For a PERESTALTIC pump to work correctly and accurately, these three conditions must be met.
1. The pump shaft must turn a very accurate controlled amount, and must do so repeatedly.
This is what electronic controls and a digital shaft encoder do. We control to 159 pulses per output revolution. In other words, we can divide a turn into 159 parts. We can turn the roller set an exact amount every cycle, with about 2 degrees of resolution.
2. The TUBE must then have time to open fully, and there must be enough time for the viscous liquid to flow into this cavity.
If a pump runs too fast, high viscosity liquid will not have time to enter the cavity in the pump tube. Even though a "FULL VACUUM" can exist in this cavity, normal atmospheric pressure, 14.7 PSI, is the only pressure that exists to move the liquid. This is why short distances and large diameters are essential for correct flow of liquid TO the pump.
Once liquid enters the pump head, the rollers can exert high pressures to move it forward, even through smaller diameter tubes.
When considering length and diameter of suction or deliver tube, consider these facts. With a fixed pressure to move the liquid (14.7 PSI for example), viscous liquids flow:
a) 2 times faster through a tube 1/2 as long.
b) 16 times faster through twice the diameter.
3. The TUBE must be closed fully by the rollers to ensure that no liquid leaks backwards.
Pump tubing is manufactured with a 1/16 inch wall thickness, .0625 inches.
Perestaltic pumps are manufactured to hold clearance between rollers and pump head of .095 inches. When compressed, the two wall thickness add up to .125 inches. The clearance of only .095 assures the pump tube is compressed to full closure. It is over-compressed somewhat to get full seal off at the "corners" of the tube; the edges when compressed.
If the rollers are worn or the tube is not up to full wall thickness, then the tube walls will not close off the I.D. fully and the unit will "leak" liquid backward.
The best test to see that the pump is doing this correctly is using an automotive manifold vacuum gage. This will read to a full 30 inches of Mercury. A pump working correctly will pull this gage all the way to FULL 30 inches of vacuum even when the pump is running very slowly. If the tubing has a "leak" (rollers not fully compressing the tube) then the gage will flutter. If only one roller is bad the gage will advance on two, and return on the one bad one. It is quite easy to see if the rollers are bad using this gage. Run the pump SLOWLY to observe pump performance.