ODOROMETER
Instruction 0023-9125 Page 5
4. THRESHOLD ODOR DETERMINATION
1. Have the observers sniff the unodorized air stream.
2. Adjust the gas flow rate to the estimated threshold value and let the observers sniff again to see if they can
smell any foreign odor.
3. Adjust the gas concentration upward or downward, as indicated by the above test, and have the observers sniff
again.
4. Repeat this procedure until the threshold value is obtained for each observer. It is best to take several
readings on both sides of the threshold and to run an occasional blank using air with no added gas.
5. Calculate the average threshold value of each observer. Then calculate the total average value, using these
individual averages. This procedure is advisable because considerable variation is found among individuals in
detecting threshold value. Thus, if the observers take different numbers of readings, a “straight” average of all
these values would give more weight to the observer with the larger number of readings.
6. It is essential for this type of determination that the odor level rating be based on the first sniff only, because
fatigue is very rapid in this range.
5. ODOR CHARACTERISTICS AT STANDARD CONCENTRATION
1. Have the observers sniff the unodorized air stream.
2. Adjust the gas rate to the desired concentration.
3. Have the observers sniff the effluent air stream, rating the odor intensity as:
(1) absent
(2) barely detectable
(3) readily detectable
(4) strong
(5) very strong (obnoxious).
4. If desired, a description of the character of the odor, such as onion, garbage,
refinery, etc., can also be obtained.
5. Repeat the test after a short time to check the first determination.
6. RANGE OF MEASUREMENT
The instrument has two ranges, obtained by using a lightweight glass and a
heavier metal float, both in the same metering tube. Take readings on the glass
float when both are within the range of measurement; take readings on the metal
float when the glass float reaches the top of the tube. Useful ranges are
approximately 0.04 to 0.4% for the glass float, and 0.2 to 1.1% for the metal float.
When taking readings, observe the FLOWMETER graduation corresponding to the
bottom of the ball float. Find the gas concentration corresponding to this reading by
looking at the calibration chart on the inside of the front cover. This chart has been
drawn to give the percent gas concentration over the useful ranges for natural gas
with a specific gravity of 0.620 relative to air. Concentrations of gases of other
densities may be read using the correction factors shown on the graph below the
calibration chart.
(2 pages)
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