Table of Contents

Tube Size


Dimensions of tubes

There are three basic dimensions of tubes: the outside diameter (OD), the inside diameter (ID), and the wall thickness (WT). These dimensions collectively constitute the tube size. The measurements are specified and expressed in inches or fractions of an inch.

An IV set is a medical device that uses a tube as one of its parts. However, the specifications depend on how the tube will be used. Thus, every factory may use different tube sizes depending on the nature of the operation. Check other factors affecting the IV set

Outside diameter (OD)

The OD of a hollow circular tube measures the outside edges of the tube passing through its center. In another saying, it is the distance from one outside edge of the tube to the opposite outside edge, measuring straight across the face of the tube or across a cross-section that runs perpendicular to the tube’s long axis. If you have access to the end of the tube, you can measure its OD with a ruler or measuring tape. If you cannot reach its ends or the tube’s diameter changes, calculate the OD using the tube’s outside circumference. OD is also known as the outside diameter or external diameter.

The OD should not be confused with the nominal diameter (ND). The ND is a dimensionless number that defines the tube size, while the corresponding OD is really measured and usually a bit larger than the ND.

For example, for a 4″ schedule 80 tube, 4″ is the nominal size, while the actual outer diameter is 4.5″. However, in medical device parlance, the OD of the tube used for the IV set is 4.1 mm.

Inside diameter (ID) 

The ID of a hollow circular object like a tube is a measurement of the distance of a straight line from one point on the inner wall of the object to an opposite point also on the inside, with the line passing through the center. ID is also known as inner diameter or internal diameter.

The ID depends on the object’s OD and wall thickness. The OD and WT are essential in deriving the value of the ID. The ID of a tube is calculated by subtracting twice the tube’s wall thickness from the total external or outside diameter. This calculation removes the tube’s walls from the measurement, leaving only the inside diameter.

In the field of medical devices, the ID of the IV set tube is usually 3.0 mm.   

Wall thickness (WT)

The distance between the tube’s inner and outer diameters is called the tube’s wall thickness. The said dimension is a crucial factor in the design and construction of the tube system, and manufacturing engineers measure it using millimeters or inches. Engineers can determine the wall thickness by subtracting the ID from the total OD divided by two.

Functional importance of the dimensions

Outside diameter

The OD plays a vital role in finding out if the IV set tube fits into proper connections and equipment in the clinical environment or hospitals.

Inside diameter

Inside diameter measurements are crucial for flow and volumetric calculations. The inside diameter of a tube must be large enough to accommodate the expected flow rates and volumes. Inadequate inside diameters will result in a decreased flow rate.

The flow rate of the IV fluid is significant in intravenous therapy. Medical staff adjust the flow to a particular rate required in every medical situation. Suppose the ID of the tube is much less than what is necessary. A patient may die from inadequate fluid entering his circulatory system.

In the paper titled “Evaluation of 3-mm diameter intravenous tubing for the rapid infusion of fluids,” published in “Emergency Medical Journal, Volume 4, Issue 3” on September 01, 1987, by G.D. Cross, the use of 3-mm-diameter (ID) tubing was compared to the conventional 4-mm tubing and both tubes were connected to standard large-gauge intravenous cannulae. The lengths of the intravenous tubes were limited to between 80 cm and 200 cm. The cannulae used in the study were gauges 16, 14, and 13. The study revealed that using 3mm-diameter infusion sets, with even shorter lengths, limited the maximum flow the IV set can deliver through the gauge 16 cannula or larger. The reduced performance of the 3-mm tubing makes it unsuitable in emergency medical situations when rapid infusion of fluids is needed.

Table 1 shows the findings of the above-cited study. 

Table 1. Flow rates in millilitres per minute through the cannulae and tubing combinations.

Wall thickness

The right tube wall thickness is essential for a system’s safe and effective operation. If properly and adequately designed, it ensures that the tube can withstand the strains and pressures it will be subjected to during its operating life, avoiding pipe failures, leaks, kinks, and other potential risks. Aside from providing overall safety and dependability of the system, proper tube wall thickness guarantees that the tube meets the necessary safety requirements and regulations for a particular industry or application.

Determinants of wall thickness   

Medical technicians consider several factors when designing and manufacturing an IV tubing. These are (1) the internal pressure of the fluid being conveyed through the IV tubing, (2) the IV tubing construction material’s strength, (3) the diameter of the IV tubing, and (4) the nature and properties of the material being conveyed in the IV tubing.

Internal pressure  

The material conveyed through the tube, such as the IV fluid, exerts a particular amount of pressure. The said pressure varies from one material to another, such as pure fluid or blood. Medical technicians have references in the manufacturing industry for the IV tubings that state the internal pressure from a particular substance being conveyed in the tube. If a substance exerts higher internal pressure, the wall thickness must be increased to prevent the tube from exploding. The wall thickness must be increased as the internal pressure rises.

Tube’s material

Medical technicians use different kinds of materials to produce the tube. They provide lesser wall thickness for tubes made of more robust materials than those fabricated with weaker materials.

Diameter of the tube

The diameter of the IV tubing also affects the wall thickness. IV set tubes with greater diameter require greater wall thickness to withstand internal and external forces acting on them.

Characteristics of the material being conveyed    

The necessary wall thickness may also depend on the characteristics of the liquid being conveyed through the IV tubing. Blood has greater viscosity than pure fluid, so blood transfusion requires an IV tubing with a relatively thicker wall to prevent tube failure.

Last words on tube size

The principle governing the operation of tubes, whether the big ones used in manufacturing industries or the smallest ones used in medical devices such as IV sets, is the same. The design and manufacturing of a particular tube depend on how a company will use it. For instance, the tube in the IV set used in clinical settings and hospitals must have the specifications to meet the expected fluid delivery for a crucial medical situation. Medical workers must avoid discrepancies in the tube’s design and expected output during operation.   

Company Introduction

Established in 1994, Liaoning KANGYI Medical Equipment Co., Ltd is a leading medical manufacturer in Liaoning, China. Certified by the Chinese National Medical Products Administration, ISO 13485, and the European Union’s CE mark, quality is our cornerstone.

Specializing in disposable medical products such as IV sets and syringes, and medical parts, our annual production exceeds 600 million units. With a workforce of over 300, including 50 in technical roles, our facilities cover 20,000 square meters, including a 100,000-class purification workshop.

liaoning kangyi medical equipment co. ltd
Liaoning KANGYI Medical Equipment Co., Ltd

Committed to customer satisfaction, we prioritize clear communication and efficient order processing. We invite collaboration and look forward to providing high-quality products and service tailored to your needs. Contact Us Now!



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