Zeroing at barometric pressure, at the local airport

Many Fluke Calibration pressure measuring instruments require periodic zeroing of the reference pressure transducer to correct for drift that occurs naturally over time. For the transducers that are intrinsically absolute, the zeroing process is often performed by comparison to local barometric pressure. This is a relatively simple task for laboratories that have access to a precision in-house barometer. With the exception of a few low pressure ranges, zero by comparison to an in-house barometer is the recommended method. 

 

If you don’t have a barometer, it might seem reasonable to use the barometric pressure value provided by a weather reporting service or from a regional airport. In many applications this is not recommended at all.  But for some applications--such as the down-hole tool industry where high pressure calibrators are used--it could be a stop-gap solution until you can get a barometer.  Before you make a choice like this, however, you should understand a few things about barometric pressure and how it is reported.

 

First and foremost, barometric pressure is always changing. It changes when the wind blows, when the air changes temperature, when the laboratory air conditioner turns on and off, or when the doors of the building open and close. On stormy days the barometric pressure is said to be “low,” and it changes to “high” on clear sunny days. Atmospheric and environmental conditions are always changing and, therefore, it becomes important to know the true barometric pressure at the moment of zeroing.

 

Second, airports do not commonly provide the true barometric pressure; they provide a barometric pressure corrected to sea level. It is a way of indicating to pilots the effects of current airport weather conditions (for example, a storm in the area with lower density air, or a sunny day with higher density air). This value is known as altimeter setting pressure. The corrected value is used with the true barometric value to set a pilot’s altimeter, correcting for local weather conditions.

 

An airport might also provide to the general public a second barometric value known as the station pressure. This can be considered as the raw or true barometric measurement at the airport. Be careful to understand which value is being provided.

 

Google the term, “Barometric pressure at DIA” and compare it to the information provided at https://www.crh.noaa.gov/bou/include/webpres.php?product=webpres.txt

 

The following values were from Denver International Airport (DIA):

29.68 InHg	Altimeter Setting Pressure
24.24 InHg	Station Pressure
(29.92 InHg Standard Barometric Pressure @ sea level)

 

The airport’s altimeter setting pressure is 29.68 InHg and is near, but slightly different from, barometric pressure that would be seen at sea level under normal conditions. This indicates to the pilots that a low pressure weather system is over DIA. Notice how the station pressure at DIA is 24.24 InHg. This is the true local barometric pressure at the airport and is a result of the high elevation of DIA and the current weather conditions. The difference between the corrected altimeter setting and station pressure is 5.44 InHg (2.67 psia). Using the wrong barometric value to zero a reference pressure transducer could have a significant effect on the calibration! This is an extreme example and, in a way, makes it easier to discern between altimeter setting and station pressure. The problem can become more difficult to distinguish for airports at lower elevations, such as Phoenix, Chicago, or even Seattle – yes, Seattle-Tacoma international airport sits at 132 m above sea level, a potential for 0.46 InHg (0.229 psia) difference.

 

In short, you must know the true local barometric pressure when you perform an absolute mode zeroing of a reference pressure transducer. Ideally, you’d use an in-house barometer for this task. If you don’t have one, then you could use the Station Pressure from a local airport as an approximate value for local barometric pressure. But you should treat this option as a stop gap until you can get a barometer. It adds uncertainty and could potentially result in an out-of-tolerance condition for your instrument.

The potential risk diminishes for high pressure hydraulic devices, such as the Fluke Calibration PPCH Automated Pressure Controller/Calibrator and 7615 Hydraulic Pressure Controller, because the uncertainty for the reference pressure transducer is typically much larger for high pressure devices than would be the case with lower pressure gas operated instruments. Therefore, the potential relative error from using the Station Pressure is reduced.

 

For more information on this topic or for pressure metrology related topics please feel free to contact Fluke Calibration’s pressure experts at +1.877.355.3225 or info@flukecal.com.