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Stability – The flow meter must stabilize for at least 3 minutes at zero flow, especially
following a high flow or overflow condition. This will allow all parts of the sensor to come to
thermal equilibrium resulting in the best possible zero value.
3.4.2. Adjusting Zero
The pre-conditions required for a zero check must also be followed when making a zero
adjustment. The zero adjustment is a digitally controlled “reset” type operation. When
commanded, the meter initiates an internal routine that performs the following sequence:
measure the current flow reading, store it in nonvolatile memory as a zero offset, and remove
this value from all subsequent readings.
This offset value can be accessed via the “S40” software query. The reported value is relative
to an internal, un-spanned sensor voltage. As an interpretation guideline, an offset that
exceeds 0.15 volts typically indicates that a faulty zero value is present.
There are three different methods to activate the zero reset function--manually, digitally, and
electrically.
Manually – With the electronics enclosure cover plate removed, a pushbutton switch on the
upper board is pressed.
Digitally – A “ZRO” (“*[address]ZRO” for RS485) command is received properly by the flow
meter’s main processor.
Electrically – An external contact closure generates continuity between pins 8 and 9 of the
terminal strip.
3.5. High Pressure Operation
When operating at high pressure, the meter’s performance can be affected in two distinct and
separate ways—a zero shift and a span (calibration) shift.
3.5.1. Zero Shift
The zero offset can occur as the result of natural convection flow through the sensor tube if
the instrument is not mounted in a level orientation with flow horizontal. This natural
convection effect causes a zero shift proportional to the system pressure. The overall effect is
more pronounced for gases with higher density. Normally the shift is within the allowable zero
offset range and can be removed by activating the zero reset at the operating pressure.
3.5.2. Span Shift
NOTE
If the instrument is inadvertently or improperly zeroed, for
example while flow is passing through the instrument, the flow
reading is subtracted from all future flow readings. This will
produce large flow indication errors.
CAUTION
Accessing the manual zero pushbutton requires removing the
enclosure front plate. Care must be taken when reinstalling
this plate to ensure that the sealing gasket is properly
positioned and the fasteners are secure to maintain an IP65
compliant seal.