Can a sheet ever be perfect? If so, who is qualified to pass that judgment? Is it the manufacturer, who is trained in fabric production? The merchant with the experienced staff, who buys sheets year after year? Is it the critic with no formal techniques but a strong sense of touch and a notebook? Or is it a series of mechanical and thermal test modules that simulate how our bodies perceive the sensation of comfort? We are convinced it is the last of these.
What is Comfort?
Comfort is a highly subjective perception. When we touch a fabric, the receptors in our skin are stimulated and the neural information is subjectively interpreted, i.e. our perception of sensations.
Neurophysiology of sense perceptions. Hatch et all, 2000.
Over the years, researchers have invented various objective methods to quantify the perception of sensations. The goal was to measure and predict the perception of comfort in fabric. Nike, Under Armour and other sports apparel giants developed tests in the 1980s to scientifically evaluate the “fabric hand,” a term of art in the textile industry that describes fabric quality and performance.
PerfectLinens.com uses these precise and objective testing instruments to evaluate the comfort of sheets. However, your best guide for touch is your own, not ours. We are a guidepost. We bring consistency and a uniform, easy-to-understand scoring system to measure and describe comfortable sheets. We’ve solved the math of comfort for sheets. More importantly, we help you know which sheet is right for you. We believe the first step in choosing a sheet is deciding what type of comfort you want.
Once you know a sheet is scored and evaluated, how do you find those favorite sheets that mesh with your specific preferences? There are some shorthand terms that apply when describing the attributes of comfortable sheets, and these terms are a great way to identify sheets you like and express those preferences with others. Superior sheets can best be identified by top scores within each "touch" descriptor. Here are eight common touch descriptors:
Temperature Cooler Warmer
Breathability Stuffy Arid
Smoothness Slick Textured
Body Delicate Beefy
Weight Heavy Light
Suppleness Crisp Creamy
Friction Slippy Grabby
Behavior Snug Airy
With low forces applied, as when touching and manipulating fabrics, instruments with mechanical sensors can quantify each of the touch descriptors.
Temperature measurement is accomplished by inserting a precision-sized sheet sample between a heated surface and a cool water bath. Instruments measure the amount of power required to increase the water temperature a specified amount. Low power requirements means the sheet is conducting heat and will feel cooler to the touch. High power requirements mean the sheet is impeding the flow of heat and it will feel warmer.
A sheet’s ability to diffuse moisture into the environment is measured by vapor transmission tests. The simplest tests utilize dishes of water that are covered by various sheet samples. Each is weighed and placed on a vibration-free turntable that rotates to ensure all dishes are exposed to the same average ambient temperature. The dishes are re-weighed in 24-hour intervals to calculate the rate of moisture vapor loss. Dishes that lose the most weight indicate greater diffusion of moisture vapor through the sheet.
Surface smoothness is evaluated by stretching a sheet and rolling a miniscule ball across the surface. A sensor measures the amplitude of the ball's movement as it rises to trace the contours of the yarn or falls into the gaps between yarns. Measurements of the distance between the ball’s upward and downward movements provide a separate wavelength image of the sheet’s surface.
To evaluate softness in both directions – top-to-bottom and side-to-side -- the sample is rotated 90° and retested. Because some sheet weaves (satin, knit for example) create different surfaces on the face and the reverse, the tests are repeated on the opposite side.
Body is a combined measure of weight and thickness. The dimensions of a sheet sample are precisely measured. The sample is placed in a weighing dish that prevents the edges from unbalancing the measuring device. Multiple weight tests are conducted and the results are averaged.
As fabric absorbs water from the ambient atmosphere, the relative humidity and temperature are tightly controlled within an enclosed environment. Airflow is also eliminated during measurement.
Sensors are lowered onto a sheet swatch and force is applied. Thickness is measured under maximum compression for the sheet type and also at zero pressure. Changes in the thickness of the sheet are measured and the two measures are averaged.
The pliability of a sheet is evaluated in part by fabric-bending tests. The edges of a sample are mounted to the vertical instrument to eliminate the effects of gravity. One edge is held stationary while the other edge rotates forward and backward around a circular arc. Sensors measure the amount of force required to bend the fabric in both the forward and backward movements.
To measure a sheet’s ability to bend, top-to-bottom as well as side-to-side, the sample is rotated 90° and retested.
Pliability / Shearing
Shear tests complement fabric-bending tests in evaluating pliability. A sheet sample is clamped on opposing edges, which move in parallel and perpendicular directions. The stress in two simultaneous directions causes the sample to deform in an angular shape. The yarns slip and slide past each other in response to the angular forces. Sensors measure the ease or difficulty with which the yarns adapt to the angular stress. Sheets with higher thread counts or tighter weaves have more ‘locking’ intersections. The yarns are less able to adapt to the stress and feel generally less pliable.
Surface friction tests are conducted on a sheet sample that is stretched on a rack. A sensor rests upon the fabric surface and is pulled across it; the amount of force required measures the resistance of the sheet’s surface. Higher forces indicate a rougher fabric. The tests are repeated after the sample is rotated 90° to evaluate surface friction in both the top-to-bottom and side-to-side direction.
Sateen and knit constructions produce different surfaces on the face and the reverse of the sheet, so friction tests are repeated in both directions on the opposite surface. It is important to recognize which side is touching the skin because surface friction is very different from one side of those sheets to the other.
ElasticityStrength and elasticity tests characterize the “give” of fabric under pressure. A sheet sample is clamped on opposing edges and force is applied as the clamps move apart. Sensors measure three fabric responses to the stretching force: 1) the maximum force before the yarns break and the sample bursts – tensile strength; 2) the percentage of stress at maximum force without bursting -- elasticity; 3) the percentage of recovery of the yarns when the applied force is removed -- resilience.
Strength and elasticity tests are conducted both in the top-to-bottom and side-to-side directions.
Correlation analyses of the three fabric responses provides a summary evaluation of the sheet. The factors that are included in the correlations are the i) number of yarns (thread count); ii) the size of the yarns (denier); iii) the fiber (i.e. cotton; polyester; microfiber polyester, silk, flax); iv) the quality of the fiber (for cotton: Supima; Pima: Egyptian; Upland) and v) the yarn quality (spinning wind-down; plies).