This post is a continuation of the the post below:
https://kishorekaruppaswamy.wordpress.com/2017/01/02/instrumenation-working-principle/
https://kishorekaruppaswamy.wordpress.com/2017/01/02/instrumenation-working-principle/
TIPS AND TRICKS IN FIELD INSTRUMENTATION
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
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)
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
6.calibration checking of capillary type LT if you dont have any instruments for checking
Suppose that a capillary type LT is mounted on a tank having range of -1200mmh2o as LRV and URV -60 mmh2o and you need to check whether the transmitter is Ok
We know that the transmitter is mounted with HP side to high pressure side and LP tapping to low pressure side,Firstly isolate the process line,vent and drain the process inorder to release any trapped pressure inside the flange.the transmitter will show 0% reading ie(-1200mmh2o),
Now measure the tap to tap length and mark the corresponding 25%,50%,75% and 100% level .Remove the LP flange of the transmitter(with HP flange of capillary intact) and keep it near(parallel) to HP tapping, the transmitter will show 100%.Now lift the LP capillary flange to 25% above from the HP tapping (where we marked before as 25%),now the transmitter will show 75% (not 25%)
Next keep the transmitter LP capillary flange at 50% marking the transmitter will show 50% reading.Similarly when we place at 75% the transmitter will be showing 25%
Finally if we place at 100% marking the transmitter should show 0% that is -60mmh2o
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