In chemical laboratories the weighing process is the first step in virtually any analysis. Errors or unmet standards in this early stage can multiply throughout the lab workflow, creating costly rework and lost batches.
Assemble the correct containers to receive your weighed material and use forceps, pipettes or spatulas of the proper size for each. Record the weight immediately as soon as the reading stabilizes.
Weight Measurement
Weighing is a process that measures the heaviness of an object. Regardless of its size, every object has a weight that is determined by the force of gravity. The strength of gravity varies between geographic locations. This is why weighing equipment should be calibrated to its location on a regular basis with the use of calibration weights.
It is also important to ensure that a balance is protected from environmental factors such as wind or heat. Additionally, the lab should follow certain procedures for weighing samples to prevent errors. For example, a sample that is hot or warm should be cooled prior to weighing and all plastic and glass containers used for weighing must be closed. These precautions prevent moisture and other contaminants from interfering with the measurement.
For routine weighing, it is recommended that the balance be tared with a reference weight and then perform (typically) 10 replicate weighings. This will help the user determine the instrument’s repeatability and sensitivity, as well as eccentricity or cornerload error.
Calibration
Calibration helps you cut down on variation and get accurate results. It also assists you avoid fees and legal action by showing that your weighing process adheres to essential regulations.
During calibration, you compare the performance of your weighing instrument to a known standard measurement to find out its accuracy. Traceability is an important part of the calibration process and it allows you to establish a link between your measurements and the International System of Measurement (SI).
The dead weight method is the most popular way of calibrating a scale. This involves placing certified test weights on the weighing scale until the material plus the weights total agrees with the capacity of the scale. If you want to save time and money, you can use a live weight calibration method wherein a pre-weighted person acts as the reference load. This can also help you avoid errors caused by the transferring of weights (which can lose or gain weight during transportation). This method produces a similar accuracy to the deadweight calibration.
Error Reduction
When weighing samples, use tongs or gloves to prevent fingerprints on the glass jar and avoid touching the scale. Doing so prevents a possible magnetic effect that can affect the reading, which can be corrected by demagnetization.
The floor or structure that supports your weigh vessel should be strong enough to support the weight of the vessel and any equipment resting on it, without flexing. This eliminates unwanted side loads that can affect the weighing system’s accuracy.
Large temperature changes can cause the materials in your weigh vessel to expand and contract, which can also affect the weighing system’s accuracy. To minimize this effect, choose a weighing system with load cells and mounting hardware that can handle the expansion and contraction of your weigh vessel.
Sensitivity is the dominant component of measurement uncertainty at the high end of your weighing system’s range, so sensitivity tests should be conducted regularly on your balance. A sensitivity test will quickly indicate if your balance is experiencing mechanical problems.
Recording
If you’re using a weighing system to measure level or inventory, a local display and manual control may be necessary. These applications don’t involve time-critical weight cutoffs and do not rely on precise measurement of a volatile, corrosive or conductive substance.
The weighing system typically includes one or more load cells that support (or suspend) a weigh vessel or platform, a junction box and a weight controller. When a load is applied to the vessel or platform, each of the load cells sends an electrical signal proportional to the load to the junction box. The junction box sums the signals from each of the load cells and then transmits them to the weight controller, which converts the summed signal into a weighing readout.
Moisture that enters the weighing system’s junction box can wick between the load cell excitation lines and the load cell signal lines, causing a capacitance between them. This introduces electric noise to the weighing signal, reducing its accuracy. To reduce this effect, seal all openings in the weighing system enclosure.