Dp type transmitters are popular in industries for the measurement of process parameters like level, flow density, Differential pressure etc.we know every instrument has its own range of measurement and it is an important data for calibration and other purposes.Any device need proper calibration for its correct working .This article describes negative range dpt calibration without using vacuum source.
Dp transmitter
Dp type transmitters are popular in industries for the measurement of process parameters like level, flow density, Differential pressure etc.we know every instrument has its own range of measurement and it is an important data for calibration and other purposes.
Calibration Range
Range of DP transmitter is the differential pressure corresponds to the 0% and 100% of the process input respectively .In most of the cases the range is positive.We can calibrate a DP transmitter by applying the DP range . Lower range and upper range of differential pressure is applied (LRV and URV)to HP side of transmitter leaving LP side open to atmosphere and current output is adjusted to correct value .
Why we open LP port to atmosphere.????
we know differential pressure is the difference in pressure acting at high pressure port and low pressure port of the transmitter .
Or DP = HP-LP.so in order to calibrate a DP transmiter with a given DP range, we should create a differential pressure equal to that range.That is we should create a pressure difference between HP side and LP side This is easily accomplished by referencing LP side to zero and apply pressure in HP port,
.As DP=HP -LP
=HP- 0
=HP
So anything we given to HP port is the DP provided LP is opened to atmosphere .So we only need to connect the pressure source to hp port only.
(Note: Here HP and LP is only to indicate the pressure applied on transmitter HP port and LP port. HP may not be always positive .It may be positive or negative depending on the conditions .)
DP with negative range values
How the range gone to a negative value in some applications.
In some closed tank level measurement using Differential pressure transmitter and LP side of transmitter filled with liquid or condensate.Here the head on LP side is more and obviously DP range will be a negative values as DP=HP-LP.
Ex.Boiler drum level transmitter is an example for transmitter with negative range values
How to do negative range dpt calibration ??
How we will calibrate a DP transmitter with a negative range.???
Suppose we have a DP transmitter which must be calibrated for a range of -6000mmwc to -500mmwc.Here LRV is -6000 mmwc and URV is -500mmwc.What is the calibration procedure.
Your procedure may be as follows
Connect vacuum source to HP side of transmitter
Open the LP port to atmosphere
Apply , -6000 mmwc on HP port adjust output to 4ma or 0 %
Apply -500 mmwc on HPport and adjust output to 20ma or 100%
Using a vacuum pump to create a negative pressure or vacuum is quite logical.So nothing wrong with the procedure.But it is not applicable all the times.what about if the range of the transmitter is -12000 mmwc to -500mmwc.It is not possible to apply this LRV as the maximum possible vaccum is -760mmHg or -10332mmwc(in gauge scale).Below that there is no value and it is meaningless..and practically it is not possible to get -760mmHg.And it is very hard to achieve even -755mmHg.
Do you really need a Vacuum source to create negative DP????
Let’s see how we can create a differential pressure of -10000mmwc
Method 1
The conventional method creating vacuum is connecting a vaccum source like a vaccum pump . LP port is opened to atmosphere .And apply a vaccum on HP side by a vaccum pump or any other vaccum source .
.Then
DP =HP – LP
= -10000 – 0
Here vacuum applied on HP side pulls the sensor diaphragm and creates a proportional output .see figure below .
Method 2
Does vacuum pump necessary to create a negative differential pressure Or differential pressure in vacuum range.????
No….DP is only a difference in pressure.It may be positive or negative depending on the pressure applied on HP port and LP port.
We know.
DP = HP -LP .
from this equation we can see that for DP to become negative it is not necessary to be the HP side pressure negative.A higher positive value of pressure on LP port will make the DP negative .see below examples.note eq2 and eqn3
DP = HP – LP
-1. = -1 – ( 0 ) ……..eq1
-1. = + 1 – (+2 ) ……..eq2
-1. = 0 – (+1) ………eq3
From eqn2 and eqn3 we can see that pressure applied on LP side is more than that on HP side causing DP to become negative .This simple logic is employed for calibration of DP transmiter. whose range has negative values .
The differential pressure is acted upon the DP cell .A pull on HP side (vacuum)or a push on LP side (positive pressure ) or vice versa ,both creates the same effect in the movement of the DP cell sensor .
If we referenced HP to zero or HP port opened to atmosphere, then ,
DP = HP -LP
=0 -LP
Or DP=- LP
DP = negative value of pressure applied on LP port (HP open to atmosphere)
So the procedure is as follows . Instead of applying a vaccum to HP port apply numerically the same value of positive pressure on LP side keeping the HP side opened to atmosphere
Thus the DP of -10000 mmwc can also acheived by applying a +10000 theto LP port leaving HP port opened to atmosphere.
DP = HP – LP
= 0 -10000
= – 10000 mmwc .
The positive pressure applied on the LP port push the sensor diaphragm(HP open to atmosphere) and creates a negative DP .The effect is same when we apply vacuum (pull on the sensor diaphragm) on HP side leaving LP open . previous method .see above figure
Calibration procedure
Suppose the range is -12000 to 500mmwc
The procedure is as follows.
LRV = -12000 mmwc
URV = -500 mmwc
1.Connect pressure source to LP port of transmitter. Open the HP port to atmosphere
2.apply 12000mmwc ,(+12000mmwc to LP side not vacuum . ) adjust output to 4 ma. ( 0%)
3..apply 500mmwc (+500mmwc)adjust output to 20ma. (100%)
Note: The above method is the preferred one because it only needs pressure source which has positive range capabilities .Also differential pressure can be created between HP port and LP port which has a value above absolute vacuum .This cannot be done using a vacuum source only .