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Cleanroom Weighing: Choosing the right scale for the most critical applications

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A Step by Step Guide

To alleviate the adding of pollutants, equipment used within cleanrooms such as scales, balances and tank weighing systems are required to emit only limited amounts of particulates into the air. Choosing the correct weighing equipment for your facility is therefore essential to help keep cleanroom environments within governmental, GLP/GMP and other regulatory standards.

When it comes down to choosing what scale is right for your cleanroom environment you must define certain criteria and understand the area of installation.

What type of Cleanroom classification is in your facility?

Cleanrooms, despite their nomenclature, are often where low-levels of environmental pollutants such as airborne microbes, dust, and chemical vapors are usually present. This is despite the fact that these areas are filtered with HEPA or ULPA filters and passages are sealed with airlocks. In addition, people entering a cleanroom are required to wear cleanroom suits, gloves, masks, and hairnets to prevent the spread of particulates. However, it has been determined that the average person gives off 1-5 million particles greater than 0.3µm each minute!

Therefore, as you can see, keeping equipment pollutants within an acceptable range is essential in cleanroom design and construction. The following tables illustrate the US FED STD 209E Cleanroom standards by showing the maximum number of particles of the indicated size per cubic foot. (this type of classification table was canceled November 29, 2001 and replaced with the ISO 14644-1 standard) Reference:

ISO14644-1 Standard
Cleanroom Class Comparison

Once the cleanroom classification is determined by the number of allowable particles (Class 100, ISO Class 5 for example) the type of weighing equipment can then be sourced that meets the needs of the environment. These requirements are primarily based on the equippments material construction and whether or not it has moving parts (lift deck system, portable scale, etc.)

What are the Basic Legal Requirements 
of weighing in a cleanroom?

The weighing instruments used must have appropriate capacities and accuracy levels. Also, these instruments must also be calibrated regularly with records of calibration required (3.4 EU GMP Guide). Because weighing has significance for subsequent processing and for product quality, the equipment should be checked frequently, based on the workload of the application. (this needs to be defined by a specific process S.O.P. (standard operating procedures).

Permissible tolerance limits; i.e., the deviation from nominal values, must be defined for each weighing range, with allowance of the uncertainty measurement specified. (pharmaceutical guidelines such as USP 25 Section 41 are typically the standard in determining the tolerances required).

Understanding the actual “accuracy” for a specific scale located in a cleanroom is just one requirement. The real problem arrises when a scale needs to not only meet the accuracy specified, but also must handle a strict cleaning regiment. There are several major scale manufacturers that can satisfy most high accuracy requirements.

The EU GMP Guide states, for example, that weighing instruments are to be cleaned after each work-order specific set of weighing tasks has been completed, to avoid transfer of materials to subsequent orders. In addition, the undersides of the scales and the bottom of the pit or floor under the scale must be cleaned because these areas are more prone to contamination.

If you visit or work in a cleanroom area, take a look at the scales, balances and load cells (usually under mixing/holding tanks, if present) at the facility. Visually inspect their surface and areas underneath. Are they made of stainless steel and do they look easy to dissenfect? Do they have lots of corners and crevices where cantaminates can hide?

The answers to these questions are usually, “Yes the scales are stainless, but no, they look very difficult to dissenfect.” The fact is that most scales are not designed specifically for cleanroom environments, rather they are adapted to “fit” the guidelines discussed earlier. To answer the stated guidelines, scale manufacturers are now beginning to engineer solutions specifically for the cleanroom environment right from the start. These solutions combine the finest materials used in product construction with specific finishing techniques that can meet the most demanding applications in higher class cleanroom environments.

How scales are designed and constructed for cleanrooms?

Stainless Steel Basics

Most weighing equipment have no problem being outfitted in Class 100.000 and 10.000 cleanrooms. However, higher class areas require equipment with superior grade stainless (better finishes, lower surface roughness) and easy cleaning solutions.

The preferred material for product surfaces that come into contact with products in pharmaceutical applications is austenitic stainless steel in the AISI 300 series (e.g., AISI 304, 316, 316L, or 316Ti).

In addition, pharmaceutical and medical technology industries require very strict hygiene regulations to their cleanroom environments. As a result many companies prefer stainless steel with electropolished surfaces because of its highly smooth surfaces that are easy to keep clean. Electropolishing meets the strictest hygiene requirements, because its surfaces are smooth enough that microorganisms (nor residues they would feed on) can take hold. Moreover, it is highly corrosion-resistant and can withstand even the most aggressive cleaning agents used in sterile environments.

Stainless Steel: Grades and Performance

Stainless steel is the preferred material due its resistance to chemically aggressive, aqueous solutions. Most grades of stainless steel contain 12% chrome and a maximum of 1.2% carbon by weight. The high corrosion resistance of stainless steel is due to a passive layer that forms on the surface of the metal. This consists of a metal oxide or hydroxide layer, rich in chrome, only a few Angstrom units thick, that separate the metal from the attacking medium. The passive layer on stainless steel is not constant; over time it tends toward a state of equilibrium, in composition and structure, with the ambient media. Thus, a passive layer once formed cannot be transferred to another medium. If the passive layer is damaged; e.g., mechanically, it generally reforms spontaneously. If a particular medium cannot form an adequate passive layer, or if the existing passive layer is penetrated or completely destroyed by a given chemical, corrosion damage may ensue.

