BEngTech and NZDE Page 14 MG5026 and DE5417-Final Examination Question 5 15 (a)

Question

BEngTech and NZDE Page 14 MG5026 and DE5417-Final Examination Question 5 15 (a) A fow measuring system uses an orifice plate flow element which yields a differenpal noun tuom pressure of 0-8 kPa for a flow of 0-20 litres/min. The transmitter is ranged so that the 0-8 kPa input the orfice plate tapping points gives a 4-20 mA output, and its output signal is electronic square root extractor with input and output ranges of 4-20 mA fed to an in Calculate and state the differential pressure (hPa), the transmilter output (mA), and the square root extractor output (mA) for a flow of 12 litres/min. (4 Marks) (i) Calculate and state the flow rste (itres/min), the differential pressure (kPa), and the 31 May 2017

Explanation / Answer

Q = K*(?P)^0.5

Q1/Q2= (?P1/?P2)^ 0.5

(20/12 ) = (8/?P2)^0.5 --equation 1

?P2 = 2.87 Kpa

Transmitter output

For 8 unit change in pressure the change in output is 16 units

So for 2.87 units change in pressure we get a change of 16*2.87/8 = 5.76

Therfore output = 5.76 +4 = 9.76 mA

Square root extractor output equation

Output= 4+ 4* sqrt(linear output -4)

= 4+ 4* sqrt(9.76-4)

= 13.6 mA

2. Square output = 4+4* sqrt(15.6-4) = 17.62 mA

For analysis we use 15.6- 4 = 11.6 mA

For x unit change in pressure the change in linear output is 11.6mA

So ?P =11.6/2 = 5.8 kpa

From equation 1

We get 20/Q2 = 1.17

Q2 = 17.02 L/min

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