Spring time means spring cleaning! Baby birds are hatching, the days are getting longer, there are butterflies everywhere and you feel inspired to do a good deep clean of your lab scales. We can’t help you sing like Snow White so that animals come help you clean but we can help you get your scales looking their best!
In this article, we’re using the term “weight” even though “mass” is technically the correct term.
Day-to-day and Deeper Cleaning
- Use a small balance brush to gently sweep any residue from the balance.
- Moisten a thin wipe (like a Kimwipe) with purified water and carefully clean the pan and around the scales. Wipe dry.
- Remove the pan to facilitate more thorough cleaning.
- If you’ve moved your scales and it has a leveling bubble, check it’s centred.
- Thank your scales! Okay maybe this is a step too far but, hey, can’t hurt.
- Press down on the balance pan as this can damage the internal mechanism, particularly for scales that measure very small weights. Be gentle!
- Turn the scales upside down and shake it to help remove residue. Don’t be abusive
- Drop the scales or put it down on a surface roughly. Imagine the scales is a sleeping baby: only move it if you have to and do with all the tender care with which you’re capable.
- Use cleaning fluids that are harsh like ethanol as it will react with agar, salts and other common ingredients.
- Leave salts on the scales. This can corrode the scales resulting in poorer performance.
Checking Accuracy: Why it’s Important and How to Check
Ensure your scales are completely free of residue before moving on to check the scales’ accuracy.
Accuracy is how reliably the scales is at measuring the weight of a sample. This can be deduced by a number of tests and each is designed to help you ensure your scales are functioning at peak efficiency. Your scales should also be left on for an hour minimum and ideally overnight before it is used.
1. The Repeatability Test:
If the minimum weight capacity of your scales is 1 g and the max is 100 g, measure out a weight somewhere on the upper end of this, say 75 g. You’re looking for a weight value of 50–100% of the weighing capacity of the instrument. Ideally you should use standard weights to do this but if you’re stuck, improvise, but be aware that not having a sample of known weight means you’ll be able to spot random error but not systematic error. Please note that your sample must have reached thermal equilibrium before it is used.
Whatever you’re using will be referred to as the “sample” here. Each time you weigh a sample, your scales should give you the same weight within a certain margin of error. This accuracy is stated in your scales manual or check the manufacturer’s website. Place and remove the sample 10 times and record each reading as soon as the balance indicates that the reading is stable. Work out the range of values and see if it is within the acceptable margin of error. If it is, great! If not, it’s time to calibrate your scales.
2. The Pan Position Error Test
Your scales should be able to weigh your sample accurately even if you’ve placed the sample off-center. To test this, note the weight value given when your sample is placed in these positions on the scales’ pan:
Illustration by author
Again, look at the range to tell you how consistently your scales are weighing your samples.
3. The Linearity Test
If your scales pass this test then you should be good to go. Here, you have two samples of different weight. Take two samples each with a weight of half the weighing capacity of the scales. Add sample 1 to the scales and note the value. Zero the scales. Add sample 2. Note the value. Add sample 1 alongside sample 2. If the value changes outside of the acceptable range, it’s calibratin’ time, YEE-HAW!
Calibrating a Balance: How Often and Just Plain How
When to Calibrate
Times to recalibrate your scales:
- When you first buy it.
- When you move it somewhere new.
- When there is a significant change in ambient temperature.
- When the scales have been unplugged for a while.
- If the accuracy is off.
How To Calibrate
Your balance may come with built-in calibrating abilities, called internal calibration. External calibration is also useful and for this you will need weights of known weight.
Once your weights have reached thermal equilibrium, place a thermometer near the balance and record the ambient temperature. Record the balance’s location, model, serial number, scale range, resolution, and the temperature sensitivity coefficient (you may need to consult the balance manual).
Record the balance reading for 10 successive loadings of the balance with the same weight (or group of weights), removing the load between each reading as you would for The Repeatability Test. If you usually tare before each weighing then do so here but don’t if you don’t! Calculate the repeatability as the standard deviation ? R of the 10 balance readings. This is also the standard uncertainty uR due to repeatability. That is:
The STDEV function in Excel can be used to calculate uR. Also calculate the error in the balance reading for this load as the average balance reading r minus the known mass of the weight (or group of weights) from their calibration certificate. This calculation can also be repeated using a sample of half the weight and done in conjunction with The Pan Position Error Test and The Linearity Test.
If you’re getting some unexpected results, it’s time to get your scales serviced.
For a more extensive guide on the ins and outs of everything you need to know to keep your scales in tip-top condition, see UKAS.