The EPA recommends a flow rate of 2 ft./s or greater to minimize the relative difference in velocities of suspended sediments. They also recommend a tubing diameter of 1/4 in. as well, but not for a particularly well documented reason. One unanswered question brought up in the design review meeting was the impact of tubing diameter on the quality of water samples. This short writeup discusses some of the considerations involved in the decision to use 1/4'' OD Teflon tubing for future sampler designs.
A paper published in 1985 collected some data on this subject [link to paper]. It discusses several trials where different concentrations of dissolved organic compounds were passed through tubing of varying diameters and materials at a known rate. The concentrations of the passed solutions were measured and recorded.
There are two obvious factors involved: the tubing absorption rate and friction. Absorption is based on contact time, tubing area, tubing material, and the analyte. Absorption is not a concern in suspended sediments but is critical when the analyte is dissolved carbons and gases. Contact time is based solely on flow rate.
The friction coefficient is decided by the material and the force of friction will be proportional to the tubing diameter and flow rate. The force of friction will reduce the flow rate of the sample water, increasing contact time between the tubing wall and the sample water. Lower flow rates also reduce the accuracy of suspended sediment sampling where particle mass is a dominating factor that creates a differential velocity between the particulate sizes, shown by this study [link] mentioned to me by Dr. Babbar-Sebens.
The results of the experiment suggest the diameter of the tubing has a lesser effect than the material of the tubing on absorption rates for inner diameters between 1/4'' and 1/2'', however increasing the tubing diameter decreases the absorption rate for the same material. This effect is unexplained in the paper and the relationship between tubing diameter and sample quality has very little research behind it. It is likely that the largest factor in choosing the tubing diameter is the maximum particulate size of suspended sediments, which requires a minimum diameter cross section throughout the entire hydraulic system including the solenoid valves and pump tubing.
Both the study mentioned above and this other study [link] show that teflon tubing absorbs the lowest proportion of dissolved organics. The EPA in this paper [link] also suggest that a high velocity decreases the slime buildup against the inner surface.
Because large suspended sediments (1mm+ diameter) are beyond the scope of the current sampler, Teflon tubing with a .17'' ID and .25'' OD will replace the current 3/16'' ID Silicone tubing. Compression fittings will have to be used rather than barbed fittings. The change in diameter of the tubing will increase the velocity significantly and the teflon tubing will have much lower coefficient of friction, improving small particulate movement in sample water. Teflon tubing will be nearly impermeable to gases and will absorb extremely low amounts of dissolved compounds in the sample water, making it ideal for most of our intended applications, such as sampling for dissolved organics or volatile isotopes.