Dwyer 641RM User Manual

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Span Setting
The Series 641RM Air Velocity Transmitter has been cali-
brated for standard sea level conditions. As a mass flow
device it will always read the air velocity for standard condi-
tions. Density changes due to barometric or absolute pres-
sure are not corrected automatically. The span setting
allows correction for altitude or other static pressure condi-
tions that affect the density of the process air. This parame-
ter 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. It is not required to use a known air
flow velocity to make this calibration setting. The calibration
settings of the velocity and mA outputs are independent of
each other. 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
It is not required to use a known air flow velocity to make
this calibration setting. The calibration settings of the veloc-
ity and mA outputs are independent of each other. 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 milliamme-
ter 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 Series 641RM Air Velocity Transmitter 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. If too much force is applied during cleaning, the
sensor of the Series 641RM Air Velocity Transmitter may be
damaged. Therefore, a trained technician should perform
the cleaning operation. A jet of air may also dislodge the
accumulated buildup. However, the sensor is delicate and
this operation should be done carefully with clean regulated
air. Using a shop air supply may exert enough force to dam-
age the sensor. Most air supplies of this sort will also con-
tain water or oil that could damage the sensor. Technical
grade denatured or isopropyl alcohol may be used where
the dust accumulation does not respond to brushing. Do
not use water. Always disconnect the power when perform-
ing a cleaning operation.

Aside from field calibrations for span, 4 mA and 20 mA set-
ting as described above, the Series 641RM Air Velocity
Transmitter cannot be fully factory calibrated in the field.
Because of specialized computer instrumentation required,
these units must be returned to Dwyer Instruments for fac-
tory calibration. Contact customer service to receive a
return goods authorization number before shipping.

Printed in U.S.A. 1/05

FR# 443309-00 Rev. 1

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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]

641-RM bulletin 2/1/05 11:53 AM Page 4