Micro Motion RFT9709 Instrukcja Obsługi Pobierz pdf (Strona 152)

138

Using ProLink

Software with Micro Motion

Transmitters

Calibrate Menu

continued

Density calibration

14.4 Density calibration

Density calibration adjusts the factors that are used by the transmitter to

calculate density. Fluid density, calculated by the transmitter, is inversely

proportional to the square of the sensor tube frequency. The ProLink

program supports three types of density calibration: one-point, two-point

(or dual-point), and third-point.

During two-point density calibration, the user commands the transmitter

to measure the tube period when the flow tubes contain a fluid with a

reference low density (usually air) and when the flow tubes contain a

fluid with a reference high density (usually water). The transmitter then

calculates the density of the process fluid from the following equation:

Where:

D = Calculated density at measured temperature t

= Reference density of low-density calibration fluid at calibration

temperature t

= Reference density of high-density calibration fluid at

calibration temperature t

= Measured tube period squared at measured temperature t,

corrected to 0°C

= Tube period squared at D1, corrected to 0°C

= Tube period squared at D2, corrected to 0°C

Two-point density calibration is preferably performed under zero flow

conditions.

One-point density calibration is an option when two-point calibration is

not possible. However, because one-point calibration is performed with

only one reference density, two-point calibration is recommended for

increased accuracy.

Third-point density calibration is available only for the RFT9739. As the

flow rate approaches the maximum flow rate of the sensor, the angular

momentum of the fluid can alter the tube period, which causes an

increase in the density value measured by the sensor. An increase in the

flow rate causes a proportional increase in the angular momentum of the

fluid. A third-point density calibration accounts for the effect of flow on

the tube period at high flow rates.

Third-point density calibration is desirable if the process exceeds or

often approaches the flow rate listed in

Table 14-1

.If the process

remains below this rate, calibrating a third density point is unnecessary,

since angular momentum of the fluid will have a minimal effect on the

flow tube frequency. Use the data listed in

Table 14-1

to determine

whether third-point density calibration is desirable.

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