Published 12th February 2017 by Andy Connelly. Last updated 8th May 2017 by Andy Connelly
The laboratory balance is so often the forgotten object that sits in the corner of the laboratory. Forgotten, that is, until it is called on to give an accurate measurement. Then all hell breaks loose as it is found to be dirty and the mass just will not settle, it just keeps wondering. Eventually it is given up as a bad job and forgotten about again. There are few things more frustrating than a balance that is not behaving itself and usually this is due to neglect.
DISCLAIMER: I am not an expert on analytical balances. The content of this blog is what I have discovered through my efforts to understand the subject. I have done my best to make the information here in as accurate as possible. If you spot any errors or admissions, or have any comments, please let me know.
Types of balance
Analytical balances are designed to measure small masses from around 320g to sub-milligram. They are very sensitive pieces of equipment so need to be treated with care. The main types of laboratory balance are (masses stated are general values only):
- Top-pan balance (200g – 0.001g)
- Analytical balance (320g – 0.0001g)
- Microbalance (6g – 0.000001g)
- Ultra microbalance (6g – 0.0000001g)
Here we will mainly look at analytical balances as see in Figure 1. These are sensitive enough that they normally require a transparent enclosure which blocks air currents and prevents dust collecting (see Figure 1). To perform well they have to be level and so are fitted with level indicators (usually a spirit level). Apart from these two important factors the basic operation is very similar to your electronic kitchen balances at home.
Balances do not directly measure mass; they measure the force (weight) that acts downward on the balance pan. Most analytical balances are electromagnetic balances and so measure this weight using an electromagnet. Figure 2 shows the electromagnetic servomotor which generates a force counter the weight of the mass being measured. The electrical current required to generate this force is proportional to the weight and so can be used, with appropriate calibration, to calculate the mass. This mass is then displayed on the screen. To signal when the weight and electromagnetic forces are equal many balances have a “null detector” that uses a light source and detector.
The use of the electromagnetic means that balances that have been turned off (at the wall) should not be used straight away after turning back on. You should wait at least thirty minutes for the electromagnetic field to stabilise (times may vary depending on manufacturer and model). It also means that placing any magnetic materials or magnets near the balance could cause problems for the balance.
Operating an analytical balance
The following is a basic procedure for using an analytical balance. Before using a balance it is a good idea to check the calibration with a check weight (see below).
- Check the balance is level using the level indicator (see Figure 1). If the bubble is not in the centre adjust the level, normally by twisting the feet, until the bubble is in the centre of the inner circle.
- Check that the balance is on and that the door is closed. Press the “Tare” button and wait 5-10 secs for a ‘*’ or similar symbol to appear in the upper left/right hand corner of the display, and the mass to read 0.0000 g.
- Open the door and place a weigh boat, weight paper, or other container on the centre of the balance pan (ideally with tweezers or similar).
- Close the door and wait for the digital readout to stabilize (‘*’).
- If you do not wish to include container mass in your measurement then press “TARE” to reset the mass to zero (see Step 2 above),
- Remove the container from the balance and add the substance to be weighed. Avoid adding substances on the balance pan as this can result in contaminating the balance.
- Return container to balance and wait 5-10 secs (may take up to a minute) for the mass reading to settle.
- If the mass reading is unstable it may be due to static electricity build up or other issues– see trouble shooting section.
The accuracy and precision of an analytical balance must be guarded and checked at regular intervals. There are many factors that govern whether an analytical balance behaves itself:
- Gravitational acceleration differences across the globe mean that the balances calibration may require local adjustment.
- Temperature: Balances take time to equilibrate to laboratory temperature changes. Also, hot or cold objects can create convection currents in the air which can cause variation in mass measured. For these reasons it is important to let the balance and objects equilibrate to the same temperature.
- Moisture: Objects or materials that absorb moisture can appear to gain weight. This may particularly be an issue for objects that have recently been removed from a desiccator. Other materials may evaporate or sublime during measurement.
- Air flows: Air movement in the laboratory across the pan will cause variations in the measurement. A draft shield reduces this but it will take time for the air within the draft shield to stabilise once the door is closed. Changes in air temperature within the draft shield will also cause air movement. These changes can be due to the temperature of the mass, hands, etc. Reducing air flow incident on the balance in the laboratory is key to reducing this issue and ensuring all items entering the draft shield are equilibrated to ambient temperature – using tweezers, not your warm hands, to move items can help.
- Static electricity: This can be one of the biggest causes of frustration when using a balance. If the mass you are measuring wonders up or down and will not stabilise then there is a good chance that you have a static issue. The static-electrical field interferes with the electromagnetic field of the balance. To prevent this you can use an anti-static device which will “fire” positive and negative ions at the weight boat, powder, etc. to neutralise the static charge. The good anti-static system are incredibly effective and can save hours of pain and frustration. Anti-static plastic weigh boats or metal weigh weight boats can also help.
To help diagnose any issues you are having with a balance the check weight is a vital tool. A check weight (or calibration weight) is simply an object that has been certified to have certain mass. They are simple to use but must be handled very carefully as they are expensive and sensitive to touch and rough handling so should only be handled with tweezers and must not be dropped as this may change the mass.
There are different grades of check weights. The main accuracy classes for weights are as follows.
- Class E1 the highest accuracy class used as primary laboratory reference standard (1 kg is ±0.5 mg).
- Class E2 Used as a high precision standard for calibration of weights and special precision analytical instruments such as Ultra Micro, Micro, Semi–Micro Balances and analytical balances. (1 kg is ±1.6 mg).
- Class F1 For the calibration of some analytical balance and high accuracy top-pan balances. (1 kg is ±5 mg).
- Class F2, M1, M2, and M3 are of lower standard and not suitable for analytical balances.
It is worth checking each section of a balance pan to ensure you do not have any eccentricity error as well (see Figure 3). This type of error is the reason why it is good practice to always place items in the middle of a balance pan.
For most laboratories an annual service and calibration is sufficient. However, accredited laboratories will often check that calibration every day to ensure no error has occurred.
In between calibrations the key maintenance is to keep the balance clean. Any material that works its way inside a balance can cause major problems; especially if it is a corrosive material. Clean the balance by dusting off the stage and surrounding area with a paint brush or similar and then gently wipe down the balance, glass panels, and counter top around the balance with a lint free tissue. Ethanol can sometimes also be used depending on manufacturer and model – always check manufacturers instructions.
Balances are too important to be forgotten in the corner of the laboratory. The better they are looked after the more reliable and less frustrating they are.
- Mass is a measure of the amount of matter an object possesses. Stays constant no matter where the measurement is taken. Measured usually in kilogram or gram.
- Weight is the force due to gravity acting on a mass. The weight of an object on the top of a mountain will be less than the weight of the same object at the bottom due to gravity variations. Weight is measured in newtons (N).
- Accuracy How close to the “true” value a balance measures. The “true” value is usually taken as the mass of a calibration standard (check weight).
- Calibration The process of checking the accuracy of a balance using a check weight or similar standard material
- Capacity The heaviest load that can be measured on the instrument.
- Precision The amount of variation between repeated measurements of the same mass.
- Readability The smallest division at which the scale can be read – number of decimal places. For example, 0.1g or 0.0000001g.
- Tare The process of resetting the balance reading to zero and so removing a known weight of an object.