between the weight reading of a given mass standard (such as a calibration weight) on your weighing device and the actual value of that standard.
How to Test Your Lab Balance
There are different methods for testing a balance. We’ll cover some of the most common.
Testing Repeatability calls for repeatedly weighing a single object (equal to or nearly equal to the scale’s capacity), recording the results and then analyzing them.
In a spreadsheet program, create two columns with the headings “Full Scale” and “Zero.” Tare the scale and weigh the test object. Record the reading under the “Full Scale” column, then remove the weight (do not zero the scale) and record that reading under the “Zero” column. Repeat this test 20 times. Once you have 20 pairs of results, use the spreadsheet program to calculate the standard deviation of both columns. If the standard deviation is larger than your scale or balance’s specifications allow, the device may be operating within a suboptimal environment (see the Getting Started section above) or the instrument may need to be repaired.
Cornerload testing verifies that you’ll get the same reading regardless of where on the pan you place the object to be weighed. Create a Cornerload chart with the headings Front, Right, Rear and Left.
Select a test weight that’s 1/3 to ½ of close to your machine’s maximum capacity and weigh it in the center of the plan, then re-zero the display. Move the weight halfway from the center toward the front and record the result under the heading Front. Repeat the test at the halfway point between the center and the rear of the scale, the left side and the right side and record those results under the appropriate heading. Because tolerances vary by industry, they’re typically not advertised. If your results are outside what you expect, consult with a qualified service technician.
For Linearity — which means your scale displays the correct value throughout its entire weighing range — begin by creating a linearity chart with two columns: 0%-50% and 50%-100%. You’ll use two calibration weights, each approximately half of the scale’s maximum capacity. The two weights, which can be referred to as Weight A and Weight B, should not be switched during testing.
Zero the display, place Weight A in the center of the pan and record the reading on your chart in the 0%-50% column. Remove Weight A, place Weight B in the center, then re-zero the display while Weight B is still on the pan. Place Weight A back on the pan along with Weight B and record the reading in the 50%-100% column.
Calculate the difference between the 0%-50% and 50%-100% columns. The difference should be less than the manufacturer’s listed tolerance for linearity. If it isn’t, your balance may need servicing or replacement.
The method to test Span, which is often what people refer to when they talk about calibration, depends on whether your balance offers an internal calibration option or if it requires external calibration. Internal calibration is as simple as the push of a button.
For external calibration, you need calibration weights. There are different classes of weights that are appropriate for different uses. For more information, check out Adam Equipment’s blog post on calibration weight classes.
Select five or six weights across the analytical measurement range (AMR), tare the balance so the reading is zero, weigh each of the calibration weights and record the results. To ensure consistency, use the same set of weights each time you perform this test. Compare the results: if there’s no significant difference between the calibration weight’s mass and the balance’s reading, the test is complete. If the difference is significant, consult a qualified balance repair service.
Questions about testing your lab balance? Reach out to us with your questions and we’ll be happy to help!