Pressure Adjustments

Typically, any barometric pressure measurement that you hear (radio, TV) represnts a value that has been corrected to a value relative to sea level. Why is this? For one thing, it's a means of standardizing data. Barometric pressures are typically similar for a very large regional area (unless a storm is approaching), but there are pressure differences based on altitude, which can change dramatically over a very small distance. Additionally, virtually all aviation uses pressure altimeters to determine where the ground is (remember how the terrorists crashed the plane in Die Hard 2?). For aviation, pressure is critical.

To determine true uncorrected barometric pressure.

1. Obtain barometric pressure directly from your own mercury barometer. If you are using an anaeroid barometer (typically have a tightly coiled spring), make sure it has been adjusted to read true uncorrected barometric pressure.
2. or call a local airport or radio station
• Ask if their data is “corrected” (to sea level)
• If it is corrected, you need to UNcorrect it,
• Otherwise you can use it as is.
3. Use known O₂ saturation tables to determine the saturation point

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Uncorrecting Pressure Readings

The local airport provides you with a “corrected” barometric pressure of 29.65 [inches of Hg]. To UNcorrect this measurement:

1. Determine the altitude (in feet) of your facility/lab (you can use the altitude of the city/town/village).

Wastewater treatment plants can usually obtain this information off the plant blueprints

2. Determine the correction factor (CF):
   CF = [760 - (Altitude x 0.026) ÷ 760
   = [760 - (600 x 0.026)] ÷ 760
   = [760-15.6 ] ÷ 760
   = 744.4 ÷ 760 = 0.9795
   Therefore, true uncorrected barometric pressure = 29.65 x 0.9795 = 29.04:

   NOTE: Pressure drops by 26 millimeters (mm, about 1 inch) for every 1000 feet above sea level.
   That's why during the process, we multiply ther altitude in feet by 0.026.

3. Convert inches of mercury to mm of mercury:
   
   • Inches of Hg X 25.4 = mm of Hg.
   • Therefore 29.04 X 25.4 = 737.6
     
   • Set your barometer to read either 29.04 inches or 737.6 mm

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Determining DO Saturation Point

Determining Saturation Point with an "UNCorrected" Chart

1. We know the temperature of the calibration solution is 20.5 °C.
2. We know (from conventional DO Saturation Table) the maximum O2 solubility (mg/L) at 20.5 °C at SEA LEVEL and standard pressure is: 8.97 mg/L
3. We know the true uncorrected barometric pressure (TUBP) is 727.5 mm Hg.
4. Determine the correction factor to adjust maximum O₂ saturation to the actual pressure
• Correction Factor = [TUBP ÷ 760]
• = (727.5 ÷ 760)
• = 0.9572
5. Multiply the sea level saturation point by the correction factor
• [Max O₂ Sat. from table] X Correction Factor
• = 8.97 x 0.9572 = 8.59 mg/L

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Determining Saturation Point with a "Corrected" Chart

Once you have the true uncorrected barometric pressure, either directly from your barometer or corrected from a local source, determine the Oxygen solubility at that pressure and temperature.

1. Determine the true uncorrected barometric pressure : 737.6 mm or 29.04 inches (from the previous section)
2. Determine the temperature of the calibration solution: 20.5 °C.
3. Use O₂ saturation table to obtain the maximum O₂ solubility (mg/L) at that temperature.
4. The Corrected DO Saturation Table gives data based on pressure increments of 5 mm (0.2") and tenths of a degree C. You may have to interpolate, or estimate, the saturation point if the actual pressure falls between two column values. In this example, the actual true uncorrected barometric pressure of 727.5 mm falls between the table columns headed by 725 mm and 730 mm. In fact, its exactly halfway between the two. Therefore, if you look across from the 20.5° C row to these two columns, you find saturation values of 8.56 mg/L (725 mm) and 8.62 mg/L (730 mm). Therefore the saturation point for a pressure of 727.5 mm and 20.5° C is halway between these two values, or 8.59 mg/L.

***This is 8.59 mg/L…using a "Corrected" Saturation Table***

OK...so which is easier?

Hopefully, it's obvious that times have changed, and the newer "Corrected" Saturation Table is much more efficient… and requires less math!

