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Assessing the compression from hosiery
Explaining the testing of this specialised product.
When considering compression hosiery or any system of compression bandage, the most relevant aspect is the pressure exerted on the leg’s surface – known as ‘compression value’.
Graduated support hosiery, for example, is designed to exert a graduated compression pressure of between 6 mmHg and 12 mmHg on the legs while standing or walking, in order to relieve minor symptoms of heavy, aching legs and help prevent deterioration of the veins that could lead to more severe conditions.
On graduated compression hosiery, the degree of compression is designated by classes, depending on the pressure measured at the ankle measurement position.
The pressure exerted by hosiery can be determined using the SATRA STM 579 stocking compression tester.
History of the compression tester
The original project for the design of a hose pressure tester for measuring leg pressure from hosiery was initiated by the Hosiery and Allied Trades Research Association, which was founded in July 1949 to support the hosiery section of the textile industry by conducting and publishing relevant research, similar to how SATRA supports the footwear industry. Unfortunately, this hosiery association went out of business in the mid-1980s.
This hose pressure tester is the instrument specified in the three British standards being developed at the time: BS 6612, BS 7672:1993, and BS 7563:1995. These three standards have been recently combined into BS 661210:2018 (amended to BS 661210:2018+A1:2022), which supersedes all three previous standards. BS 661210 still specifies this specific instrument.
There was a period when this test equipment became unavailable, so SATRA started manufacturing the SATRA STM 579 in order that equipment was available for these active standards.
SATRA STM 579
The stocking compression tester (figure 1) comprising two main parts.
a) An adjustable former, comprising the following:
- a flat former, consisting of two metal bars arranged to give a simplified leg form, the movable top bar of which can be lowered by hand lever to enable the garment to be loaded
- a pivot at each end of the former which can be moved to different holes to enable the former to adopt a wide range of girths, thus enabling the testing of different sizes of hosiery
- a fixed lower bar having two curved attachments, simulating the calf and thigh, with clips attached to the movable top and fixed lower bars carrying conventional suspender-type fasteners to hold the garments, the clips being movable along the bars to hold garments at the correct position according to the size
- a traverse rail having marked positions to which the measurement head is set, the marked positions locating the measuring positions at 100 mm, 290 mm, 310 mm, 330 mm, 530 mm, 600 mm and 670 mm from the sole point (although other measurement positions can also be used)
- a sole point, representing the sole of the foot, lying at the intersection of the line of the sole of the simulated foot with a line midway between the outer surfaces of the movable top and fixed lower bars.
b) A measurement head (figure 2), having the following features:
- a movable tongue 25 mm wide and a guard plate, which prevents excessive movement of the tongue. It was designed to be sensitive to tension in the garment, only in the circumferential direction. It also has a digital display, which shows a reading proportional to the fabric tension.
Equipment for calibrating the measurement head (figure 3), comprises a pin, spring, tape, reference weight of mass (400 ±2 g) and a small screwdriver for adjusting the two potentiometers.
Annex A of BS 661210 – ‘Graduated compression hosiery, anti-embolism hosiery and graduated support hosiery. Specification’ describes the method of test for the compression value of compression and support hosiery.
The garment is loaded onto the adjustable former of the stocking compression tester, simulating the wearing of the garment on a human leg. A measuring head pressed against the stretched fabric at various points along the length of the adjustable former is used to determine and display a value for the circumferential tension in the fabric. This value is converted to a value for compression pressure.
The SATRA STM 579 stocking compression tester can also be used to measure the pressure exerted on the leg using elastic methods – for example, compression bandage systems. It can measure how the pressure levels vary at different locations of the leg with different circumferences and with different tensions on the bandages. Bandages will often incorporate user instructions to help ensure that they are wrapped to the correct tension. The STM 579 machine can be used to check the accuracy and ease of use of these applications.
The BS 661210 test method incorporates detailed calibration instructions specific to this type of machine, so only this method of measurement can be used. Prior to commencing the test, the measurement head shown in figure 2 must be calibrated using the pin, spring, tape and reference weight of mass (400 ±2 g) – see figure 3. First, the head is moved into contact with the fixed lower bar and the potentiometer is adjusted to give a reading of 0.00. Then, with the tape and weight in place, the head is moved to firmly contact the tape (this first contact is to stabilise the tape and no reading is taken) and the potentiometer is adjusted to obtain a reading of 1.82.
The British standard requires a sample set of 15 individual test pieces, which need to be conditioned in a standard atmosphere of 20 ±2°C and 65 ±5 per cent relative humidity (RH) for not less than four hours before testing. The testing must be conducted in the same temperature and humidity conditions.