Chrome is the decisive alloy in determining the ability of a stainless steel to form a passive layer. Chrome content above the level of 12% will suppress the formation of rust under normal atmospheric corrosive influences. A further increase of the chrome content or, under certain circumstances, the addition of molybdenum or other alloying elements, will extend the corrosion resistance of the stainless steel so that it can withstand far more aggressive conditions. Only the alloying additions dissolved in the metal are effective in achieving passivation. This is why the best corrosion resistance is provided by matrices that are free of segregation and are not depleted of chrome or molybdenum due to precipitations or formation of inter-metallic phases.

The correct heat treatment for achieving optimum material structure is described in data sheets on the particular materials. Stainless steels can suffer from wear corrosion on the surface and various forms of local corrosion. Surface-wear corrosion can be expected primarily when working with acids and strong alkalis.

The Stainless Surface (weighing pan)

It is essential that equipment surfaces be easy to clean. All surfaces that come into contact with the product must be completely resistant not only to the product itself, but also to the detergents and other agents used in cleaning, disinfecting and sterilizing the equipment. Furthermore, these surfaces must be made of a non-absorbent material and must conform to the specifications dictating the permitted surface roughness. The surface characteristics must conform to the defined mean roughness Ra, and must be free of defects such as holes, brush marks or grooves, or fissures. For large surfaces, Ra should be ≤ 0.8 µ. Surfaces that do not touch the product must be smooth enough to enable easy and thorough cleaning.

Ra refers to the average roughness of the steel across its surface and is also known as Arithmetic Average (AA) or Center Line Average. The average roughness is defined as the area between a roughness profile and the mean line, the following graphic illustrates this, along with the mathematical representation (integral of the absolute value of the height over a given length)

The following methods are used to ensure surfaces with low roughness characteristics:

Bead Blasting

The abrasive polishing agents used on unalloyed metals cannot be used to polish stainless steel, as the iron particles rubbed off in polishing might lead to extraneous rust on stainless steel. To avoid corrosion and extraneous rust, it is also important to ensure that polishing agents used on stainless steel are free of both iron and sulfur.


Electropolishing also called chemical polishing or finishing, is ideal for polishing metal parts that cannot be mechanically polished; for example, parts that have complex forms or thin walls, or that bend easily. In the Electropolishing procedure, the part to be polished is immersed in a special bath and made anodic. The introduction of a cathodic metal to the bath causes the metal ions to be removed from the surface of the anodic test piece, which both decreases its surface roughness and significantly increases the proportion of chromium on the surface, which in turn improves corrosion resistance. The advantages of an ultra-smooth surface is clear, it prevents deposits that could otherwise lead to crevice corrosion. Only a perfectly smooth, bright surface is effectively resistant to the corrosion that can originate in crevices or holes. The surface roughness of electropolished stainless steel is as low as 0.2 to 0.3 µm.

Stainless Steel Before Electropolishing
Stainless Steel After Electropolishing

Mechanical Designs of Cleanroom Scales and Platforms

Constructing weighing devices with stainless steel is only one part in developing a scale suitable for cleanroom environments. Typical bench and floor scales used in everyday weighing applications (i.e. shipping scales) are usualy acquired with one thing in mind and that is how cost effective it will function. This is an excellent strategy when an application doesn’t require high accuracy or sanitary washdown specifications. However, this “typical” scale should never be used in a cleanroom or even an environment where sanitation is critical.

Combics Bench Scale with open frame design

Scales specifically designed for cleanroom environments usually have several characterisitics in common and they are as follows:

  • Open Frame Design for easy cleaning and dissefection (see picture)
  • Limited corners and crevices to elimnate gathering points for bacteria
  • Large Platforms must be equipped with some type of pressure lifting device to allow cleaning underneath the platform.

The pictures found below also illustrates how a low-profile platform scale is specifically designed for cleanroom areas. It is constructed completely of AISI 304 electropolished stainless steel, with rounded corners, and has an integrated pressure lift system satisfying basic cleanroom needs.

Pharmaline platform scale with integrated pressure lift system in CLOSED weighing position
Pharmaline platform scale OPENED to 90 degrees for easy cleaning below the scale.


Finding the right scale for a cleanroom area (as we have determined by reading this article) is not as easy as it looks. There are many factors that first must be investigated to determine if a weighing instrument will not only satisfy your accuracy needs, but also satisfy stringent cleaning regiments. To eliminate any doubts you might have, carefully inspect the scales in your current cleanroom environment and ask yourself the following questions:

  • What resolution to capacity ratio are the scales? Take the maximum capacity of the scale and divide it by the resolution (readability). If this number is less than 30,000, then you should investigate a higher accuracy alternative that would be within your budgetary requirements.
  • Does the scale look “Easy to clean?” Every scale looks simple to clean on the surface, but what does it look like when you lift the weighing pan off? Does it have an open frames design and are there large amounts of corners and crevices? Do you have a floor scale with no means of cleaning below the unit? If your scale is located in a sanitary area and it is not easy to clean, then a scale replacement should be considered.

In summary, before you purchase weighing equipment for a cleanroom it would be wise to consult an expert that has experience in the specific application your facility requires. Most major scale manufacturers have free phone or email consultation. In addition, some will even provide on-site consultation that can design a system to your particular needs and one that will meet SOP or regulatory requirements at minimal cost.

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