Silicone tube with three holes, flow rate, pressure I have a silicone tube -- a saline solution flows in from one end, and then flows out of three holes of equal diameter and equal distance from each other that are along the side of the tube. What can I do to make the pressure at the three holes equal?
 A: If injected from a single end, and assuming a constant diameter pipe, the pressure decreases linearly along the length of the pipe due to friction.  Therefore equal diameter holes will not yield equal flow rates or pressures.
For equal flow rates:
To control the flow rates, you must control the injection pressure (ex. reservoir with regulator valve).
Specify a desired flow rate (per hole or total) where $Q_\text{total}=3Q_\text{hole}$.  For each hole location along the length of the pipe, $\ell$, calculate the pressure.
$$ P = P_\text{injection}-\frac{8\mu\ell Q_\text{total}}{\pi r_\text{pipe}^4} $$
Next, apply a loss coefficient depending on the cross sectional geometry of the holes (sharp-edged, rounded, etc.) to obtain the pressure at the holes.

$$P_{hole} = P*K_{L}$$
Next, for the desired flowrate, $Q_{hole}$, and known exit pressure, solve for the radius required at each hole:
$$P_\text{hole}-P_\text{exit} = \frac{8\mu\ell Q_\text{hole}}{\pi r_\text{hole}^4} \rightarrow r_\text{hole} = \left(\frac{8\mu\ell Q_\text{hole}}{\pi(P_\text{hole}-P_\text{exit})} \right)^{1/4}$$
For equal pressures:
The only practical way I can think to accomplish this is adding regulator valves to each hole, with an injection pressure high enough to overcome frictional losses to the furthest hole.
A: I'm guessing the flow rates in your system are slow so the flow is laminar. In that case the flow is described by the Hagen-Poiseuille equation:
$$ \Delta P = \frac{8\mu\ell V}{\pi r^4} $$
where $\Delta P$ is the pressure drop, $\mu$ is the viscosity of your saline solution, $\ell$ is distance along the pipe, $r$ is the pipe radius and $V$ is the volume flow rate. To get the pressure difference between the first and last hole set $\ell$ to the distance between them.
You can't do much about the viscosity of the saline, and I'm guessing the distance between the holes is fixed so you can't change $\ell$. Likewise the volume flow rate $V$ is probably dictated by the experiment. So the only variable under your control is the pipe radius $r$. The good news is that the pressure difference is very sensitive to $r$ because it varies as $r^{-4}$. So if you double $r$ you reduce the pressure difference by a factor of 16.
If the equal flow rate is really critical you could change the pipe geometry to reduce the difference in length between the holes. Something like this:

A: If your objective is to obtain precise and equal flow rates through the three holes however constrained by serial arrangement you sketched then you need to progressively increase the radii of each duct that are downstream of one another.
