 | Pascal’s principal says that in a closed container a change in pressure at one location will be experienced equally in any other part of the pressure circuit. This principal allows pressure metrologists to simply connect a device under test (DUT) to a reference, change the pressure to a value they want to test, then compare the reference and device under test. But I probably should not have used to the adjective “simply”. There is a lot to consider when deciding how to connect the DUT to the reference. Here is what I like to think about when getting ready to design a pressure circuit. |
| Pressure hardware choices include tubing, fittings, adapters and valves. |
Safety
First and foremost the most important aspect when designing pressure systems is safety. Safety is achieved by ensuring the rating of all pressure components is acceptable, the connections are connected properly and that the hardware has not aged significantly.
Many times, when we are in a hurry and trying to meet schedules, assumptions are made about the pressure rating and age of tubing, fittings, adaptors and valves. Never lose sight of the pressure rating of your hardware. Not all hardware will have the pressure rating stamped or engraved on it. So my suggestion would be to organize hardware that is used for pressure calibrations by the rating and the type of media it is used with such as gas, oil or water. Also, it is always a good idea to label the hardware with the correct pressure rating if it is possible. And finally, inspect the threads of the hardware to ensure they have not worn or deformed and throw away those that look questionable.
It is true that gas pressure is more dangerous than hydraulic pressure due to the stored energy of compressed gas. If other effects do not get in the way, many pressure metrologists will try to use hydraulic fluid to calibrate instead of gas. This is fine as long as the devices being calibrated are compatible with the fluid being used. Also if the intent of the device under test is to be used in gas you would have to be able to clean it out. Water or fluorocarbons are sometimes used to help ease the task of cleaning of a device calibrated in a hydraulic fluid.
Internal diameter
As long as there are no blockages in the pressure hardware Pascal’s principal will be true. However the amount of time for a pressure change to be experienced at another location in the pressure circuit will depend on the internal diameter of the tubing. In a pressure circuit pressure changes are transmitted to other locations. It is a little similar to the transmission of a sound wave. If there are restrictions in the pressure circuit this transmission can be “interrupted” and pressure will not stabilize.
A good rule to follow is to use the lowest inside diameter that is possible without causing delayed responses in pressure circuits.
Flexibility
If you have to connect and reconnect a lot of different types of devices under test to your reference sometimes it is convenient to use flexible pressure lines. This is fine, but beware of the fact that this type of tubing tends to flex when exposed to high pressure and make it difficult for a pressure circuit to become stable. Also many flex lines that go to high pressure have very small internal diameters and will restrict lower pressures and also causing pressure instabilities.
Thermal Properties
Simply put, you want something that reacts to ambient temperature quickly for high pressure applications (>100 psi, 700kPa), such as stainless steel, and for very low pressures (below 5 psi, 35 kPa) something that can insulate ambient temperature from the pressure circuit. ¼ inch PFA tubing is fine for pressures up to 100 psi with the proper compression fittings. High pressure tubing is almost invariably stainless steel anymore.
Ease of connection
Quick connectors are designed so that fittings can be snapped together in an instance. This is convenient for a high volume pressure calibration lab. But normally you would have to connect the quick connect fitting to the device anyway so is not always a great time saver. And quick connectors are prone to leaking. My advice would be to use easy to use dependable fittings such as compression fittings, such as coned and threaded and flare fittings (AN4 as an example) that many pressure hardware manufacturers offer. Tapered NPT (National Pipe Thread) fittings are limited in pressure range and are cumbersome because you have to use a sealant such as Teflon tape.
Valves
There a few types of valves that can be designed into a pressure circuit. Primarily these are ball, needle, stem and gate valves. In general needle valves are great for high pressure applications, but can cause changes in pressure as they are closed or opened, and can be over tightened. This is true also to some extent with stem valves. Ball valves are nice for low pressure because they are not restrictive, do not change the pressure significantly when opening or closing, and cannot be over tightened, but tend leak more frequently. Gate valves are not very common but give the same attributes as a ball valve but go to higher pressures and leak less frequently.
Filters
It is always a desire to protect our references from contamination that may be in the devices we test. If a filter is put between the reference and test in a pressure circuit there is a good chance that the filter will collect contamination over time (that’s what they are supposed to do) and will create a significant restriction and cause pressure errors. A good way to not have to use filters is to vent pressure away from the reference as is shown in the figure below.
Vent pressure away from the reference to avoid the use of filters which may cause errors.
Take the time to understand your pressure hardware
And that’s just a summary. It is true that being a pressure metrologist usually means you are a very special plumber. It seems that almost all pressure accidents are with beginning technicians or metrologists that do not take the time to understand what they are using. Sometimes this comes when we are in a hurry. Think about the fact that if you are in a hurry, and do not take the time for your pressure hardware, you could have a leak or instabilities while calibrating, or worse yet experience personal injury, in which either case the calibration doesn’t get done anyway.
Related information
- Learn more about selecting a pressure calibrator.