Gently Vacuumed:

A term widely used, but rarely measured!

"Carefully vacuumed" is often used to describe the initial cleaning treatment of accessioned museum objects and carried out throughout the Smithsonian Institution museums and in museums, archives, and historical societies around the country. However, there is no clear definition or measurement of what constitutes the appropriate vacuum suction, or what level of vacuum suction is used. With volunteers, contract workers, collection managers, conservators all carrying out this procedure, a series of experiments is being devised to provide some guidance and understanding of this primary cleaning procedure.

Intermural: Manufacturers of vacuum cleaners used a standard test promulgated by the American Society for Testing Materials (ASTM), #F-558-98 "Standard Test Method for Measuring Air Performance Characteristics of Vacuum Cleaners" to measure the suction strength of a vacuum cleaner. This testing describes the suction power, defined as a pressure difference measured in inches or millimeters of water. A "water lift" gage or water manometer is used to measure how many inches of water the vacuum would pull up in a tube. Because the density of water is affected by temperature, altitude, barometric pressure, and humidity, these factors are calculated into the final test method result. In this way a manufacturer in Denver or in the Alps can compare his product with one operating at sea level on a rainy day-local conditions are explicitly discounted.

Water Manometer
Water Manometer measures pressure in inches (or cm) of water column

To measure the volume of air pulled through a hose or an opening, an anemometer is used to measure air flow. This air velocity is described either in cubic feet of air per minute (cfm) or in cubic meters per second (cms). The value will depend upon the size of the opening. Manufacturers 'correct' this measurement to account for deviations from standard temperature and pressure conditions.

Water Manometer

Because these values are also affected by the wattage or electrical input to the vacuum cleaner motor, they are also corrected by voltage and wattage readings. In order to adjust the "speed" on the vacuum cleaner in museum laboratories, a motor speed control device may be added or purchased already attached to some vacuum cleaner models. Technically, these are not 'rheostats' but variable speed control devices (VSCD).

Intramural: For internal comparisons among conservators and volunteers at the same museum, the extensive calculations need not be made, but the principles and the equipment used for testing by manufacturers provide a basis for comparing different vacuum cleaning styles and techniques. In conservation laboratories, variable speed control devices (VSCD) are used to reduce the electrical input, but the manometer and anemometer have not been regularly employed. In a series of experiments, the water manometer and anemometer were used to answer some basic questions.

Does holding the vacuum cleaner slightly off the textile make any difference? How about pressing the brush into the textile? Does the variable speed control setting control the suction power adequately to define the vacuuming quality by specifying this setting? To answer these questions, the water manometer was used to measure the suction strength at the edge of the metal nozzle and at various small distances, setting the variable speed control device at different levels (Chart 1). You can see that the difference in suction strength was quite small if the nozzle was distanced from the measuring device (or textile) but became very pronounced the more closely the manometer was to the nozzle. At the setting of 60, the variable speed control device reached the outside limit of the manometer's measurement scale 0.25 cm from the nozzle's edge.

Effect of Distance on Nozzle Edge

What effect does pressing the brush against the textile produce? Is this controlled by using a variable speed control device? With an upholstery brush (4cm long bristles), the water gage was used to measure at different VSCD levels (see Chart 2). When the brush is not pressed, the difference in the vacuum motor speed setting is not very important; if the operator is pressing down on the textile, the suction strength exerted is increased tenfold. The operator's hand will dramatically affect both the level of cleaning and the force exerted on the textile. A lowered variable speed control can reduce the level of increase, but the operator of the vacuum cleaner nozzle controls the suction strength to a much greater degree.

Effect of pressing down on brush

Does the suction strength vary from the center of the nozzle to the outside edge as much as it does from the distance away from the nozzle? Does the center of the nozzle area pull more suction? Or is the distance away from the nozzle more important? With the water manometer, the suction strength is consistent throughout the opening of the nozzle (see Chart 3). The distance of the nozzle away from the textile is the most important factor in determining the pull exerted on the textile.

Effect of Location

Is the water gage affected by the angle of the measurement? How much care must be taken when using it to get an accurate reading of suction strength? The end of hose attachment should be held in the same way for each reading (see Chart 4). There is a distinct difference between the measurements made parallel to the direction of the suction and those made perpendicular. This is true at all settings on a variable speed control device.

Effect of variable speed

Would it be better to measure with an anemometer? Would a measure of air flow be more precise? Would it show the difference between using an upholstery brush and a bare nozzle? It's possible to measure the air flow in terms of velocity at the lower variable speed settings (Charts 5 & 6) with the type of anemometer routinely used to measure the air flow for 'elephant exhaust trunks' and fume hoods (Figures 1 & 2) but only at lower variable speed settings. The slight reduction in air flow with an upholstery brush is monitored; it remains less than the difference produced by increasing the variable speed setting.

Air Flow chart

Air flow chart

Does the anemometer show any difference in air flow with a screen to protect the textile being vacuumed? Does the screen protect the textile against excess suction?
Yes (see Charts 7 & 8). The screen reduces the air flow especially when measured on the 'outside' where the textile would be. There is a 33% reduction in air flow because of the screen. However, if the upholstery brush is pressed down into the textile, the screen has very little protective affect: the pressed brush exerts a 578% increase in air velocity upon the textile as measured by the anemometer.

Air flow at brush attachment

Air flow at brush attachment

Summary: It is not enough to specify the vacuum cleaner, variable speed control setting, and type of screening. The most important aspect of 'gentle vacuuming' is the hand of the person doing the cleaning. Manometers measuring suction strength and anemometers measuring air flow can help to establish the proper method for vacuum cleaning delicate, unique antique textiles. Routine intermural exercises can assure volunteers, collection managers, and conservators that they are providing a consistent and appropriate treatment for fragile textiles.


#F-395-00 "Standard Terminology Relating to Vacuum Cleaners," Annual Book of ASTM Standards 2002, vol. 15.08. West Conshocken, Pennsylvania: 2002.

#F-558-98 "Standard Test Method for Measuring Air Performance Characteristics of Vacuum Cleaners," Annual Book of ASTM Standards 2002, vol. 15.08. West Conshocken, Pennsylvania: 2002.

Pessa, Joanna, and Christine Richardson, CAL Report # 5432, "Summer Internship Project 1993," pages 28-30, Appendices E & F.

Stavroudis, Chris, "Never Mind the Bollocks, Here's the HEPA Chart," WAAC Newsletter, vol. 24 #2 (May, 2002):13-15.


Prepared by Mary Ballard, Senior Textile Conservator, 2003