Analysis of the classification of zero point shift and range calculation of Rosemount differential pressure transmitters.

For the application of differential pressure transmitters, if the measurement conditions are met, generally no migration technology treatment is required. However, in practice, the use of differential pressure transmitters often involves considerations such as maintenance and installation. To facilitate operation, this can lead to a difference in the horizontal level between the pressure tapping point and the measuring instrument, or when the measured medium has certain limiting factors, such as being highly corrosive or having excessive viscosity. This can hinder the entire pressure measurement process and even affect the accuracy of the instrument's measurements. Therefore, to achieve accurate measurement with differential pressure transmitters, zero point migration is usually performed. The technical editor from Shaanxi Huibo Electromechanical will introduce the classification of zero point migration and how to calculate the range using the Rosemount differential pressure transmitter as a reference, hoping to provide some help and assurance for the functionality of the Rosemount differential pressure transmitters used in the future.
　　1. Concept of Zero Point Migration
Zero point migration, simply put, is a numerical change guarantee measure centered around the measurement starting point to ensure the effectiveness of the basic quantities of the differential pressure transmitter (mainly including range, measurement accuracy, etc.) during measurement. At this point, we need to compare zero point adjustment and zero point migration, as these two concepts have significant similarities. Their function is to ensure that the lower limit of the transmitter's measurement signal matches the input signal, while the difference between the two mainly depends on whether Xmin is 0. If Xmin is 0, it is called zero point adjustment; otherwise, it is zero point migration. Migration is generally divided into two situations: one is positive migration, where the starting point of migration is chosen as the end point, and the value is 0. If the end measurement value gradually shows a positive value, this is what I refer to as positive migration. The opposite situation is the second form of migration, which we call negative migration.
　　2. Classification of Zero Point Migration
Essentially, zero point migration includes three forms: no migration, positive migration, and negative migration. Since no migration is relatively rare, this article only introduces positive and negative migration of the Rosemount differential pressure transmitter.
　　(1) Positive Migration
It is a common phenomenon in actual measurements that the installation position of the differential pressure transmitter differs from the measurement liquid level height, as shown in Figure 1, which illustrates a form of migration.

Figure 1: Principle Diagram of Positive Migration of the Transmitter
From Figure 1, we can observe that the container is an open container, h is the height difference between the installation position of the differential pressure transmitter and the measurement liquid level, and we can derive the function relationship ΔP = ρgH + ρgh related to the pressure difference ΔP. Assuming that we want the output pressure of the Rosemount differential pressure transmitter to be greater than 4mA, in addition to ensuring that a portion of the static pressure remains in the positive pressure chamber of the Rosemount differential pressure transmitter, we also need to assume that the installation position of the Rosemount differential pressure transmitter and the measurement liquid level are at the same horizontal level, meaning H is 0. When H takes the maximum value, we can again obtain the equation ΔP = ρgH + ρgh. At this point, the output pressure of the transmitter has exceeded the limit value of 20mA, indicating that the static pressure generated by ρgh is an excess part that must be eliminated, thus leading to a type of migration known as positive migration.
　　(2) Negative Migration
As shown in Figure 2, this is the principle diagram of negative migration. If the pressure tapping chamber of the Rosemount differential pressure transmitter is injected with liquid or gas from a closed container during use, the measurement pipeline will suffer significant damage and may even experience severe corrosion due to the flow of liquid or gas within the closed container. Therefore, isolation tanks are installed between the positive and negative pressure chambers of the Rosemount differential pressure transmitter and the pressure tapping point, and a density of ρ1 isolation liquid is injected.

Figure 3: Principle Diagram of Negative Migration of the Transmitter
If we assume H to be 0 and the maximum value respectively, we can obtain two different numerical situations for the pressure difference ΔP. This also precisely indicates that when H is 0, 4mA is the limit value of the Rosemount differential pressure transmitter, while when H is at the maximum value, the actual density of the isolation liquid far exceeds the expected value. Therefore, when at the highest position, due to the pressure in the negative pressure chamber being greater than that in the positive pressure chamber, the actual output pressure value of the liquid level instrument does not match the theoretical calculation, which severely affects the balance between the liquid level and the output pressure of the transmitter. After analysis, to maintain the relationship between the actual liquid level and the instrument, the static pressure from the negative pressure chamber's pressure tapping pipeline must be removed, which is the fundamental measure. During this process, the negative migration technology of the Rosemount differential pressure transmitter is required, where the migration amount can be considered as ρ1gh.
　　3. Range Calculation of the Rosemount Differential Pressure Transmitter
Measurement Schematic:

Condition: P is the pressure on the Rosemount differential pressure transmitter
P+ is the pressure in the positive pressure chamber of the Rosemount differential pressure transmitter
P- is the pressure in the negative pressure chamber of the Rosemount differential pressure transmitter
h is the height of the measured liquid
h1 is the total height of the measured liquid
h2 is the height from the measuring instrument to the low position of the measured liquid
ρ is the density of the measured liquid
ρ1 is the density of the silicone oil in the dual flange gauge
1. When the pressure tapping pipe is filled with the measured liquid
P = P+ - P- = (hρg + h2ρg) - (h1ρg + h2ρg)
= (h - h1)ρg
Rosemount differential pressure transmitter range:
When h = 0, P = -h1ρg
When h = h1, P = 0
Example: As shown in the figure above, the total height of the measured liquid h1 is 12 meters, the density of the measured liquid ρ is 0.86×103kg/m3, g is 10m/s2, and the pressure tapping pipe is filled with the measured liquid. Let's calculate the measurement range of the Rosemount differential pressure transmitter.
According to P = (h - h1)ρg
When h = 0, it is the lower limit of the measurement range of the Rosemount differential pressure transmitter.
P lower limit = -h1ρg = -12m × 0.86×103kg/m3 × 10N/kg
= 103200N/m2 = 103200pa = -103kp
When h=h1, it is the upper limit of the measurement range for the Rosemount differential pressure transmitter.
P upper limit = (h - h1)ρg = 0 × 0.86 × 103 kg/m3 × 10 N/kg = 0 kp
The range of this table is from -103 kp to 0 kp.
2. When the Rosemount differential pressure transmitter has a double flange pressure tap with silicone oil inside.
P = P+ - P- = (hρg + h2ρ1g) - (h1ρ1g + h2ρ1g)
= hρg - h1ρ1g
Rosemount differential pressure transmitter range:
When h=0, P = -h1ρ1g.
When h=h1, P = (ρ - ρ1)h1g.
Example: As shown in the figure above, the total height of the measured liquid h1 is 12 meters, the density of the measured liquid ρ is 0.86×103kg/m3, g is 10m/s2, and the pressure tapping pipe is filled with the measured liquid. Let's calculate the measurement range of the Rosemount differential pressure transmitter.
According to P = hρg - h1ρ1g.
When h = 0, it is the lower limit of the measurement range of the Rosemount differential pressure transmitter.
P lower limit = -h1ρ1g = -12m × 0.94 × 103 kg/m3 × 10 N/kg.
= 112800 N/m2 = 112800 pa = -112.8 kp.
When h=h1, it is the upper limit of the measurement range for the Rosemount differential pressure transmitter.
P = (ρ - ρ1).
P upper limit = (ρ - ρ1)h1g = 12 × (0.86 × 103 kg/m3 - 0.94 × 103 kg/m3) × 10 N/kg.
=-9600 N/m2 = -9600 pa = -9.6 kp.
The range of this table is from -112.8 kp to -9.6 kp.