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CALIBRATION AND INITIAL SETTING OF CAPILLARY TYPE LEVEL TRANSMITTER:
If we are using capillary type level transmitter like Rosemount 3051CD series,
We can follow simple initial setting as follows:
Install the level transmitter to the flanges provided i.e. LP side of the transmitter to top flange and HP side of the transmitter to the lower flange
Keep the transmitter open to the atmosphere i.e. LP and HP side vented to atmosphere
Power up the transmitter, the reading which is shown in the transmitter which will be generally a negative reading like for example -300 mm WC is the LRV value and note down this value
Now the URV value is simply LP tap to HP tap (or centre to centre of the two capillary flanges ) measurement multiplied by specific gravity of the liquid inside vessel added to the LRV value For example if HP to LP distance is 600 mm and specific gravity of liquid inside vessel is 0.89 then URV is calculated as follows URV=LRV+(600*0.87)= -300+(522)=222mm WC
After setting LRV and URV close the HP vent (drain) point and LP vent point.
Line up both HP and LP side, the reading showing in the LT will be the process level in percentage.
Take the necessary permit to work, take override if necessary
Isolate the process i.e. close both LP and HP side and vent the both to atmosphere, Now the reading in LT should show 0% if not trim the transmitter
Make an arrangement like that of a manometer, or a PVC transparent tube through which we can see water, and connect this to the drain(vent) point of the transmitter, measure the tap to tap point of LP and HP side and mark the corresponding 0%,25%,50%,75% and 100% in the PVC tube
Now fill the water up to 25% and note the reading in LT it should show 25% and 8 mA. Similarly fill for 50% and reading should be 12mA,75% for 16 mA and 100% water in PVC tube for 20 mA
Do span trim if necessary after filling 100% water, but if error is below 5%,it is advisable to not disturb the transmitter
After calibration box up the transmitter, close the vent plugs and line up both LP and HP side
N.B: For zero trimming;change the LRV to 0 mmh2o and URV to USL(upper sensor limit) or a value below USL;bring the LP side remote seal to the HP side capillary remote seal,keep it parallel horizontally and see the transmitter reading,it should show 4 mA or 0% ,if not zero the transmitter using HART. Upper sensor limit is nothing but the maximum upper range value written in transmitter tag plate which a transmitter can sense. USL is a maximum positive value of range after that the transmitter cannot sense.
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 from bottom tapping point .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