different rates. When weighing a sample, this variable response to temperature produces unreliable data. It is recommended to keep a constant temperature at all times in an environment where weighing instruments are kept. When room temperature changes, allow the instrument to equilibrate for 24 hours. Air Currents or Drafts The flow rate of ambient air should be minimized to get nd stable results with weighing equipment For balances with a readability of 1 mg, an open draft shield (glass cylinder) will suffice. Below 0. 1 mg, a closed draft chamber is needed These shields or chambers should be as small ible eliminate convection currents within the chamber to minimize temperature variation and internal draft problems. Magnetic Forces and Magnetic Sample Magnetic forces are produced when a sample is magnetized or magnetizable, which means it contains a percentage of iron, cobalt or nickel. Magnetic effects manifest themselves in the sample's loss of reproducibility. But unlike electrostatic forces, magnetic forces can yield a stable measurement. Changing the orientation of the magnetic field(moving the reagent sample) relative to the weigh system causes the irreproducible results magnetic effects are thus difficult to detect unless the same sample is weighed more than once. Placing an inverted beaker or a piece of wood between the sample and the pan can counteract the magnetic force. Some instruments allow for below balance weighing, in which a hook used to attach magnetic samples lies underneath the weigh pan at a safe distance in order to eliminate magnetic effects. Gravitational tilt a balance must be level when performing measurements on the weighing pan. Gravity operates in a direction that points straight to the center of the earth. Thus, if the weigh cell in not directly in this path, the weight will end up somewhat less. For example, say we weigh a 200g sample that is 0. 2865(angle a) out of paral lel. we have Apparent weight weight * cos a Apparent weight = 200*cos 0. 2865=199.9975g This result represents a 2.5mg deviation. This is a significant quantity when working with analytical samples How to Properly Use and Maintain Laboratory Equipment 53different rates. When weighing a sample, this variable response to temperature produces unreliable data. It is recommended to keep a constant temperature at all times in an environment where weighing instruments are kept. When room temperature changes, allow the instrument to equilibrate for 24 hours. Air Currents or Drafts The flow rate of ambient air should be minimized to get quick and stable results with weighing equipment. For balances with a readability of 1 mg, an open draft shield (glass cylinder) will suffice. Below 0.1 mg, a closed draft chamber is needed. These shields or chambers should be as small as possible to eliminate convection currents within the chamber to minimize temperature variation and internal draft problems. Magnetic Forces and Magnetic Samples Magnetic forces are produced when a sample is magnetized or magnetizable, which means it contains a percentage of iron, cobalt, or nickel. Magnetic effects manifest themselves in the sample’s loss of reproducibility. But unlike electrostatic forces, magnetic forces can yield a stable measurement. Changing the orientation of the magnetic field (moving the reagent sample) relative to the weigh system causes the irreproducible results. Magnetic effects are thus difficult to detect unless the same sample is weighed more than once. Placing an inverted beaker or a piece of wood between the sample and the pan can counteract the magnetic force. Some instruments allow for below balance weighing, in which a hook used to attach magnetic samples lies underneath the weigh pan at a safe distance in order to eliminate magnetic effects. Gravitational Tilt A balance must be level when performing measurements on the weighing pan. Gravity operates in a direction that points straight to the center of the earth. Thus, if the weigh cell in not directly in this path, the weight will end up somewhat less. For example, say we weigh a 200 g sample that is 0.2865° (angle = a) out of paral￾lel. We have Apparent weight = weight * cosa Apparent weight = 200 *cos0.2865 = 199.9975 g This result represents a 2.5mg deviation. This is a significant quantity when working with analytical samples. How to Properly Use and Maintain Laboratory Equipment 53