In measured observations, odor determinations are much to be preferred to taste determinations. There is no method  for  measuring  tastes  quantitatively. The threshold odor test is the most widely used method  of  determining  odor  levels.  It  consists  of comparing different dilutions of the sample (diluted with  odor-free  water)  to  an  odor-free  standard.  The dilution at which the odor can just be detected is called the threshold point. The odor at the threshold point is expressed quantitatively by the threshold number. This is simply the number of times the odor-bearing sample is diluted with odor-free water. For example, if odor- bearing water requires dilution to ten times its volume with odor-free water to make the odor just perceptible, its threshold number will be 10, A more concentrated odor-bearing water will require dilution to 100 times its  volume  to  make  the  odor  just  perceptible;  its threshold number will be 100. Here are some basic principles of measuring odor values consistently. 1. Some practice with the test is desirable to develop   consistent   threshold   sensitivity.   The consistency   can   be   developed   readily   in   most individuals. An acute sense of smell is not essential. 2. An adequate supply of freshly prepared odor- free water must be available before starting the test. 3.   All glass must be clean and free of odor. Rinse all glassware several times with odor-free water before each test and between dilutions. 4.   Tests should be run in a room free from foreign odors. Odors caused by fresh paint, volatile solvents, tobacco smoke, food, and the like, will decrease the accuracy of the observations. 5. Each dilution should be compared with the odorless  standard  to  check  judgment  and  minimize reliance on odor memory. Equipment The following items of equipment are needed to carry out the threshold odor test: Six  500-ml  Erlenmeyer  flasks  with  ground  glass stoppers Two thermometers (0°C-110°C) One 250-ml graduated cylinder One 100-ml graduated cylinder One 50-ml graduated cylinder One 25-ml graduated cylinder One 10-ml Mohr pipette One large hot plate One odor-free water generator (fig. 7-17) Several large flasks for collecting and heating odor-free water PROCEDURES In carrying out the threshold test, determine first the approximate range of the threshold odor number. Carefully follow these steps. 1.   Add 250-ml, 63-ml, 16-ml, and 4-ml portions of the  odor-bearing  water  to  separate  500ml  glass- stoppered Erlenmeyer flasks. 2. Dilute the last three to 250 ml with odor-free water. 3.   Add 250 ml of odor-free water to another flask that will be the reference for comparison. 4.   Heat the flasks to 140°F (60°C) on a hot plate. 5. Shake the odor-free flask, remove the stopper, and sniff the vapors. 6. Do the same with the flask containing the least amount   of   odor-bearing   water   and   observe   by comparison whether it contains an odor, and, if so, what type of odor. (See table 7-2.) 7.  Repeat  steps  5  and  6  and  use  the  sample containing the next higher concentration of the water sample. 8.  Continue  the  process  until  all  dilutions  have been  observed. 9.   Record which flasks contain an odor and which do not. Experience will enable an operator to estimate the Figure 7-17.—Odor-free water generator. 7-25