Dwyer 641B User Manual

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Span Setting
The 641B has been calibrated for standard sea level condi-
tions. As a mass flow device it will always read the air veloc-
ity for standard conditions. Density changes due to baro-
metric or absolute pressure are not corrected automatically.
The span setting allows correction for altitude or other stat-
ic pressure conditions that affect the density of the process
air. This parameter allows for a ±50% adjustment in the
span value.

To make the span adjustment you will need to know either
the absolute static pressure or the corrected velocity of the
process air. Set the air velocity to a known value, ideally
about 3/4 of the full-scale range value. Press SELECT until
the SPAN LED indicator is illuminated then press ENTER.
The SPAN LED will begin to blink. Adjust the control for the
desired velocity then press and hold the ENTER button until
all of the LED’s flash, indicating the new value was saved. If
you know the absolute static pressure you can compute the
corrected velocity using the following equation:

Where:
P0 is the standard pressure of 29.9 in. Hg. or 760 mm Hg.
PA is the absolute pressure reading
Vrdg is the indicated velocity
Vcor is the corrected velocity

4 mA Setting
To make this setting you will need a milliammeter connect-
ed in the current loop. Press SELECT until the 4 mA LED
indicator is illuminated then press ENTER. The milliammeter
will now read approximately 4.0 mA. Adjust the control for a
4.0 mA reading on the milliammeter. Press and hold ENTER
to save the new setting. Pressing SELECT before pressing
ENTER will restore the previous calibration value.

20 mA Setting
With the milliammeter connected in the current loop, press
SELECT until the 20-mA LED indicator is illuminated. Press
ENTER to begin adjustment of the 20-mA set point. The 20
mA LED will now be blinking. Adjust the control until the mil-
liammeter reads 20.0 mA. Press and hold ENTER to save
the new setting. Pressing SELECT before pressing ENTER
will restore the previous calibration value.

Restoring Factory Default Settings
The 4 mA, 20 mA, and Range settings override factory
default values. To restore these to the factory default set-
tings, start with the unit in the RUN mode. Press and hold
the ENTER button. The RUN LED indicator will begin to
blink. After about 2.5 seconds all LED indicators will flash
indicating the factory settings have been restored. Range
and Filter settings are not affected by this operation. If you
are unsure whether any have been altered, press the
SELECT button six times to sequence through all settings.
When you return to the RUN mode, the RUN LED indicator
will blink several times if either the 4 mA, 20 mA, or span
settings have been changed. The RUN LED will otherwise
remain on.

MAINTENANCE
In general the 641B should require very little maintenance.
In some installations dust may accumulate on the sensor
over time. This can be removed by carefully brushing the
probe with a small camel hairbrush. A jet of air may also dis-
lodge the accumulated buildup. Technical grade denatured
or isopropyl alcohol may be used where the dust accumu-
lation does not respond to brushing. Always disconnect the
power when performing a cleaning operation.

Aside from the adjustments described above, the 641B
cannot be field calibrated. Because of specialized comput-
er instrumentation required, these units must be returned to
Dwyer Instruments for factory calibration.

DWYER INSTRUMENTS, INC.

Phone: 219/879-8000

www.dwyer-inst.com

P.O. BOX 373 • MICHIGAN CITY, INDIANA 46361, U.S.A.

Fax: 219/872-9057

e-mail: [email protected]

Printed in U.S.A. 2/05

FR# 02-443205-20

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rdg

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641B bulletin 2/8/05 11:04 AM Page 4