How to select and check ULPA differential pressure instrument?

The differential pressure gauge seems to be a small piece, but in the operation and maintenance of ULPA, it is often the main basis for judging the status of the filter element. If you choose the wrong meter, or if you don't check it for a long time, the readings may lead you astray.

The key to model selection is not to 'buy a meter that can display the pressure difference', but that the range, accuracy, interface and display method must match the actual working conditions. The focus of the inspection is not just to look at the readings, but to first confirm that the instrument itself, the pressure pipeline and the signal link are all normal.

The larger the measuring range, the better

When selecting a differential pressure gauge, the measuring range is an item that needs to be determined first.

The measuring range should cover the working range. From the initial resistance of the new filter element, to the final resistance agreed upon in the project, to possible instantaneous fluctuations, they should all be within the range. If the measuring range is too small, the pressure difference will hit the meter and the reading will be meaningless.

It is also not good to have a measuring range that is too large. If the actual working pressure difference is only 200 Pa, but you choose a meter with a range of 0 to 5000 Pa, the pointer or number will jump within a small range, the resolution will be limited, and the early dust accumulation trend may not be visible.

A more reasonable approach is to choose according to the range of 'initial resistance to final resistance' and leave an appropriate margin. For example, the initial resistance is about 200 Pa, the final resistance is about 500 Pa, and the range is 0 to 1000 Pa or 0 to 1500 Pa. The specific values ​​are confirmed according to the actual working conditions of the project.

It also depends on the object being tested. The numerical ranges of the filter element body pressure difference, the filter section pressure difference and the whole machine pressure difference are very different. The pressure difference of the whole machine is usually higher than the pressure difference of the filter element itself. When selecting tables, the same set of measuring ranges cannot be mixed. For the specific definition of measuring points, please refer to the relevant instructions for differential pressure measuring points. involving ULPA ultra high efficiency filter or For HEPA high-efficiency filter monitoring, the principles for range selection are the same.

Accuracy and resolution must match the intended use

If the accuracy is not enough, early changes cannot be seen; if the accuracy is too high, the cost will go up, and it may not be necessary on site.

Differential pressure gauge for trend monitoring with emphasis on repeatability. Whether multiple readings under the same working condition are consistent is more important than the number of decimal places of the absolute value. If the reading fluctuates every time, the trend cannot be judged.

Differential pressure gauges used for alarms or interlocks , with emphasis on reliability near the set point. The reading near the alarm value should be stable and should not be triggered frequently due to slight fluctuations.

For differential pressure sensors with signal output , it is also necessary to confirm whether the output signal type, power supply method and control system interface match. If there is a deviation between the output signal and the displayed value, it needs to be calibrated according to the instruction manual.

The accuracy is not as high as possible, but sufficient and stable. Stability under on-site working conditions often affects actual use more than nominal accuracy.

Zero point inspection cannot be omitted

If the differential pressure gauge is used for a long time, the zero point may drift. If the zero point is inaccurate, all readings will have a fixed deviation.

The inspection method is very simple: make sure there is no pressure difference on both sides of the filter and see if the meter displays zero. If it is not at zero position, adjust or record the deviation value according to the instructions.

When to check zero point? After installing or replacing instruments; before restarting the system after a long-term shutdown; when abnormal readings are found. If the system cannot completely stop the air flow, you can use bypass or pressure equalization to create zero pressure difference conditions. The specific method should be implemented according to the equipment instructions.

The reasons for zero point drift may come from many aspects. The aging of the instrument itself, vibration of the installation location, ambient temperature changes, and power supply fluctuations may all affect it. If you find that the zero point drifts frequently, you need to check the installation and usage conditions instead of repeatedly zeroing.

How to check the fault of pressure pipe

The pressure tube is the most problematic link in differential pressure measurement, but it is often ignored during troubleshooting.

The pipe is detached or leaking. Loose joints of the pressure-conducting pipe and damaged pipe walls will distort the pressure transmission and cause low readings or jumps. Check whether the joints are tight and whether there are any cracks in the pipes.

