Viscosity Measuring Cup Fresh Air Breathing Systems Figure 8-8.  Breathe-Cool II® supplied air respirator system with  Tyvek® hood. Figure 8-9.  Charcoal-filtered respirator. Figure 8-10.  A Zahn cup viscosity measuring cup. Fresh air breathing systems should be used whenever coatings  are being sprayed that contain isocyanides. This includes  all polyurethane coatings. The system incorporates a high- capacity electric air turbine that provides a constant source of  fresh air to the mask. The use of fresh air breathing systems  is also highly recommended when spraying chromate primers  and chemical stripping aircraft. The system provides cool  filtered breathing air with up to 200 feet of hose, which allows  the air pump intake to be placed in an area of fresh air, well  outside of the spraying area.  [Figure 8-8] A charcoal-filtered respirator should be used for all other  spraying and sanding operations to protect the lungs and  respiratory tract. The respirator should be a double-cartridge,  organic vapor type that provides a tight seal around the nose  and mouth. The cartridges can be changed separately, and  should be changed when detecting odor or experiencing nose  or throat irritation. The outer prefilters should be changed if  experiencing increased resistance to breathing.  [Figure 8-9]  This is a small cup with a long handle and a calibrated orifice  in the bottom that allows the liquid in the cup to drain out  at a specific timed rate. Coating manufacturers recommend  spraying their product at a specific pressure and viscosity.  That viscosity is determined by measuring the efflux (drain)  time of the liquid coating through the cup orifice. The time  (in seconds) is listed on most paint manufacturers’ product/ technical data pages. The measurement determines if the  mixed coating meets the recommended viscosity for spraying.  There are different manufacturers of the viscosity measuring  devices, but the most common one listed and used for spray  painting is known as a Zahn cup. The orifice number must  correspond to the one listed on the product/technical data  sheet. For most primers and topcoats, the #2 or #3 Zahn cup  is the one recommended.  [Figure 8-10] To perform an accurate viscosity measurement, it is very  important that the temperature of the sample material be  within the recommended range of 73.5 °F ± 3.5 °F (23 ºC ±  2 ºC), and then proceed as follows:  1.  Thoroughly mix the sample with minimum bubbles. 2.  Dip the Zahn cup vertically into the sample being  tested, totally immersing the cup below the surface. 3.  With a stopwatch in one hand, briskly lift the cup out  of the sample. As the top edge of the cup breaks the  surface, start the stopwatch. 4.  Stop the stopwatch when the first break in the flow of  the liquid is observed at the orifice exit. The number  in seconds is referred to as the efflux time. 5.  Record the time on the stopwatch and compare it to  the coating manufacturer’s recommendation. Adjust  the viscosity, if necessary, but be aware not to thin  the coating below recommendations that could result  in the release of VOCs into the atmosphere above the  regulated limitations. 8-9