Archimedes-born in 287 BC – was a practical man and took full advantage of Laws and the repeatable behavior of Physics. He invented a variety of machinery including a screw pumping device that lifted water from a river to irrigate field crops. Today we employ the same laws and engineering to manufacture a variety of pumps for many different applications. All of these pumps make life easier and surely Archimedes would approve. Regardless of the type, under some circumstances they still leak or appear to leak, what can we do about it?
The first experience most of us have with leaks is a bicycle tire, pushing our bike home and looking for dad to help us fix it. The usual reply is, “I’m going to show you how to do this once and then you fix the rest yourself”. Depending on your age and experience you usually get the idea of fixing a flat but along the way you are introduced to a bicycle pump. By design they are a simple pressure generator, but to a young child trying to fix a flat, they can be an inexhaustible supply of leaks and “How does this thing work?”. So being the budding instrument technician that you are, you begin to take the bicycle pump apart to figure out what is going on. Skip ahead past two school graduations to a career and you have a hand pump in front of you giving you grief. Where is it leaking and how can I find out fast if it truly is leaking?
The quickest way to check a pneumatic or hydraulic pump for leaks is to employ the adiabatic temperature affect. Try generating about two-thirds of a pumps rated pressure and then rapidly open and close the vent valve or quick release. If the pump is not leaking and the Laws of Physics hold true-which they will- the pressure in the pump will drop quickly and then rise and become stable. By simply applying the same gas and fluid laws we can actually do the reverse. Generate pressure rapidly, stop quickly and the pressure will fall. Many inexperienced first time user do this and expect the pressure to stabilize immediately. This is simply not the case; the simple generation and introduction of heat will affect the pressure which is exactly what the user should expect to see. In fact, a skilled technician familiar with these simple principles can quickly stabilize a pump to a target pressure point for calibration. Beyond this simple lesson though, there are other factors that can eliminate the complexities of dealing with a leaky pump.
For the first time purchaser of a new pump, there are questions you should ask of your supplier about the pumps they sell. Definitely ask your supplier if they have a service department that will help you troubleshoot a faulty pump and do they keep repair kits on hand. Also, be sure to ask if the pump contains standard O-rings compatible with your pressure media and does the pump include a repair manual for ordering replacement parts. This is a must; standard o-rings are a readily available item and are often cheaper to get than the repair kit itself. A pump with Asian or European sized o-rings is a recipe for a four to six week time frame on the repair and or paying extra for an over priced repair kit.
Very often there is more than one choice of pump, do you purchase a pump for one specific range or a multifunction pump. Although the Multi-function has the all in one appeal, they often have the downside of complex valves and the possibility of oil inclusion into the pneumatic side of the pump during repair and operation. The introduction of liquid head pressure into pneumatic measurements should be avoided. Liquid head pressures can cause havoc with measurement and give the appearance of a leak that is not there.
Other sources for pump leaks are the connection fittings. The best pumps have fitting that do not require wenches or thread tape to ensure leak proof operation. Many pumps are sophisticated hand tight fittings, utilizing o-ring seals, to handle vacuum all the way to several thousand PSI. The easier it is to connect to your device under test (DUT), the faster and more accurate your calibrations will be. Also, try to avoid a pump that uses push connection fittings, especially for low pressure. These fittings fatigue very easily with repetitive use and will eventually leak.
So in conclusion, just like your bicycle, the best way to prevent leaks with any pump is a little preventative maintenance. Good cyclists always clean and lubricate their chains and likewise a good technician treats their pumps and instruments with respect. Take a look at them occasionally and make sure the o-rings are not cracked or dried out and that any moving parts actually move without strain. If you have a pump that moves freely and doesn’t leak, you will have a happier trip home at the end of the day.
Written by: Nick Grayson, Cameron Instruments Inc.