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Calibration - putting it into perspective

• Pressure drops 26 mm Hg (~ 1.0 inches) every 1000 ft
• Maximum DO saturation drops roughly 0.3 mg/L each 1000 ft
• Barring abnormal storm systems, daily pressures fluctuate roughly ± 10 mm (0.4 inches)
• Around 20°C, saturation point drops about 0.1 mg/L for each 0.5 degree rise in temperature

With the advent of DO meters containing "on-board" barometers, many of these issues are beginning to disappear. Nevertheless, it is important for people to understand how important pressure changes are in the calibration process. Remember that historically, operators were provided with a very simplistic chart of DO saturation based on temperature and a correction factor based on altitude was employed. This approach gave no consideration to pressure changes as the saturation table was based on standard pressure at sea level (760 mm)

Remember: you calibrate on day 0 AND day 5.

• What if samples go in under a low pressure, but come out out under a high?
• What if samples go in under a high pressure, but come out under a low?

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Pressure changes- another perspective

This all may be easier to “see” if we put it in different terms.

We'll use the analogy of a scale…

Little does George know that someone--let's call him Rick-- is aware of George's weight concerns and has re-set the scale to read 2% high. George still weighs 171, of course, but due to calibration error, he thinks he's gained a few pounds.

George pledges to start a crash diet. George is not a good dieter, however, and for every meal of celery and diet soda, he sneaks in a big old piece of apple pie a la mode. So, the reality is, that George hasn't lost a pound. But he doesn't know that yet. In fact, when he steps on the scale a week later, it reads 167.5.

(this time Rick re-set the scale to read 2% low)

Remember: George’s weight (which has NOT changed from the original 171) now reads 167.5 because of the "error" in the scale's calibration.

Feeling much better weight-wise, George gives up the celery and diet soda (but not his cherished apple pie--in fact, he now feels he can afford an even bigger slice!). A week later (and unknowingly up 2 lbs from his original weight to 173 lbs.) George checks in with the scale again-- to see a weight of 176.5 lbs!

Once again, that trickster, Rick re-set the scale to read 2% high).

… and George checks in for some much needed therapy.

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Why saturate your dilution water before calibration?

1. It Provides a KNOWN standard to evaluate calibration.
• If you KNOW the temperature is 20.5°C...
• If you KNOW you shook the solution vigorously, then the solution SHOULD measure 8.52 mg/L
• If the meter registers substantially different value, you know to initiate corrective action.
2. It establishes the point at which supersaturation occurs.
• If you KNOW the temperature is 20.5°C...
If sample DO_I (at 20.5°C) is 9.5 mg/L, suspect supersaturation

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Pressure Considerations- Other Altitudes

If your lab was at/in:

• Denver, CO (5280 ft)

  • Average pressure = 24.7 inches
  • Maximum O₂ saturation, 20^oC = 7.48 mg/L

• Pike's Peak (CO) (14197 ft)

  • Average pressure = 15.47 inches
  • Maximum O₂ saturation, 20^oC = 4.68 mg/L

  NOTE: since GGA typically uses up about 4.0 mg/L of oxygen, one could not even obtain an acceptable GGA as the final DO would be less than 1.0 mg/L!!!

• Mount Everest (29028 ft)

  • Average pressure = 0.20 inches
  • Maximum O₂ saturation, 20^oC = 0.06 mg/L

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Calibration - Final Thoughts

• Check your meter’s accuracy with a ‘0’ DO standard

Add an oxygen scavenger (e.g.,~ 2% sodium sulfite) to dilution water.

• Calibrate your barometer
• Most anaeroid barometers need to be calibrated initially
• Set it against true uncorrected local barometric pressure
• Know what reasonable barometer readings are
• Normal is 29.9; range ~29.6 - 30.2 inches Hg (752-767 mm Hg)… at SEA LEVEL!
• If you are in Merrill, for example, at 1300 ft. altitude, this range changes
• Rarely (sea level) do readings exceed 30.4 inches Hg (773 mm Hg)…except for occasional arctic highs in January
• Rarely (sea level) do readings fall below 29.5 inches Hg (749 mm Hg)…except for occasional sever low pressure storm systems
• Does this REALLY affect results?
• November 10, 1998; major Wisconsin low pressure system
• Pressure readings as low as 28.5 inches Hg (724 mm Hg)...that's corrected to sea level! (which would make them about 27.5 inches/700 mm in most areas of Wisconsin.
• Amounts to a change in maximum O2 solubility of 0.4 mg/L
• Local labs reported an inability to obtain a stable DO reading; "the meter kept dropping"

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