The pipe is clogged. Dust, condensed water or foreign matter entering the pressure guide pipe will cause blockage. After blockage, the reading response is slow or remains unchanged for a long time. You can disassemble the pipeline and check whether it is smooth.

The pipeline is connected incorrectly. If the high and low voltage sides are connected reversely, the reading will be negative or obviously abnormal. When multiple devices share a differential pressure gauge, confusion in the corresponding relationship is more likely to occur. It is recommended to check each device after installation.

Water accumulates in pipes. If the moisture content in the gas is high, water may accumulate at the low point of the pressure pipe, affecting pressure transmission. When laying out pipelines, try to avoid obvious low points or provide drainage measures.

The pipe is bent or stressed. The hose is pinched or broken, which will also affect the reading. Check the direction of the pipeline to make sure it is not squeezed by structural parts. For specific measuring point layout principles, please refer to the relevant instructions for differential pressure measuring points. Device-side applications such as The EFU equipment-end fan filter unit has a compact internal space and the pipelines are more likely to be squeezed or worn. Special attention should be paid during installation and inspection.

Readings and filter status should be judged separately

When the differential pressure gauge reads abnormally, do not directly attribute it to the filter element.

First confirm that the instruments and pipelines are normal. Check the zero point, pressure tube, wiring, and signal output one by one. If there is something wrong with the meter itself, the readings cannot be used as a basis for judgment.

Confirm again that the working conditions are consistent. Are there any changes in air volume, speed, and operating mode? Depending on the working conditions, the differential pressure readings are inherently different. For specific judgment methods, please refer to the relevant instructions for a sudden increase or decrease in pressure difference.

Finally, compare the filter curve. On the premise of confirming that the measuring points and working conditions are consistent, compare the readings with the product resistance data. If it is indeed too high or too low, re-evaluate the condition of the filter element.

In a constant air volume system, the speed and power also depend on it. While the pressure difference rises, if the rotational speed continues to increase, it means that the control system is compensating for the change in resistance. At this time, the pressure difference alone is not enough, and it must be judged together with the control output. Supporting For the FFU fan filter unit system, it is recommended to record the pressure difference, rotation speed and power in the same table to facilitate correlation analysis. involving HEPA/ULPA air filters are the same. The maintenance judgment and data comparison method for

Project judgment example

The differential pressure gauge of a certain equipment showed close to zero for a long time, and the operator thought that the filter element had no resistance.

The inspection found that the pressure-conducting pipe was pressed by the cable inside the equipment, and the pipe was broken and the pressure could not be transmitted. After reorganizing the pipeline, the pressure difference reading returned to about 180 Pa, which is consistent with the initial resistance data of the filter element.

Another item, the differential pressure gauge reading is much higher than similar equipment. The investigation found that the zero point of the instrument had drifted and the actual pressure difference was normal. After calibrating the zero point, the readings return to a reasonable range.

These two cases illustrate: when the pressure difference is abnormal, check the instrument and pipeline first, and then check the filter element. Only if the instruments and pipelines are normal can the readings be of reference value.

Selection and Acceptance Checklist

About Aili Industrial’s differential pressure monitoring package

The selection and inspection of differential pressure instruments seems to be a matter of maintenance, but if the measuring points, pipelines and instrument interfaces are taken into consideration during the equipment design stage, a lot of trouble will be saved in the future.

Aili Industrial has independent manufacturing capabilities in filters, fans, control systems and sheet metal processing. In the supporting design of equipment-side air supply units and filters, the pressure difference monitoring solution can be incorporated into the overall layout. For equipment that needs to distinguish the pressure difference between the filter element and the whole machine, the location of the pressure port and the installation location of the instrument can be clarified in the drawing stage; for projects with signal output, you can check whether the sensor interface and the control system match. When the filter is matched with equipment such as FFU and EFU, the recording format and alarm settings of the pressure difference data can also be confirmed uniformly.

Explaining the monitoring requirements, operating conditions and control methods during the inquiry will help to consider the differential pressure instruments and pipeline layout together during the planning stage. The specific configuration is subject to the drawings and technical documents confirmed by the project.