Principles of Measurements Flashcards

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Questions and Answers

Prescription balance can be either:

Class A (Class III)

Class B

Both Class A (Class III) and Class B (correct)

None of the above

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?

120 g Signup and view all the answers

What is the error of measurement for a Class A (Class III) prescription balance?

5% or less Signup and view all the answers

What is the sensitivity requirement for a Class B prescription balance?

30 mg or less Signup and view all the answers

Which type of balances is considered more convenient and self-calibrating?

Electronic balances Signup and view all the answers

What does a torsion balance contain?

2 pans, dial-in weight of a weighbeam with rider, index pointer, leveling screws or feet, arresting knob Signup and view all the answers

How often should calibration be performed on balances?

Regularly Signup and view all the answers

Which of the following is NOT a test for calibration?

Weight limit test Signup and view all the answers

Does the sensitivity requirement of a balance change when the measured weight is changed?

False Signup and view all the answers

As sensitivity requirement increases, what happens to percentage error?

Increases Signup and view all the answers

What happens to percentage error as quantity to weigh decreases?

Increases Signup and view all the answers

Since error over ___ is not allowed, what is generally the minimum weighable quantity (MWQ) for a Class A prescription balance considering SR = 6 mg?

5% Signup and view all the answers

If a balance with lower sensitivity requirement is chosen, what happens to minimum weighable quantity (MWQ)?

Decreases Signup and view all the answers

How do you weigh something that exceeds the maximum capacity of the balance?

Weigh multiple times Signup and view all the answers

How do you weigh something that's below the minimum weighable quantity (MWQ)?

Use aliquot method Signup and view all the answers

What are the three components you are weighing out in the aliquot method for solids?

Drug, diluent, aliquot Signup and view all the answers

What must you observe to avoid parallax error when weighing?

Observe straight at the bottom of meniscus Signup and view all the answers

What are the steps in the aliquot method for liquids?

Measure more than the desired quantity, add water or other vehicle, measure the required aliquot volume Signup and view all the answers

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.