# Measuring the pressure through a 0.04" ID 1/16" OD metal tube

Hello, this is my first question so I hope someone can offer some advice. I would like to measure the pressure through a 0.04" ID 1/16" OD metal tube. The liquid running through the tube may block it (the liqud has bits in), so I need to measure if there is a decrease in pressure as it flows through.

I understand that I will need a 3 way connector, fluid in, fluid out, and a screw for the pressure gauge at the top. As well as an appropriste pressure gauge. Could someone please recomend a pressure gauge and connector from the RS website?

The experiments will be conducted at room temperature and pressure and the fluid will be pushed into the metal tube with a syringe.

Thank you for any help / advice.

January 28, 2020 11:30

Hi Richard, interesting question and here are a few more...
What is the liquid and the the length of the pipe, also is the syringe pressed manually or by a syringe pump?
What is the flow rate?
You can use an online pipe flow calculator to find the normal operating pressure expected http://www.pressure-drop.com/Online-Calculator/ and then select a gauge to cover say 10x this.
Other thoughts are you need to be able to purge air from the system as this will compress just like car brakes when they need bleeding and the pedal becomes spongy.
If driven by a syringe drive does this have any indication of the load being driven? That could provide an alternative to measuring the fluid pressure.

January 28, 2020 11:30

@Boss Thank you for your reply. The liquid is a mixed solution that will precipitate inside the capillary tube, so for example may be calcium chloride (in one syringe) and sodium bicarbonate solution (in another syringe). They are pressed by syringe pump. The flow rate is in the region of 1 mL / hour. For this system I have read that there could be a 1 psi difference (or less) in the system which could be at 5800 psi, which makes things a little more complicated. However, I will at least try to see what is available. The air will be purged as the liquid moves through the tube and exits the other end into a beaker. Really just trying to get an idea of a cheap and cheerful solution in terms of pressure gauge and which connector I would need to connect to the tube. If you might suggest any product with the above information it would be appreciated. Also, very interesting idea at the end there! If the tube becomes blocked, if the syringe drive had a pressure measurement it would read as having to exert more pressure to reached the required flow rate. I will look further into this aspect, thank you.

January 29, 2020 10:15

@Richard28 this sounds like HPLC or something similar? About 25 years ago my team developed a leak detector which was at the injection port of an HPLC system. At least then, detecting blockages by pressure rise was not possible/easy ( from memory it was around 2000psi?) and as soon as there was a blockage the pressure rapidly rose and a leak occured and it seemed it was always this location that leaked so that was the work around to detect and halt the system. I would be suprised if between flow and a blockage you only saw 1psi difference, that would imply there was a return flow path from the pump outlet back to the source? Not sure I contribute anything further but would be interested in the outcome.

January 29, 2020 10:14

@Boss it's for a scale blocking system (like this: https://www.chandlereng.com/products/viscosity-or-rheology/dynamic-scale-deposition-loop-system/model-5400). The way it works is you inject 2 solutions that precipitate (block) the small test coil (0.04" ID). A leak detector is a good idea, thank you. I could block the coil to see where the leak occurs, then look for a similar leak during the experiment. Although, the tube may not become completely blocked, hence the need to measure changes in pressure. But, I thank you very much for your help! Thank you.