Selecting Tubing for Medical and Laboratory Equipment

Deciding factors to consider when choosing metal tubing for mission-critical equipment

By Dennis Gudgel,
Plymouth Tube Company

Designers of chromatography and other analytical laboratory apparatus and medical instrumentation are often challenged when it comes to selecting the right tubing for their equipment. In demanding lab and production applications, high quality tubing is the key to maintaining the quality and purity of chemical processes and testing. Tubing that does not meet the specifications of the application can create quality problems, contaminate processes, and result in frequent and costly replacement rates because of interior tube corrosion.

On the other hand, the right tubing can significantly reduce costs for medical equipment buyers by minimizing contaminants, improving production yields, and reducing reject rates. Here are a few of the factors medical device designers and OEMs should consider as they source tubing for their equipment.

Material, grade and dimensional precision

Tubing designed for medical, pharmaceutical and laboratory applications demands top quality materials to ensure high purity delivery of gases, solvents and chemicals. For best results, many manufacturers choose precision cold drawn stainless steel tubing that has been designed, finished, packaged and tested to extremely tight tolerances and stringent standards of cleanliness and interior smoothness.

Cold drawn seamless stainless steel tubing offers dimensional control and consistency that cannot be matched by welded or hot-finished tube. The physical properties of cold drawn tube--such as tensile strength and formability to accommodate connectors and fittings--are also superior. In addition, cold drawn tubing may be specified in a wider range of custom sizes and chemistries, should the application demand.

Typically, precision tubing for laboratory equipment is available in diameters ranging from 0.25 to 1.0 inches (OD) and with wall thicknesses ranging from 0.035 to 0.083 inch. Stainless steel grades will include TP304, TP304L, TP316 or TP316L, but other grades may be specified depending on the application and the liquids, solvents or gases involved in the equipment's process.

Tube finishing

Tubing for medical and laboratory equipment requires exceptional cleanliness and smooth surface texture of the internal diameter (ID) to ensure high-purity delivery of gases, solvents and chemicals. Typically, bright annealed (BA) stainless steel tube is specified. But beyond that, there are additional finishing processes that can be specified, depending on application requirements. For an extra degree of cleanliness, the bright annealed (BA) tube can be thermocouple cleaned. And for the highest degree of smoothness and cleanliness, the tubing can be electropolished after cleaning. For highly critical applications, these additional finishing processes, described below, are key to the long life and reliable performance of tubing.

Thermocouple Cleaning

Thermocouple cleaned is defined as being free of all drawing compounds, carbon, dirt, dust and other contaminants. In this process, a mild acid is used to clean the inside and outside of the tubing, followed by multiple water rinses using deionized water that meets purity level standards established by Balazs Analytical Laboratory. A test for this degree of tubing cleanliness is done by passing a swatch of lint-free yarn or cloth soaked in acetone through the tube. After cleaning, the tubing should be capped or otherwise protected before shipping to ensure cleanliness is maintained through delivery. A nitrogen purge and capping can also be specified.

Electropolishing

For extra clean and smooth tubing, thermocouple-cleaned BA tubing is electropolished to impart a chromium-enriched interior surface that delivers several key benefits. The microscopically smooth interior surface dramatically improves cleanability and reduces particulation. It also minimizes moisture adsorption and out-gassing while improving corrosion resistance, resulting in lower levels of contamination. The higher purity level of gases and/or chemicals delivered through an electropolished tube can significantly improve yields.

For best results after electropolishing, wet and final processing of the tubing should be performed in Class 100 clean rooms with Class 10 zones. In evaluating tube cleanliness, designers should look for a moisture, oxygen and hydrocarbon analysis of less than 1 ppm for both BA cleaned and electropolished tube. For particle count analysis, electropolished tube should test at less than one 0.1 micrometer particles/0.1 cubic foot. For BA cleaned, it should be less than ten 0.3 micrometer particles/cubic foot.

Traceability, certification and testing

To ensure all quality standards and product requirements are met, be sure that the selected tubing supplier is ISO 9002 certified. All manufacturing quality processes should be fully documented--from raw materials to finished product--and the order should be fully traceable throughout every process, with specifications, certifications and supporting documents kept on file for review if required.

In addition, visual inspection and comprehensive testing should be a routine part of every tubing supplier's process. Upon visual inspection, tubing for medical and lab equipment should exhibit an exceptionally smooth and mirror-like interior surface condition to enhance the free flow of gases and liquids while resisting contamination and corrosion.

All tubing should be inline eddy current tested for phase amplitude detection of discontinuities. Tubing for highly critical applications may also be ultrasonically scanned in longitudinal and transverse directions, and the grain structure may be inspected using photomicrography. And, profilometer testing should be performed to ensure that the tubing meets the customer's required R_a smoothness standards. R_a is the universally accepted symbol for the arithmetic average roughness height parameter used to specify a surface texture. For bright annealed tubing, the R_a rating for ID surface finish should consistently fall within the 4-8 range. For high performance, critical application tubing, look for ID surface finish of 12 R_a or better. If uncertain as to the R_a demands of the specific application, the tubing supplier should be able to help.

Added value for end users

Specifying high quality tubing for medical devices and other analytical equipment pays off in several ways. Obviously, quality components will contribute to the quality, integrity and performance of any OEM's products. But, beyond that, taking the time and care in the product design and development phase to select the right quality tubing for the intended application also adds value for customers. Equipping mission-critical products with exceptionally efficient gas and liquid delivery systems will ensure that end users achieve maximum production yields with fewer performance problems, and will reduce the need to replace tubing due to contamination or corrosion.

For more information:

Circle 450 - Plymouth Tube Company