Calibration & barometric pressure
Typically, any barometric pressure measurement that you hear (radio, TV) represents 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. For aviation, pressure is critical.
To determine true, uncorrected barometric pressure:
- Obtain barometric pressure directly from your own mercury barometer. If you are using an aneroid barometer (typically have a tightly coiled spring), make sure it has been adjusted to read true uncorrected barometric pressure.
- 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.
- Use known O2 saturation tables to determine the saturation point
Uncorrecting pressure readings
The local airport provides you with a "corrected" barometric pressure of 29.65 [inches of Hg]. To UNcorrect this measurement:
NOTE: Pressure drops by 26 millimeters (mm, about 1 inch) for every 1000 feet above sea level.
26 ÷ 1000= 0.026. That's why during the process, we multiply the altitude in feet by 0.026.
- 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. Otherwise, the internet is a resource which can help you quickly find altitude.
- 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
- Convert inches of mercury (Hg) to mm of mercury (Hg):
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
Determining DO saturation point
Determining saturation point with an "uncorrected" chart
- We know the temperature of the calibration solution is 20.5 °C.
- 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
- We know the true uncorrected barometric pressure (TUBP) is 727.5 mm Hg.
- Determine the correction factor to adjust maximum O2 saturation to the actual pressure:
- Correction Factor = [TUBP ÷ 760]
- = (727.5 ÷ 760)
- = 0.9572
- 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
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.
- Determine the true uncorrected barometric pressure: 737.6 mm or 29.04 inches (from the previous section)
- Determine the temperature of the calibration solution: 20.5 °C.
- Use O2 saturation table to obtain the maximum O2 solubility (mg/L) at that temperature.
- 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, it's 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 halfway between these two values, or 8.59 mg/L.
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!
Calibration - putting it into perspective
|Low Pressure||Normal||High Pressure|
|Sea level||750 mm (29.58")||760 mm (29.98")||770 mm (30.38"|
|1000 ft altitude||724 mm (28.56")||734 mm (28.95")||744 mm (29.35")|
- 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 under a high?
- What if samples go in under a high pressure, but come out under a low?
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… …and let's say this guy —we'll call him George— weighs 171 lbs but feels like he needs to lose a few pounds
"George" steps up on the scale and weighs in at 174.5 lb. Little does George know that someone —let's call him Rick— is aware of George's weight concerns and has adjusted the calibration of 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 carrot sticks, Saltines 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. Unfortunately for George, Rick has been up to shenanigans again and this time has re-adjusted the scale calibration such that it now reads 2% low.
Remember: George's weight (which has really never 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 carrot sticks, Saltines 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.
Why saturate your dilution water before calibration?
- It provides a known standard to evaluate calibration:
- If you know the temperature is 20.5°C...
- ...and you know the barometric pressure is 740 mm Hg (29.13 inches)...
- ...and you know you shook the solution vigorously, then the solution MUST measure 8.73 mg/L (laws of physics are in play)
- If the meter registers a substantially different value, you know to initiate corrective action.
- It establishes the point at which supersaturation occurs:
- Again, if you KNOW the temperature is 20.5°C...
- ...and you know the barometric pressure is 740 mm Hg (29.13 inches)...
- Then if your sample DOI (at 20.5°C) measures 9.5 mg/L (or any value greater than 8.73 mg/L), suspect supersaturation.
Pressure considerations - other altitudes
If your lab is located:
- in Denver, CO (elevation 5280 ft)
- Average barometric pressure = 24.7 inches
- Maximum O2 saturation, 20oC = 7.48 mg/L
- ...and you're working DO range for samples would be 7.48 − 1 = 6.48 mg/L
- on Pike's Peak (CO) (elevation 14197 ft)
- Average barometric pressure = 15.47 inches
- Maximum O2 saturation, 20oC = 4.68 mg/L
- ...and you're working DO range for samples would be 4.68 − 1 = 3.68 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!!!
- on Mount Everest (elevation 29028 ft)
- Average pressure = 0.20 inches
- Maximum O2 saturation, 20oC = 0.06 mg/L
- If you're even thinking of setting up BODs here, your brain is already suffering from hypoxia.
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 aneroid barometers need to be calibrated initially
- Set it against true uncorrected local barometric pressure
- Know what represents reasonable barometer readings
- 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 (at sea level) do readings exceed 30.4 inches Hg (773 mm Hg)…except for occasional arctic highs in January.
- Rarely (at sea level) do readings fall below 29.5 inches Hg (749 mm Hg)…except for occasional severe low pressure storm systems.
- Pressure really does 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 just kept dropping"
Copyright 2006. University of Wisconsin Board of Regents.
Unauthorized use prohibited without the expressed written consent of the UW, State Laboratory of Hygiene and the Wisconsin DNR - Laboratory Certification & Registration Program.
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