Model 2000 Flow Computer Instruction Manual
2.0 GENERAL DESCRIPTION
Model 2000 issue 21 Page No
28
2.4.3.1. DIFFERENTIAL PRESSURE MEASUREMENT
2.4.3.1.1. DIFFERENTIAL PRESSURE UNITS
The Units of differential pressure used in the Model 2000 can be set to be any of the following bar, mbar, psi inches of water
gauge or mm of water gauge.
In the case of analogue transmitters it is assumed that the 4-20mA current value is calibrated in the required units. However,
where HART transmitters are used with digital communication, the units value is read from the transmitter and compared
against the value selected, if the value read does not agree with the value selected then an accountable Unit alarm is raised.
2.4.3.1.2. HART TRANSMITTER DIFFERENTIAL PRESSURE INPUTS
The Model 2000 is designed to be used with either one single (high range) or two (high and low range) differential pressure
transmitters using the HART communications protocol. Typical transmitters are the Rosemount Model 3051C Differential
Pressure. All communication with the transmitters is done digitally and the analogue signals from the transmitters is set to the
minimum value and therefore cannot be used for monitoring purposes. Where two transmitter operation is selected the
differential pressure from both the high range and the low range will be displayed, the in use differential pressure is
determined by the following selection method.
If the low range d.p. value is below 98% of d.p. low max then the d.p. low range value is used.
If the low range d.p. value is above 98% of the d.p. low max. then the d.p. high range value is used. Once the
d.p. high range has been selected the d.p value must fall below 95% of the d.p. low max before the
unit switches to using the d.p. low range value.
WARNING:- The transmitters are connected in parallel as a multidrop system and each transmitter must have a different
address.
It is essential that the transmitters have their addresses programmed and the burst mode operation turned off before they are
connected to the Model 2000 Flow Computer.
For Rosemount Transmitters the Rosemount Smart Family Interface Model 268 is used to programme the transmitter. Refer
to the Rosemount operating manuals for the programming procedures.
To interface with the hazardous area where intrinsically safe transmitters are used a digital safety barrier must be used a
typical barrier is a Measurement Technology Ltd barrier Model MTL3046B or MTL 5042.
2.4.3.1.3. ANALOGUE TRANSMITTER DIFFERENTIAL PRESSURE INPUTS
As an alternative to the HART transmitter inputs described in paragraph 2.5.3.1.2 the Model 2000 can also accept current
input signals for differential pressure (Low range and High range) which are fed into an single sided amplifier and having a
response time of approximately 470ms.
Input current: 4mA. to 20mA.
Input resistance: 100Ω
The Model 2000 can be used with either one single (high range) analogue (4-20mA) differential pressure transmitter or two
(one high range and one low range) analogue (4-20mA) differential pressure transmitters. Where two transmitter operation
is selected the differential pressure from both the high range and the low range will be displayed, the in use differential
pressure is determined by the following selection method.
If the low range d.p. value is below 98% of d.p. low max then the d.p. low range value is used.
If the low range d.p. value is above 98% of the d.p. low max. then the d.p. high range value is used. Once the
d.p. high range has been selected the d.p value must fall below 95% of the d.p. low max before the
unit switches to using the d.p. low range value.
To interface with the hazardous area where intrinsically safe transmitters are used a safety barrier must be used.
2.4.3.2. MASS FLOW MEASURMENT LINEARISATION
A facility is provided which gives the option of linearise the calculated mass flow rate. The options available are:-
off No correction.
5 pnt Linearity correction using 5 point interpolation.
The mass flow calculated is characterised by entering five co-ordinates for flow and corresponding error values which are
stored in the Model 2000 memory. A linearity correction is applied to the mass flow by interpolating between the co-ordinate
points. Correction is applied to the mass flow rate and mass total flows, subsequently as the volume and energy
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