__TIPS AND TRICKS IN FIELD INSTRUMENTATION__

**For**the resistance across the white and common terminal, then the temperature can be calculated simply by__RTD Pt 100__measurement,measure

Temp=(resistance measured across terminal minus 100)/0.385

for example

if the resistance across white and red terminal is 126 ohm, then

the temperature measured is 26/0.385=67.53 degree centigrade

This follow the same eqn, R=Rₒ(1+αT) remember that this is only applicable for PT 100 not other types

__For calibration of -100 mmH2O to -10 mmH20 range capillary type using pressure pump not vacuum pump__

The values for 25%,50%,75%,100% are as follows:

0%————- -100 mm h2o

25%———– -77.5 mm h2o

50%——– -55 mm h2o

75%——— -32.5 mm h2o

100%—— -10 mm h2o

First find the Span=URV-LRV=-10+100=90

Then divide this by 4 as we are calibrating for 4 values namely 25%,50%,75%,100%

i.e. 90/4=22.5

then the 4 points can be calculated as follows

0%(4ma)————- 0 mm h2o i. e LP and HP open to atmosphere

25%(8ma)———– -0+22.5=22.5 mm h2o (apply 22.5mmh2o to HP side not LP here LP is open to atmosphere.)

50%(12ma)——– 22.5+22.5=44 mm h2o (apply 44mmh2o to HP side not LP)

75%(16ma)——— 44+22.5=67.5 mm h2o (apply 67.5 mmh2o to HP side not LP)

100%(20 ma)—— 67.5+22.5=90 mm h2o (apply 90 mmh2o to HP side not LP)

:__Calculation of flow m3/hr from differential pressure values mm h20 if both ranges are known__

We know that the flow equation is related as follows

Q=k√∆p

Here Q is the rate of flow: k is the Bernoulli’s constant; and ∆p is the differential pressure

Consider for instance the D.P. transmitter is of range 0 to 120 mm H2O and the DCS range of

0 to 1500 m3/hr

Then the next step is to find the Bernoulli’s constant

i.e. Q=k√∆p

1500=k√120 (here we consider span URV values to find Bernoulli’s constant)

k=1500/√120

k=136.936

Once we get Bernoulli’s constant we can calculate any flow rate if we know the D.P.

For e.g.

If differential pressure is 90 mmH2O

Q=k√∆p becomes

Q=136.936√90

= 136.936*9.486 = 1298.9 m3/hr

Thus we can calculate any flow rate if we know the transmitter and DCS range.