What is the difference between a pressure guide tube and a capillary differential manometer?

As far as I know, both can adjust the range

This post was last edited by ylb913 on 2015-12-26 at 12:09. The capillary is a self-sealing system that isolates the system being measured. The pressure guide tube should cover the capillary, but the uncovered portion is open to the measurement system. The medium inside the capillary must not have its pressure affected by thermal expansion and contraction; of course, there will be a larger amount of pressure-transmitting medium in certain areas, but the proportion of such pressure-transmitting medium inside the capillary is very small, just as we can see in thermometers and glass tube thermometers. This requirement does not apply to the open ends of the system under test. ——The zero point can also be adjusted for the capillary through migration. I have a practical experience in this regard: the pressure measurement points on one of our atmospheric storage tanks tend to get clogged. The measurement range is -1000 to 3000 Pa, and this issue occurs 2 or 3 times a year on average; however, it hasn’t happened in recent years. Once, when I was trying to clear it out, I used a wire to pry, and accidentally poked the corrugated plate at the measurement point (it was just a very light, vertical poke), and the pressure reading shot up to tens of thousands in an instant. I didn’t dare to tell the technicians; I called them and asked them to remove the gauge head, reset it to zero, and then reinstall it.

The pressure guide tube has three valve assemblies; does the capillary have any? Therefore, it cannot be zeroed at operating conditions. But if you remove the double flanges and place them on the same plane, and there is a zeroing menu option available for adjustment with respect to the atmosphere, then you can zero it out

The zero pressure difference of the pressure guide tube is related to the density of the medium. The zero point of the capillary is set at the time of manufacture based on calculation formulas, but it can also be adjusted

Capillaries are thin, resulting in greater transmission lag.

The zero point of the capillary type can also be adjusted. It’s just that when the differential pressure across the capillary is zero for the medium being measured, the pressure difference experienced by the transmitter is not zero; therefore, the zero point in this case is not the numerical value zero, but rather a certain number, and that number is negative. So, the zero point here is not zero, but rather the lower limit. When it is found that the differential pressure variation of the capillary is inaccurate in field use, modifying the configuration requires adjusting the lower and upper limits. Since the pressure measurement ports at the top and bottom of the capillary are fixed, its range is also fixed; if the lower limit changes, the upper limit must change as well, so that the range can remain constant.

Capillaries mostly use migration to set the zero point, as they have built-in pressure after installation, which requires migration to be performed. After the pressure tap is installed, it can be zeroed out in conjunction with the three-valve assembly. However, pressure tapping pipes are generally used in conjunction with orifice plates. The orifice plate is designed based on the operating conditions, so the differential pressure value remains roughly constant and there is no need to adjust the range. However, if the pressure tap is used to measure differential pressure rather than in conjunction with an orifice plate, it is still possible to adjust the range, perform zero adjustment, and make offsets

The capillary is a self-sealing system that isolates the system being measured. The pressure guide tube should cover the capillary, but the uncovered portion is open to the measurement system. The medium inside the capillary must not have its pressure affected by thermal expansion and contraction; of course, there will be a larger amount of pressure-transmitting medium in certain areas, but the proportion of such pressure-transmitting medium inside the capillary is very small, just as we can see in thermometers and glass tube thermometers. This requirement does not apply to the open ends of the system under test. ——The zero point can also be adjusted for the capillary through migration.