When we have a doubt, indicating value of instrumentation will need to inspect the instrument, the most commonly used method is to check the zero point is correct, this is the most basic relates to whether the measurement factors, we are often ignored the most basic problem, led to a small problem, large processing results.
So this article will focus on zero, help you learn and improve.
The instrument zero, refers to the lower limit (instrument measuring range
The instrument under the certain accuracy can measure the minimum)
Common operation 1 meter zero, zero zero namely the false data, like a pressure transmitter
to atmospheric pressure should be displayed when zero, but the table is not false data shows zero can be zero.
2, moving zero meter zero migration: instrument input the output curve of arbitrary parallel moving.
Is the instrument of measuring range of parallel moving.
Zero migration is commonly used in production to change instrument measuring range.
First moving zero is because measurement point on the data of different migration.
Such as double flange level meter installed after he will be measured when the empty cans to because of capillary pressure and the zero drift.
This time you need to zero point for migration.
Second instrument to use after a period of time can sometimes produce zero drift, the phenomenon of migration, cause range becomes large or small, affect measurement accuracy.
And by adjusting the return;
Zero migration into no transfer, negative transfer and positive transfer three conditions: no migration will pressure chamber with container respectively connected to the lower and upper pressure points, and ensure the positive pressure chamber with zero level contour;
Connected to the negative pressure chamber and the upper container pressure points of pressure containers filled with liquid level above the same gas in the tube, because the gas density than the liquid much smaller, the pressure points and static pressure difference between negative pressure chamber is small, can be ignored.
A differential pressure transmitter is the pressure of the suction chamber of P + and P - respectively
, there are: P + = P0 + Hg, P -
= p0△P = P + -
= Hg can be seen, when H = 0, delta P = 0, differential pressure transmitter is not subject to any additional static pressure;
When H = Hmax, delta P = delta Pmax.
This suggests that the differential pressure transmitter without migration.
Positive transfer in the actual installation of differential pressure transmitter, often cannot ensure the liquid level transmitter
and zero in the same horizontal plane, set connected to the negative pressure chamber and the upper container pressure points of pressure in the tube filled with gas, and ignore the gas produced by static pressure, the differential pressure transmitter is under negative pressure chamber pressure respectively: P + = Hg + Hg + P0P -
= P0 so delta P = P + -
Hg + Hg = = Hg + C, visible, when H = 0, delta P = C difference variable by an additional positive pressure difference function.
Make the output I>
To make H = 0, I = 4 ma, we have to try to eliminate the role of C.
0, so need to positive transfer.
Negative transfer when the space above the liquid in the container is condensable gases, such as water vapor, to maintain the pressure chamber of liquid column height constant, or corrosive measured medium, often in the differential pressure transmitter is respectively equipped with isolation between negative pressure chamber and take pressure points to fill, and filled with spacer.
The density of a spacer for & rho;
2, P + = h1g + Hg + P0P -
= h2g + P0 delta P = h1g + Hg - so
B = h1g - B type
H2g visible, when H = 0, delta P = -
0, differential pressure transmitter by an additional differential effect, make the output I<
To make H = 0, differential output I = 4 ma, elimination -
The role of B.
Due to the migration amount is negative, so called negative transfer.
Instrument zero check and zero method for pressure, differential pressure transmitter for the most part shipments after checking method of zero.
1, check pressure transmitter, and zero check the zero point, the first discharge is divided, the pressure transmitter is in a state of no pressure.
For pilot pressure tube longer, transmitter installation position is lower than the sample point of steam or liquid pressure measurement, even if the discharge pressure in addition to, there will still be ling condensate in the guide tube or fluid, due to the effect of liquid column static pressure transmitter output current will be greater than zero, it is need to be aware of, sewage can be taken to solve.
As well as to implement, the original ever take positive transfer measures to counteract the effects of static pressure, in order to avoid wrong judgment, and the normal zero adjustment in confusion.
2, check of differential pressure transmitter and zero zero point of differential pressure transmitter, to turn off three valve set pressure valve;
To open the balanced valve;
After the positive pressure valve closed.
Open the balance valve of positive and negative role is to make the transmitter measuring chamber pressure differential is equal to zero.
For no migration of transmitter, observe whether its output is 4 ma, otherwise should be zero.
Has been using the migration of the transducer, and its positive and negative measurement chamber pressure differential is equal to zero, the output is not 4 ma, but according to the range of the migrated range, to calculate how much is the output current should be, and can determine the transmitter according to the performance is stable.
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