Monitoring HEPA Filter Loading Through Pressure Differential

NOTICE: General Reference Only

The information provided in this article is for general reference purposes only and does not constitute project-specific guidance. SLD Technology products are engineered to order, and their installation must comply with all applicable codes, standards, and project-specific specifications. In all cases, the approved project documentation and requirements take precedence over any general information contained herein. For project-specific instructions, contact your SLD Project Manager or the SLD Support Team at: support@sldus.com.

Applies to: AirFRAME®, AirFRAME-LT®, and LEDiffuser® units with HEPA filtration

Summary

This article explains how to install upstream and downstream pressure probes on an AirFRAME®, AirFRAME-LT®, or LEDiffuser® unit and use the pressure differential (ΔP) across the HEPA filter to determine when the filter is loaded and due for replacement.

IMPORTANT: For Reference Only.
The guidance in this article is provided as general reference information. Installing the pressure probes, monitoring the differential, and determining when to change the HEPA filter are the responsibility of the customer or their qualified personnel and fall outside SLD Technology's scope of supply. Follow your facility's own procedures, the filter manufacturer's instructions, and any applicable regulatory protocols, and consult a qualified professional for your specific installation.

How pressure differential indicates filter loading

A HEPA filter works by trapping airborne particulate as air passes through it. Over time, that captured material accumulates and restricts airflow, increasing the resistance the air must overcome to pass through the filter. That rising resistance shows up as an increasing pressure drop — the differential pressure, or ΔP — between the two sides of the filter.

By comparing the current differential against a clean-filter baseline, you can gauge how loaded the filter is without opening the unit or visually inspecting the media. A doubled differential is the practical signal that the filter has reached the end of its service life.

Probe placement

The measurement requires two probes: one upstream of the HEPA filter (inside the unit) and one downstream (in the room). The difference between the two readings is the pressure differential across the filter.

Upstream probe (before the HEPA filter)

Install the upstream probe so it senses pressure inside the unit, on the dirty-air side ahead of the HEPA filter.

The probe may be located anywhere convenient on the unit's sheet metal skin, as long as the penetration lands ahead of the HEPA filter in the airflow path.
The probe must penetrate the sheet metal skin only. Do not allow the probe to penetrate any internal tube steel. The tube steel is structural framing — piercing it can compromise the unit's structural integrity and is not a valid probe location.
Seal the penetration after installation to prevent air leakage. An unsealed penetration lets air bypass the sensing point and will produce inaccurate differential readings.

Downstream probe (room side)

Install the downstream probe in the room served by the unit — either ceiling- or wall-mounted, wherever is practical.

This probe senses pressure on the clean-air side, downstream of the HEPA filter.
Mount it in open room air, away from supply diffusers, doorways, or other localized airflow that could skew the reading.

With both probes in place, you have one reference upstream of the HEPA and one downstream. Your monitor or gauge reports the difference between them: the pressure differential across the filter.

Establishing a baseline

The baseline is the differential measured with a new, clean HEPA filter installed and the unit running at its normal operating airflow.

After commissioning or immediately following a filter change, allow the unit to run at its standard operating setpoint.
Record the differential pressure reading. This value is your baseline.
Note the date and the reading in your maintenance log.

Because differential pressure depends on airflow, always capture the baseline at the same fan speed / airflow the unit will run at day to day. If the unit runs at variable speeds, take and compare readings at a consistent operating point. Keep room doors closed when taking any reading. The downstream probe references room pressure, and an open door breaks the room's seal — the room pressure is no longer valid, and the differential reading it produces cannot be trusted. Capture the baseline with the doors closed, and take all subsequent readings the same way so they remain comparable.

Reading the differential over time

Check the differential on a regular schedule and log each reading against the baseline. The value will climb gradually as the filter loads. Trending these readings over time lets you anticipate the change-out well before it becomes urgent.

HEPA differentials are typically small — on the order of fractions of an inch of water column (in. w.c.) up to a couple of inches, or the equivalent in Pascals (Pa). Make sure the gauge or monitor has enough resolution to show meaningful change at that scale.

When to change the filter

Typically, HEPA filter replacement occurs when the differential pressure reaches approximately twice (2×) the baseline value.

A doubled differential indicates the filter is loaded and at the end of its effective service life. This 2× rule is the recognized industry 'rule of thumb' for HEPA change-out at normal design airflows. For example, if the clean baseline was 0.15 in. w.c., plan the change when the differential approaches 0.3 in. w.c.

Many facilities set an alert slightly below the 2× threshold so the replacement can be scheduled rather than performed reactively. After installing the new filter, re-establish the baseline (see above) — the fresh filter will read at or near the original clean value.

Filter service life and other factors

The doubled-differential rule is the loading-based trigger for replacement, but actual filter life varies widely with operating conditions. A HEPA filter in a low-contaminant environment can stay in service for a long time — often ten years or more, and in some documented cases beyond fifteen — while a filter under heavy particulate load will reach the 2× point far sooner.

Conditions that shorten or extend service life include the volume of outside air versus recirculated air being filtered, how often and how long the filter is exposed to aerosol or contaminant challenge, the total amount of that challenge reaching the filter face, how the contaminant is introduced into the system, and the effectiveness of any prefiltration ahead of the HEPA. Note that charged synthetic (electret) prefilter media loses efficiency over time as its electrostatic charge dissipates; a prefilter that becomes less effective passes more load downstream and can shorten HEPA filter life.

Time-based replacement in critical environments. In tightly controlled applications — pharmaceutical cleanrooms and similar critical zones — some operators replace HEPA filters on a fixed schedule rather than waiting for a pressure-drop threshold. Common guidelines run in the range of roughly five years for the most critical (e.g., Grade A) spaces and around seven years for less critical ones.

Where your application is governed by that kind of protocol, follow it in place of, or alongside, the differential trigger.

See attached Technical Bulletin from Camfil for more information.

Wet filters
A HEPA filter that gets wet should be replaced as soon as possible, and the condition that caused it corrected. A filter left wet for roughly 48 hours or more can develop mold growth within the media. A filter that has dried out after being wet may also read a higher pressure drop than it did when new, which can skew the baseline comparison.

Troubleshooting unexpected readings

Differential unusually low or near zero: Suspect a leak or bypass — commonly an unsealed probe penetration or supply air leaks. Air taking a path around the sensing point suppresses the reading.
Sudden jump in differential: Check for an obstruction ahead of or across the filter face, or a prefilter (if equipped) that has loaded rapidly.
Erratic readings: Confirm the downstream probe isn't sitting in a diffuser stream or other localized airflow, and that the unit is running at a steady operating speed.

Contacting Support

You can reach SLD Product Support by:

Submitting a ticket online: Click: "Submit a Ticket" at the bottom of this article or at www.sldus.com/support
Submit a ticket via Email: support@sldus.com
Phone: +1 (503) 744-0002, Option 2

All rights reserved. AirFRAME®, LEDiffuser®, uLED®, AirFRAME-LT® and associated marks are trademarks of SLD Technology, Inc. Unauthorized reproduction, distribution, or use of this manual or any portion thereof is strictly prohibited and may violate applicable laws.

SLD Technology reserves the right to make changes to products, specifications, or this manual without notice. The information herein is believed to be accurate at the time of publication; however, SLD assumes no responsibility for errors, omissions, or the consequences of use.