# Accuracy in Measurements

## Laboratory Procedure

I. Test of the Prescription Balance

Serial# ____________________
Model ____________________

A.  SENSITIVITY REQUIREMENT – The balance should have a maximum sensitivity of 6 mg with no load and with a load of 10 g on each pan. You don’t have a 6 mg weight in your weight set, so you will use multiples of 10 mg weights in this test.

1. Level the balance and determine the rest point. Place a 10 mg weight on one of the empty pans and again determine the rest point. How many divisions was the rest point shifted by the 10 mg weight? ___________ Place a 20 mg weight on one of the empty pans and again determine the rest point. How many divisions was the rest point shifted by the 20 mg weight? __________ Place a 30 mg weight on one of the empty pans and again determine the rest point. How many divisions was the rest point shifted by the 30 mg weight?_____

Plot “number of scale divisions shifted” versus “weight” for your three determinations. Use Excel to make a trendline through the points and by calculation, determine the weight that would cause a shift of one scale division.

2. Put a 10 g weight in the center of each pan. Level the balance to get the marker in the middle of the plate.

3. Place a 10 mg weight on one of the pans having the 10 g weight and again determine the rest point. How many divisions was the rest point shifted by the 10 mg weight? _______ Place a 20 mg weight on one of the pans containing the 10 g weight and again determine the rest point. How many divisions was the rest point shifted by the 20 mg weight? __________ Place a 30 mg weight on one of the pans containing the 10 g weight and again determine the rest point. How many divisions was the rest point shifted by the 30 mg weight?_____

Plot “number of scale divisions shifted” versus “weight” for your three determinations. Use Excel to make a trendline through the points and by calculation, determine the weight that would cause a shift of one scale division.

4. The balance passes the sensitivity requirement if the balance is shifted one scale division by 6 mg or less with no load and a load of 10 g. Does your balance pass this test? __________

B. ARM RATIO TEST – This test is meant to check the equality of length of both arms of the balance.

1. Determine the rest point of the balance with no weight on the balance pans.

2. Place 30 g of weights in the center of each pan and determine the rest point.

3. If the second rest point is not the same as the first, place a 20 mg weight on the lighter side. The rest point should return to the original place on the scale or farther.

4. Does your balance pass this test? __________

C. SHIFT TEST – The shift tests are used to check the arm and lever components of the balance.

1. Determine the rest point without any weights on the balance pans.

2. Place one of the 10 g weights in the center of the left pan and place the other 10 g weight successively toward the right, left, front and back side of the right pan, noting the rest point in each case. If in any case the rest point differs from the rest point determined in (1), add a 10 mg weight to the lighter side and this should cause the rest point to shift back to the rest point determined in (1) or farther.

3. Repeat (2) with the 10 g weight in the center of the right pan varying the position of the weight on the left pan.

4. Make several observation in which both weights are simultaneously shifted to off-center positions on their pans, i.e., both toward the inside, both toward the outside, one front and the other back, etc. If in any case the rest point is shifted from (1), the addition of a 10 mg weight should overcome the difference.

5. Does your balance pass this test? __________

D. RIDER AND GRADUATED BEAM TEST – This test detects improper graduations or rider.

1. Determine the rest point for the balance with no weight on the pans.

2. Place a 500 mg weight on the left pan and move the dial or rider to the 500 mg point, and determine the new rest point. If it is different from (1), add a 10 mg weight to the lighter side. This should overcome the difference in rest points.

3. Repeat (2) with a 1 g weight on the left pan and the dial or rider on the 1 g point. If the new rest point is different than (1), add a 10 mg weight to the lighter side. This should overcome the difference in rest points.

4. Does your balance pass this test? __________

II. Weighing and Measuring

You should spend some time practicing your techniques with each apparatus. Use the following prescription and compounding procedures as a guide to your practice. When you feel comfortable with each technique, have the instructor or TA observe your technique and then date and sign your skills check list.

 ``` Jacquelyn Hyde, MD Rx #51201 123 Upendown Rd. Nowhere, NC 27000 Phone: 555-1234 DEA# AH0079411 Name: Dan D. Lyon Date:   Today Address: 123 Jackla Lane Phone:555-5678 Rx Diphenhydramine HCl 250 mg Glycerin 5 ml Simple Syrup 30 ml Vanillin solution 67 mg% 0.2 ml Aqua Dist q.s. 100 ml M.Ft. Sol Sig: i tsp q6h Refills: 0 Product selection permitted Dispense as written ```

Compounding Procedures

1. Using the prescription balance, weight the diphenhydramine HCl. Transfer the powder into a 150 ml beaker.

2. Using an adapt-a-cap syringe, measure and transfer the glycerin into the beaker.

3. Using an adapt-a-cap syringe, measure and transfer the vanillin solution into the beaker.

4. Using a graduated cylinder, measure and transfer the simple syrup. Mix well.

5. Add the mixture to a calibrated prescription bottle.

6. Rinsing the beaker several times, bring the solution to final volume with distilled water.

7. Label and dispense the preparation to your TA.

View a video

Skills Check List (to be signed and dated by Instructor or TA)

Prescription Balance___________________

Pipets______________________________