Maya Ellison
Measuring plastic pipes accurately is crucial in plumbing, construction, and DIY projects. The pipe size is usually labelled on the pipe itself, but if not, it is important to measure the outside diameter (OD) and inside diameter (ID) to confirm pipe classification. The outside diameter is the measurement taken from one outer edge of the pipe to the other, spanning its full diameter. The inside diameter is the measurement of the inner diameter or 'bore' of the pipe, from one side directly to the other. The pipe wall thickness is also important to note, as it can impact the pipe's compatibility with fittings and accessories. The thickness is measured from the inner surface to the outer surface of the pipe. Various methods and tools can be used to measure these dimensions, including soft tape measures, calipers, and ultrasonic gauges.
| Characteristics | Values |
|---|---|
| What to measure | Outside diameter (OD), inside diameter (ID), wall thickness |
| Tools | Soft tape measure, caliper, calculator, string, ultrasonic thickness gauges, magnetic probe |
| Units | Metric (mm) or imperial (inches) |
| Conversions | Divide circumference by pi, then use a conversion chart |
| Compatibility | PVC pipes are measured by ID, other pipes by OD |
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What You'll Learn
Measuring the outside diameter
Step 1: Identify the Type of Pipe
Before measuring, identify the type of pipe you are working with. Plastic pipes, such as PVC, CPVC, or PEX, are typically labelled with the size on the side, which refers to the outside diameter. Copper, iron, and steel pipes, on the other hand, usually require manual measurement.
Step 2: Determine the Measurement Technique
You can use different tools to measure the outside diameter. A flexible measuring tape, string, or a caliper can be used. If using a measuring tape or string, you will measure the circumference of the pipe. Then, divide the circumference by pi (approximately 3.14159) to find the outside diameter. If using a caliper, simply place the caliper jaws around the pipe's outside edge to directly obtain the outside diameter measurement.
Step 3: Take the Measurement
When using a measuring tape or string, wrap it around the pipe's circumference, ensuring you measure from outside edge to outside edge. Mark the point where the string wraps around itself and then lay it flat to measure with a ruler if using string. For pipes with male threads or no threads, measure the outside diameter. For female pipes, measure the inside diameter, which is the distance across the middle of the pipe, excluding the pipe wall thickness.
Step 4: Convert to Nominal Pipe Size
The outside diameter measurement correlates with a nominal pipe size, which is the size sold in hardware stores. Conversion charts or online resources can help you convert your measurement to the nominal pipe size. This step is essential, especially if your pipe is smaller than 14 inches, to ensure you purchase the correct size for your project.
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Measuring the inside diameter
To measure the inside diameter of a plastic pipe, you can use a caliper. Insert the caliper inside the pipe and extend it to the opposite inner edge to obtain an ID reading. Alternatively, if the pipe is installed, you can calculate the inside diameter by subtracting twice the wall thickness from the outside diameter (OD). For example, if a pipe has an OD of 3 inches and a wall thickness of 0.1 inches, the ID would be 3 – (2 x 0.1) = 2.8 inches.
For very small diameter pipes, you may need to use an ultrasonic thickness gauge such as the 72DL PLUS™. This tool measures the thickness of the pipe wall by calculating the round-trip pulse transit time and using this measurement along with the speed of sound in the test material. The Magna-Mike™ 8600 Hall-effect thickness gauge is another option for accurately measuring the thickness of plastic pipes.
It is important to note that the diameter of a pipe is not always reflected in its name. For example, a 2-inch PVC pipe does not measure exactly 2 inches in diameter. Always refer to a sizing chart to determine the correct pipe size before ordering or installing a pipe.
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Using a conversion chart
If you need to measure changes in plastic pipes, you can use a conversion chart to find the nominal pipe size. This is the size you would buy in a store. The size of a pipe is not always the outside or inside diameter but a nominal diameter that depends on the type of pipe.
For male pipes, you need to measure the outside diameter, whereas for female pipes, you need to measure the inside diameter. You can then use a nominal diameter conversion chart to get the actual pipe size.
There are different pipe sizing standards, such as copper tube sizing (CTS) and the iron pipe standard (IPS), and each has its own conversion chart. Pipe dimensions can also vary based on specific standards and regulations. For example, in the European industrial market, the Nominal Diameter (ND) sizing system is commonly used, and it is based on millimetres. In the American industrial market, the Nominal Pipe Size (NPS) system is widely used, and it is based on inches.
If your pipe is smaller than 14 inches, you can use an online chart to convert your measurement into the pipe size. You can also check the size on the pipe itself, as it is often written on the side of the pipe.
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Measuring wall thickness
Measuring the wall thickness of plastic pipes is crucial to ensure compatibility and longevity for fittings, coatings, or repairs. It is also important to measure wall thickness to protect against corrosion, abrasion, erosion, or chemical attack.
