Should the concentration before and after the chemical filter be measured at the same time?

The removal efficiency of a chemical filter is calculated using the inlet concentration and outlet concentration. The formula itself is simple, but whether the data can be used depends on whether the two concentrations were obtained under comparable conditions.

It is recommended to measure the inlet and outlet concentrations in pairs. You cannot just measure them once and calculate them together. If the inlet concentration is fluctuating and the outlet sampling lags for a long time, the two sets of data may not reflect the same state, and the calculated efficiency may be higher or lower, or even an outlier with the outlet concentration being higher than the inlet. For specific efficiency calculation methods, please refer to Discussion on how to calculate the removal efficiency of chemical filters .

Consider air and instrument response

'Simultaneous measurement' is not simply looking at two meter readings, but allowing the two data to represent the same airflow during the same period.

There is a time difference in the passage of air through the system. From the inlet sampling point to the outlet sampling point, it takes a certain amount of time for the air flow to pass through the filter section. If the inlet concentration is changing rapidly, the outlet concentration reflects an earlier or later state. The time difference depends on the air volume and pipeline length.

Instrument response may also vary. Different types of testing instruments have different response times. Some have real-time readings, while others require sampling and enrichment for analysis. Different instruments are used at the entrance and exit, and the time correspondence between the readings needs to be confirmed.

Sampling methods should be comparable. The sampling flow rate, sampling time, and analysis method of the inlet and outlet should be as consistent as possible, or cross-validated. Inconsistent methods will compromise data comparability. For specific sampling point selection, please refer to Discussion of sampling points for chemical filter performance verification .

Keep the detection caliber consistent

Inlet and outlet data must be compared under the same caliber.

The units must be consistent. Both use mg/m³, or both use ppm. If one is the mass concentration and the other is the volume concentration, they need to be converted first and then calculated.

The sampling time must correspond. If the inlet is an instantaneous reading, the outlet is also an instantaneous reading. Make sure that the two instantaneous readings correspond to the same working condition. If the entry is the average of the time period, the exit should also be the average of the same time period.

Instrument calibration should be consistent. The calibration status and range of the instruments used for inlet and outlet must be confirmed. Systematic deviations between different instruments may affect efficiency calculation results. For specific differences, please refer to Discussion of why chemical filter performance reports list specific target gases .

Record operating conditions. The air volume, temperature and humidity, and production status during sampling should be recorded. These conditions vary, and so do the ways in which the data can be interpreted. For specific methods, please refer to Discussion of how to set clear success conditions for chemical filter trials .

Retain continuously changing data

Paired measurements are particularly important when the inlet concentration fluctuates significantly.

Instead of just picking the lowest exit value, you can keep data for a stable or representative period. If the inlet concentration changes periodically during the production process, sampling should cover the entire cycle or be recorded in sections according to working conditions.

Efficiency ratios also need to be interpreted with caution when the entrance is very low. As the inlet concentration approaches the detection limit, small measurement differences can significantly alter the ratio. When the outlet is lower than the detection limit, it should be stated as 'not detected' or 'below the detection limit' according to the method, and cannot be directly regarded as zero concentration. For specific differences, please refer to Discussion of what breakthrough concentration means in a chemical filter report .

Efficiency is not the only metric. The removal efficiency at a certain point in time does not represent the performance over the entire life cycle. Long-term applicability also depends on the penetration trend and cumulative load. For specific differences, please refer to Discussion of the difference between initial efficiency and lifetime efficiency of chemical filters .

Project judgment example

A project tested the removal efficiency of a chemical filter. The inlet concentration was 100 mg/m³ and the outlet concentration was 20 mg/m³. The calculated efficiency was 80%.

Further inspection found that: the inlet sampling was during the equipment start-up stage, the outlet sampling was during the stable operation stage, and the working conditions at the two time points were inconsistent. Efficiency data will only be comparable after pair sampling again under stable operating conditions.

In another project, the inlet concentration fluctuated greatly. The operator only took the lowest value at the outlet to calculate the efficiency, and the result was on the high side. After supplementing the entrance data and pairing according to the same time period, the efficiency returned to a reasonable range.

These two cases illustrate that paired measurement of entrance and exit is the basis for efficiency calculation. involving When selecting chemical filter product series , it is recommended to provide the sampling plan and testing requirements to the supplier for verification.

Key points for selection and acceptance inspection

About Aili Industrial’s testing and sampling packages

Aili Industrial has independent manufacturing capabilities in filters, motors, fans, control systems and sheet metal processing, and can provide chemical filter product series and supporting air supply equipment. For projects that need to verify the filtration effect, we can assist in checking the sampling point locations and paired measurement plans based on the target gas, system conditions, sampling methods and testing requirements. When the filter is matched with MAU, FFU, EFU and other equipment, the location and maintenance method of the sampling port can be confirmed uniformly.

Related products:Chemical filter product series、MAU box chemical filter、FFU comes with chemical filter . Providing the target gas, sampling conditions and detection requirements when enquiring, will help confirm the appropriate solution. The specific selection is subject to the technical documents and test results confirmed by the project.