The next step is to ensure that the machine is correctly set up. The calf and thigh attachment and suspender clips must be positioned in such a way that the form matches as closely as possible the intended limb size for the hosiery. The measurement positions for the ankle, calf and thigh must also be determined before testing – the size of the form should be adjusted to match the appropriate girth size for each individual measuring position.
Different techniques for loading the garments are described in the BS 661210 standard, depending on the type of stocking – for instance, thigh-length stockings and below-knee versions. Unlike hose, compression bandage systems (the loading technique for which is not described in this standard) must be hand-wrapped on the machine and with the movable top bar raised. Therefore, the amount of extension (and, subsequently, the level of pressure applied) will be determined by the accuracy of the person wrapping the bandage. The bandage should be supplied with instructions on how to achieve this in real-world situations.
When testing bandage systems, it is important to make sure that the product is stretched over the correct part of the leg appropriate to the product’s use, and the rectangular perimeter dimension is appropriate to the circumference of the part of the leg for which the product is intended.
The tension on each measurement position can then be taken with the measurement head in the vertical position. This is done by sliding the head along the traverse rail, until the edge of the base corresponds to each mark of measurement positions for the ankle, calf and thigh. The measurement head is moved forward to contact the garment, and the reading of the tension show on the digital display is recorded after two to five seconds. For each measuring position, the displayed tension readings must then be converted to compression values, expressed in mmHg.
The mathematics behind compression value
It is important to verify the equation used in these standards and how it was derived. This is important both for machine development and for the BS 661210 standard, the writing of which SATRA was involved in. This standard now includes an explanation of the mathematics.
The main equation used for measurement is the ‘Laplace equation’, which allows the wall tension to be calculated for a given pressure inside a vessel, or vice versa.
The BS 661210 standard uses the following equation to determine the compression value:
p = 4r/g (‘equation 1’)
where ‘p’ is the compression value (in mmHg), ‘r’ is the reading displayed, and ‘g’ is the girth (in centimetres).
The machine is calibrated with a tape which is tensioned with a 400 g mass, and the machine adjusted to read 182. Although it may be assumed that this is 182 N/m, it is actually 182 x 1.17808 N/m. This is because the force with a 400 g mass at standard gravity is 0.4 kg x 9.81 m/s2 (newtons), so:
force on tape = (head reading x 0.4 x 9.81)/182 = head reading x 0.0215604 (‘equation 2’)
To measure the ‘pressure’ exerted by the hosiery, the Laplace equation is used:
pressure = tension/radius of tube (‘equation 3’)
The Laplace equation allows for the calculation of the wall tension in a tube with a certain pressure inside it, so, for example, it can be used to establish how thick a boiler tube should be made. In the situation being discussed in this article, it is necessary to measure the tension of the wall and subsequently calculate the pressure that the wall is exerting. This can be done because the hose or stocking must exert an equal and opposite force.
The head is designed to measure tension in newtons per metre (N/m), which is
force/width of the head (‘equation 4’)
The head is 1 inch wide, which is 0.0254 m.
Therefore, using equation 4 and replacing the force by equation 2 and the width of the head by 0.0254 results in the following:
tension in hosiery (N/m) = (head reading x 0.0215604)/0.0254 = (head reading)/1.17808 (‘equation 5’)
Hence, the head reading is actually in units of 1.17808 N/m.
The next stage involves revisiting the Laplace equation (equation 3) and converting radius to circumference to produce the following:
pressure = wall tension (N/m)/radius of tube (m) = tension/(circumference (m)/2π) = tension x 2π/circumference (‘equation 6’)
This identifies the tension to use in equation 6, so replacing the value of the tension by equation 5 gives the following:
pressure (Pa) = ((head reading/1.17808) x 2π)/circumference (m) = (head reading x 5.334)/girth (m) (‘equation 7’)
However, while the tension is still in units of 1.17808 N/m, pressure in pascals and girth in metres, it is necessary to obtain an answer in mmHg. So, to convert pascals into mmHg, the figure is multiplied by 0.0075 (1 mmHg = 133 Pa), and by 100 to obtain girth in centimetres.
The compression pressure in mmHg measured in a garment with a girth expressed in centimetres is:
Compression pressure = (head reading x 5.3334 x 0.0075 x 100)/girth (cm) = (head reading x 4)/girth (cm) (‘equation 8’)
which is the same as equation 1.
This, at last, gives the pressure exerted by a 1 inch-wide band of the compression product.
Compression hosiery and systems of compression bandage are used in medical situations, either to reduce the incidence of deep venous thrombosis or to relieve aching legs. In both situations, it should be designed to exert a certain amount of pressure on the legs, depending on the medical condition to which it applies. This is why manufacturers should always test their product to know the compression profile.
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