# Author Archives: kishore karuppaswamy

# Tips and tricks in field instrumentation

__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 or (resistance measured across terminal minus 100)multiplied by 2.6

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 or 26 multiplied by 2.6

26*2.6=67.53

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.

4.For K type thermocouple (Chromel alumel) if the mV measured across yellow(positive) and red (negative) is x,then the temperature can be calculated as follows

Temperature=x/0.0397(millivolt measured divided by 0.0397) or x *25.2 (millivolt multiplied by 25.2)

for example if we measure the mV value across yellow and red terminal using a multimeter and found to be 0.397 then temperature can be calulated by

temperature=(0.397/0.0397)=10 degree centigrade or (0.397*25.2)=10 degree centigrade

# Data Communication protocols

## Pressure Unit Conversion

### Image

# Calibration,theory and initialization of GWR level transmitter Rosemount 5300 and Ktek 5100

and the blanking distance or blocking distance is used to ignore an extended nozzle that would otherwise cause a reflected signal at the top of the probe and result in a high level reading even when no product is in the vessel

and the Level offset is used to correct the level output of the transmitter to match the actual level in your tank or vessel.There are two cases when Level Offset can be used. One is to accommodate for a unmeasurable length at the bottom of the probe (a negative offset) The other is to accommodate for the length of the probe being shorter than the actual depth of the tank (a positive offset).

seems confusing but I will make it clear, if physical dip tape or gauge measures 1200 mmwc and transmitter is showing 1000 then put level offset as +200 mmwc

and if physical dip tape or gauge measures 1200 mmwc and actual level is 1400 mmwc then put level offset as -200mmwc then the transmitter display will show 1400-200=1200 mmwc which matches with physical reading……………..means this is a trick provided by the company to match physical reading with transmitter reading