The most common method for measuring the wall thickness of plastic pipes is to use ultrasonic gauges. Ultrasonic gauging provides a quick and reliable way to measure wall thickness without needing to access the inside diameter (ID) or cut the pipe. It can also be used to measure the thickness of in-service pipes to detect wall thickness changes during use. The measurement process involves a simple pulse/echo test where a gauge measures the round-trip pulse transit time and uses that time measurement plus the sound velocity of the test material to calculate thickness. This method only needs to be in contact with one side of the material being measured, making it a convenient option. Various transducers are used, so it is important to select a transducer according to the diameter and wall thickness of the pipe or tube being measured. For common sizes of plastic pipes, a 5-15MHz probe can be used to measure wall thickness of approximately 0.5-25mm, a 2.25 MHz probe can be used to measure wall thickness of approximately 25-80mm, and a 0.5MHz probe can be used to measure thickness over 80mm.
There are several recommended instruments for plastic pipe and tube measurements. The 45MG ultrasonic thickness gauge is a handheld device that is compatible with the complete range of Evident dual-element and single-element transducers. The 39DL PLUS™ gauge is field-proven and reliable for virtually every ultrasonic thickness inspection. It has world-class ultrasonic capability, fast scanning speeds, and fully integrated wireless connectivity. The 72DL PLUS™ advanced ultrasonic thickness gauge is another option that delivers precision thickness measurements at high speed in a portable, easy-to-use device. It is compatible with single-element transducers up to 125 MHz and is ideally suited to measure the thickness of ultra-thin materials.
For small diameter tubing (less than approximately 0.125 in. or 3 mm in diameter), the 72DL PLUS™ high-frequency thickness gauge can be used. The Magna-Mike™ 8600 Hall-effect thickness gauge is another option that uses a magnetic probe to accurately measure the thickness of nonferrous and thin materials such as plastic bottles.
Another method for measuring wall thickness is to use a piece of string. Wrap the string around the pipe and mark where it meets itself. Then, lay the string flat and measure it with a tape measure. This method may be less accurate than using ultrasonic gauges but can still provide a reasonable estimate of the pipe's circumference and, by extension, its wall thickness.
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Using ultrasonic gauges
Plastic pipes are commonly measured using ultrasonic gauges, which provide a quick, reliable, and precise way to determine the thickness of the pipe wall. This method is non-destructive and does not require access to the inside of the pipe or any cutting. It is especially useful for monitoring uniformity and ensuring the pipe meets technical specifications.
Ultrasonic gauges emit ultrasonic waves and measure the time taken for these waves to reflect off a surface and return. This time is inversely proportional to the distance between the gauge and the surface. Thus, the thickness of the pipe wall can be accurately determined. This process is known as pulse-echo detection, where the gauge measures the round-trip pulse transit time and uses this value along with the speed of sound in the test material to calculate thickness.
To use an ultrasonic thickness gauge, a small drop of coupling gel or a high-viscosity liquid couplant is applied to the test point. The transducer is then touched to the surface, and a reading is obtained. Measurements are typically recorded in a few seconds and can be stored for documentation and analysis. It is important to select the appropriate transducer for the job, considering factors such as the size of the ultrasound emitter and the strength of the signal.
There are various types of ultrasonic thickness gauges available, including pulse-echo, continuous-wave, and Doppler gauges. Pulse-echo gauges are commonly used for thicker materials, continuous-wave gauges for thinner materials, and Doppler gauges for measuring changes in frequency caused by the motion of the material. For very small diameter pipes, high-frequency thickness gauges are recommended.
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Frequently asked questions
The outside diameter of a plastic pipe is the measurement taken from one outer edge of the pipe to the other, spanning its full diameter. This is the same for metric sizes and inch/imperial sizes. You can use a soft measuring tape to measure the outside circumference and then divide your measurement by pi.
The inside diameter of a plastic pipe is the measurement of the inner diameter or 'bore' of the pipe, from one side directly to the other. You can use calipers to measure the inside diameter of an uninstalled pipe by inserting them inside the pipe and extending them to the opposite inner edge. If the pipe is installed, you can subtract twice the wall thickness from the outside diameter to get the inside diameter.
The wall thickness of a plastic pipe is the thickness of the pipe wall, measured from its inner surface to its outer surface. You can use an ultrasonic thickness gauge, such as the 39DL PLUS™ or 72DL PLUS™ gauge, to measure the wall thickness of a plastic pipe without needing to access the inside diameter or cut the pipe. Alternatively, you can use a magnetic probe, such as the Magna-Mike™ 8600 Hall-effect thickness gauge, to measure the thickness of plastic pipes.
