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# Pressure transducers are commonly used to measure pressure

## Problem 139P Chapter 3

Fluid Mechanics | 2nd Edition

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Fluid Mechanics | 2nd Edition

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Problem 139P

Pressure transducers are commonly used to measure pressure by generating analog signals usually in the range of 4 mA to 20 mA or 0 V-DC to 10 V-DC in response to applied pressure. The system whose schematic is shown in Fig. P3-147 can be used to calibrate pressure transducers. A rigid container is filled with pressurized air, and pressure is measured by the manometer attached. A valve is used to regulate the pressure in the container. Both the pressure and the electric signal are measured simultaneously for various settings, and the results are tabulated. For the given set of measurements, obtain the calibration curve in the form of P = al

+ b, where a and >> b are constants, and calculate the pressure that corresponds to a signal of 10 mA.

 Δh, mm 28 181.5 297.8 413.1 765.9 I, mA 4.21 5.78 6.97 8.15 11.76 Δh, mm 1027 1149 1362 1458 1536 I, mA 14.43 15.68 17.86 18.84 19.64
Step-by-Step Solution:

Step 1:

The pressure transducer converts the pressure into electrical signals. The variations may be calculated as current or voltage. The manometer filled with mercury is connected to the pressurized container with air and the container is connected to a pressure transducer. The values of currents are noted down with respect to pressure.

Step 2:

To calculate the pressure value corresponds to the mercury column.

The value of current corresponds to mercury column is given by the following table

 S.No 1 2 3 4 5 6 7 8 9 10 Δh, mm 28 181.5 297.8 413.1 765.9 1027 1149 1362 1458 1536 I, mA 4.21 5.78 6.97 8.15 11.76 14.43 15.68 17.86 18.84 19.64

The specific gravity of the mercury SG = 13.56

The pressure can be directly measured using P =

Where is the density of mercury

= SG x density of water

= 13.56 x 1000 kg/m3

= 13560 kg/m3

g - acceleration due to gravity, g = 9.81 m/s2

- height of mercury column in mm

Step 3 of 3

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