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Prescription balance can be either:
What type of prescription balance is required in all dispensing pharmacies?
Class A (Class III)
What is the sensitivity requirement for a Class A (Class III) prescription balance?
6 mg or less
What is the maximum weight capacity generally for a Class A (Class III) prescription balance?
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What is the error of measurement for a Class A (Class III) prescription balance?
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What is the sensitivity requirement for a Class B prescription balance?
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Which type of balances is considered more convenient and self-calibrating?
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What does a torsion balance contain?
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How often should calibration be performed on balances?
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Which of the following is NOT a test for calibration?
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Does the sensitivity requirement of a balance change when the measured weight is changed?
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As sensitivity requirement increases, what happens to percentage error?
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What happens to percentage error as quantity to weigh decreases?
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Since error over ___ is not allowed, what is generally the minimum weighable quantity (MWQ) for a Class A prescription balance considering SR = 6 mg?
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If a balance with lower sensitivity requirement is chosen, what happens to minimum weighable quantity (MWQ)?
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How do you weigh something that exceeds the maximum capacity of the balance?
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How do you weigh something that's below the minimum weighable quantity (MWQ)?
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What are the three components you are weighing out in the aliquot method for solids?
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What must you observe to avoid parallax error when weighing?
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What are the steps in the aliquot method for liquids?
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Study Notes
Prescription Balances
- Prescription balances are categorized into Class A (Class III) and Class B.
Class A (Class III) Prescription Balance
- Known as torsion balance.
- Mandatory in all dispensing pharmacies.
- Must comply with National Bureau of Standards' requirements.
- Sensitivity requirement is 6 mg or less.
- Maximum weight capacity typically reaches 120 g.
- Measurement error allowed is 5% or less.
Class B Prescription Balance
- Sensitivity requirement is 30 mg or less.
- Considered optional for pharmacies.
Torsion vs Electronic Balances
- Torsion Balances: Mechanical, do not need power, known for durability and lower cost, require multiple calibration steps, need warm-up period, and frequent calibration.
- Electronic Balances: Offer convenience, integrate electronic and mechanical features, more expensive, allow single-step calibration, include self-calibration and digital readouts, require power supply.
Pharmacy Requirements
- All pharmacies must have a Class A (Class III) prescription balance.
Components of Torsion Balance
- Includes two pans, a dial-in weight on a weighbeam with a rider, an index pointer, leveling screws or feet, and an arresting knob.
Calibration Procedures
- Calibration is essential for initial use and should be performed regularly.
- Electronic balances benefit from automatic calibration.
Calibration Tests
- Various tests for calibration include sensitivity requirement (SR) test, arm ratio test, shift test, and rider & graduated beam test or dial-in weight test.
Sensitivity Requirement Insights
- The sensitivity requirement remains unchanged regardless of the measured weight.
- It is directly related to the accuracy of measurement.
- As sensitivity requirement (SR) increases, the percentage error also increases.
- Decreasing the quantity to weigh results in an increased percentage error.
Minimum Weighable Quantity (MWQ)
- For Class A balance with SR of 6 mg, the minimum weighable quantity (MWQ) or least weighable quantity (LWQ) is 120 mg, based on an allowable error of 5%.
- Opting for a balance with lower sensitivity results in a decrease in MWQ.
Weighing Techniques
- To weigh an item exceeding the maximum capacity of the balance, divide into smaller measurable portions and weigh each separately.
- For items below MWQ, apply the aliquot method.
Aliquot Method for Solids
- Weigh a quantity greater than the desired amount of drug.
- Mix the drug with an inert diluent using geometric dilution for uniform distribution.
- Weigh out the aliquot quantity from the homogenous stock mixture to obtain the desired amount of the drug.
Key Components in Aliquot Method for Solids
- Involves weighing the drug, diluent, and the aliquot.
Measurement Accuracy
- Avoid parallax error by observing the bottom of the meniscus straight-on.
Aliquot Method for Liquids
- Measure more than the desired liquid quantity that can be accurately gauged.
- Add a vehicle (e.g., water) to achieve a suitable stock liquid mixture.
- Measure the aliquot volume from the stock liquid that contains the desired drug quantity.
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Description
Test your knowledge of prescription balances with these flashcards. This quiz covers key concepts, classifications, and requirements associated with Class A and Class B prescription balances. Perfect for pharmacy students and professionals alike.
