Xylem 6-Series Multiparameter - Manual (Page 185)

650 MDS Section 3
YSI Environmental Monitoring Systems Operations Manual 3-55
3.9 USING THE 650 BAROMETER
3.9.1 PRINCIPLES OF OPERATION
The optional barometer operates on the strain-gauge principle and is located on the internal PCB of the 650.
To make certain that the barometer is sensing true atmospheric pressure under conditions of temperature and
pressure changes, the interior of the case is vented to the atmosphere with patches of material which are
permeable to air, but impermeable to water. One of these patches is located in the rear section of the case and
the other is attached to the battery lid. The patch on the case rear is not accessible to the user, but the one on
the battery lid can be damaged by abuse. Be certain to identify the location of the patch on the interior of the
battery lid and take care not to puncture it with sharp objects or to peel it away from the plastic. If you suspect
that the venting patch on your battery lid has been damaged, make certain that you do not expose the 650 to
water (either spraying or submersion). Then contact YSI Technical Support as soon as possible for advice.
The 650 barometer reads true barometric pressure and therefore is unlikely to agree with values from your
local weather service which are usually corrected to sea level before being distributed. Note, however, that
the primary purpose of the barometer is for use in calibrating your sonde dissolved oxygen sensor and the
“true” value as shown in the 650 Status Bar is the required parameter for this procedure.
3.9.2 CALIBRATING THE BAROMETER
The 650 barometer is factory calibrated to provide accurate readings in the 500-800 mm Hg range which will
be useful for dissolved oxygen calibration at most locations. This factory calibration should provide readings
that are useful for dissolved oxygen calibration for many months with no user attention. However, if the user
wishes to either increase the accuracy of the sensor in the local barometric pressure range or to compensate for
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 3.3.5 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.
3.9.3 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.