Analytical Balance

Analytical Balance:

The Chemstack lab uses a TE214S analytical balance

Specifications:

Readability: 0.0001g

Weighing capacity: 210g

Response time average: 3 seconds

Repeatability: ±0.0001g

Linearity: ±0.0002g

There are two types of electronic balances today: (1) Electromagnetic balancing type and (2) load cell type. Both operate in different ways to achieve the same result of weighing  by measuring the downward force exerted by the object.

The balance uses a standard as a reference point to determine what the downward force is equivalent to a weight (usually in grams).  Furthermore, balances have to be calibrated at certain times  (usually once a year) in order to compensate for the drift that occurs with most electronic measuring tools.

Types of error:

  1. Sensitivity error: Difference between the measured weight and the actual weight, which reflects on the accuracy of the balance converting force to weight. The sensitivity of balances can be adjusted by the user.

  2. Linearity error: Refers to the deviations from the linear relationship between actual and displayed weight value established by the reference standard. For the Sartorius TE214S, the reference standard is 200 grams. This means that we ideally want a linear relationship from two points: starting at  0 grams displayed value and 0 grams actual value (with no weight), and a second point of 200 grams displayed value and 200 grams actual value. Deviations outside the linear relationship is lineairty error.

  3. Repeatability: is self-explanatory. It is the precision or how close the measured values are from each other. This is usually expressed as standard deviation or relative standard deviation.

  4. Eccentric Error: Refers to where the object was placed on the pan, either in the center or to any of the sides on the pan/ It is best to place objects in the middle of the weighing pan remove eccentric error.

Sources of Error:

  1. Gravitation error: from moving the equipment between distant places. The difference in latitude and/or alititude changes the sensitivity of the instrument. The sensitivity should be adjusted to compensate for this error.

  2. Temperature: Balances require time to adjust changing temperatures. Any sudden change in temperature can result in sensitivity error.

  3. Containers with varying temperatures: Because the amount of air in containers vary by temperature, different temperature of a container than the balance can result in errors. If the container is colder than the balance, it will heat inside the weighing area and cause air to expand, resulting in higher weight than actual. The opposite phenomenon occurs when the container is hotter than the balance.

  4. Air flow: Too much air moving around can disrupt the stability and repeatability of the measurements.

  5. Static electricity: occurs with dry air, and can electrically charge objects being weighed causing stability and repeatability issues. This can be eliminated by using antistaic weighing boats or humidfying the room (increasing humidity may not be beneficial to the lab).

References:​

 
  1. Harris, Daniel C. Exploring Chemical Analysis. 5th ed., W.H. Freeman and Company, 2013.

  2. “Lab Balance Sartorius TE214S.” Precision Weighing Balances, scaleman.com/sartorius-te214s-analytical-milligram-balance.html.

  3. Learning About Electronic Balances. Shimadzu, www.shimadzu.com/an/hplc/support/lib/lctalk/66/66lab.html.

  4. Morse, Douglas, and Daniel M. Baer. “Laboratory Balances: How They Work, Checking Their Accuracy.” CE Update [Generalist], doi: 10.1309/QYR5UV73FRY2YBMJ.