Critical Considerations When Selecting a Vented Balance Enclosure
Robert K. Haugen, Ph.D.
Flow Sciences, Inc.
2025 Mercantile Drive
Leland, North Carolina 28451
ABSTRACT: This paper discusses the primary benefits of the vented balance enclosure. Critical to the operation of laboratories are the safe and effective weighing of potent powder compounds, active pharmaceutical ingredients and nanomaterials. Parallel challenges for fast accurate weighing with proven safe containment for product and personnel protection create continued demand for an enclosure that meets both functionality and performance.
Flow Sciences’ first product was a vented balance enclosure. Three engineering principles guided the design of this unit:
1) Containment: The unit had to keep all powders inside the enclosure, preventing operator exposure. Personnel and/or product protection when working with harmful and toxic powders was the priority.
2) Weighing Accuracy: Air currents and fan vibration inside the enclosure could not interfere with the accuracy of the analytical scale or semi-micro balance. Under no circumstance could the enclosure be turned off to achieve a stable weighing environment.
3) Functionality: Air currents within the enclosure could not prevent manipulation of finely divided powders. Equipment maintenance such as filter replacement should never cause a breach in containment
In the age of Active Pharmaceutical Ingredients (API’s) and High Potency Active Pharmaceutical Ingredients (HPAPI’s), the equipment challenges for vented balance enclosures are even more critical today. The same three overriding engineering principles still apply:
High potency powders must be retained within the containment area and these materials should never be expelled during the weighing procedure. Turbulence outside of the enclosure becomes laminar inside by using four airfoils on all sides of the rectangular face opening as shown below paired with the slotted rear plenum.
The net result of these features is a Class 1 BSC 6 (biological safety cabinet) with proven particulate and vapor containment using ASHRAE 110-2016 and ISPE approved surrogate powder containment protocols. ASHRAE containment is routinely found to be 0.05 PPM or better; surrogate powder testing results are routinely below 10 nanograms per cubic meter. 2, 4
2) Weighing Accuracy:
In modern measurement scenarios, lab balances are required to be accurate and reproducible with deviations ranging from +0.1 mg (milligrams) to +0.1 mg (micrograms).
It is crucial in analytical environments that precise weighing takes place inside the containment area. This is accomplished with the Flow Class I BSC in the following manner:
Little to no vibration in Flow Sciences balance enclosures means balance stability is achieved while the fan-driven containment system is running under negative pressure.
In contrast, many weighing units from other manufacturers mount the fan below the filter housing and attach it directly to the containment cavity with sheet metal screws. This produces a direct contact pathway for motor vibration to be transmitted into the weighing area. Frequently in these units, fans must be turned off to get a stable balance reading.
The containment system should support, not impede, effective science. The Flow Sciences balance enclosures do exactly this in five ways:
a) Filter system should not cause particle shedding or emission back into the containment area.
Some less expensive balance enclosures actually place the fan below the filter. This creates an area of positive pressure inside the fan housing and causes loose particles to fall down onto the surface inside the containment area when the fan is switched on or off. The Flow Sciences enclosure places the fan above the filter, keeping it and the fan housing completely clean!
b) Bag-In/Bag-Out filters can be replaced without threatening lab area contamination or sample cross contamination. 3
Many less expensive balance enclosures do not offer this option. An exposed filter is difficult to remove from a lab area without room contamination. Such cross-contamination will create health issues and potentially contaminate other samples in the lab, destroying traceability.
c) Ease of Cleaning
Many balance enclosures have small holes or slots in their baffle assemblies which are very hard to clean. The sharp edges can cut skin or a glove and increase the chance for contamination. Flow Sciences uses straightforward large slotted baffles.
d) Thicker Filters mean longer life!
Flow Sciences balance enclosure filters are 4” thick to assure long life and infrequent motor RPM adjustments. Other manufacturers use thinner (2-3”) filters, which need to be replaced more frequently. Also, RPM adjustments must be made more often, and the fans produce more noise.
e) Vented Enclosure Naming Conventions:
The VE or vented enclosure is defined as an engineering control as described by NIOSH6. The National Institute for Occupational Safety and Health is the United States federal agency responsible for conducting research and making recommendations for the prevention of work-related injury and illness. The selection of the appropriate vented enclosure has created SOPs (standard operating protocols) for many pharmaceutical companies and industries that have adopted acronyms to describe the unit. The acronyms below comprise a list of commonly used terms.5
VBSE™ = Vented Balance Safety Enclosure (trademark of Flow Sciences, Inc.)
VBE= Vented Balance Enclosure
VSE= Vented Safety Enclosure
VE = Vented Enclosure
CVE = Contained Vented Enclosure
Flow Sciences has focused on three key issues affecting the functionality of balance enclosures: containment, accuracy, and functionality. We have shared with you the ways our engineering team has addressed these issues. As the sciences of measurement and containment move forward in this century, there will no doubt be more such challenges to our industry in the areas reviewed in this paper. Flow Sciences’ experienced Engineering Team will continue to develop industry leading improvements and engineering controls to address the safety and performance concerns in our industry.
- BIBO FILTER CHANGE Rev 103117 (C-8832), 1/13/2018, Flow Sciences
- ISPE: Good Practice Guide: Assessing Particulate Containment
- US Centers for Disease Control and Prevention; Primary Containment for Biohazards: Selection, Installation, and Use of Biological safety Cabinets (PDF), 2000