YSI 650 MDS - Manual (Page 58)

YSI Incorporated 58 650 MDS
the small drift associated with any sensor, the 650 provides a single point calibration routine which is accessed
in the 650 System setup menu and is described in Section 3.9 above.
The critical factor in carrying out the single point calibration is the accuracy of the “standard” barometer that
is used to determine the true barometric pressure. Common sources of this “standard” reading are high
quality laboratory barometers (either mercury-based or electronic) and data from your local weather service.
Laboratory barometer readings are usually “true” (uncorrected) values of air pressure and can be used “as is”
for a standard as long as the system is known to be accurate. Weather service readings are usually not “true”,
i.e., they are corrected to sea level, and therefore cannot be used until they are “uncorrected”. An
approximate formula for this “uncorrection” (where the BP readings MUST be in mm Hg) is:
True BP = [Corrected BP] – [ 2.5 * (Local Altitude/100)]
Overall, the key point to remember in calibration of your barometer is to make certain that your “standard”
reading is correct. If the “standard” is not accurate, you are likely to do more harm than good with your user-
calibration.
9.3 CHANGING BAROMETER UNITS
The 650 offers the user a choice of six commonly used units of barometric pressure which can be displayed
in the Status Bar. When received from the factory, the 650 will show values in mm Hg. If the user wants
to change to other units, the selection is found in the 650 System setup menu as shown below. Highlight
the selection and press Enter to show the possible units. Then highlight the unit of choice and press Enter
to activate it. The barometer reading in the Status Bar will automatically change to the newly selected unit.
9.4 LOGGING BAROMETER READINGS
To log barometric pressure readings with the 650, the following conditions (similar to those above for
logging GPS readings) must be met:
•
Barometer readings can ONLY be logged to 650 memory – the system does NOT support logging to
sonde memory.
•
The barometer readings must be logged to 650 memory from the 650 Run display along with sonde
sensor readings. This means that you must have a sonde attached to your 650 in order to log barometer
readings. Logging of barometer readings is not supported without sonde attachment even though these
readings can be viewed in the Status Bar with no sonde attached.
•
If your want to log barometer readings to 650 memory, you must make certain that the Store
Barometer option is active in the Logging setup menu of the 650.