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MODEL100 REV D.wpd
SECTION 3
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OPERATION
3.1 START UP
Typically, the instruments come from the factory set up for the flow range of interest to the customer.
Following installation all that is required is to switch on power and initiate flow in the measurable flow
rate range. Liquid meters that are not calibrated directly on the liquid to be measured are so indicated
in this manual (SECTION 6) and require an in-line field calibration.
When power is first turned on, the output reading or signal will indicate full scale. After ten to forty-five
seconds (depending on flow range adjustment) the reading will stabilize. The instrument time constant
is generally between 4 to 12 seconds. Higher average flow rates will result in an observed faster
response time for a given unit.
3.2 GENERAL INFORMATION
The Rheotherm instrument is compensated for a wide range of both ambient and flowing media
temperatures. However, abrupt changes in the temperature of the flowing material can cause the
instrument output to deviate from the true representation of flow rate. A proper reading is obtained only
when the transducer is in thermal equilibrium with the material. Typically, a 20°F abrupt change in
temperature may require 40 seconds to stabilize. Temperature ramps should be kept below 1°C/minute.
Rheotherm instruments are calibrated for a particular fluid, either at the factory or in the field. If the
fluid changes properties, the calibration changes. Therefore, once calibrated, do not allow fluid
properties such as density and viscosity to change (other than the intrinsic changes which occur with
temperature variation). If the fluid is changed, a recalibration may be attempted following the procedure
in SECTION 4.2. If this procedure does not provide a good calibration for the range of interest on the
new fluid, contact the factory.
3.3 OPTIONS (Refer to those Sections that apply to the instrument you have purchased).
1. Analog Outputs — The unit will have a non-linear 4-20 mA, 0-5Vdc or 0-10Vdc output. See
SECTION 2.3.3 for a discussion of the 4-20 mA output.
2. Flow rate display — The optional analog panel meter has a log scale so it is direct reading,
showing % of full scale flow rate.
3. Flow switch relay — Flow switches may initially indicate a high flow rate even for zero flow
when power is first applied to the unit. Correct indication of flow will result after an initial
period which can extend to forty seconds depending on where the trip value is set. The relay
in standard flow switches is picked up (energized) when the flow rate is above a set point. A
loss of flow is therefore indicated in the event of loss of power to the sensor when connected
to N.C. contacts. If the switch option has been ordered for high flow indication, the relay will
be energized with flow below the trip value. See CUSTOM INFORMATION, SECTION 6
for details on operation of the relay.