If you measure or calibrate pressure in the real world, you are at risk for contamination. And contamination can cause problems that you don’t even want to think about. A manufacturer in the aerospace industry found that out when they used their air system to calibrate a gauge that had water in it, then used it to calibrate another device that was supposed to be “dry and air pure.” That device was used on a water-free system under test and, you guessed it, water got into the system where it is definitely not supposed to be. It caused a lot of problems. The manufacturer had to clean every pressure line in their cal lab, which shut down their pressure calibration operations for a time until they could prove that all of the contamination was gone. Not a good day for those guys.
The type of contamination I’m talking about is anything that gets into your system that isn’t supposed to be there; for example, water or oil in a gas system, or the wrong oil in an oil-based system. It can damage a calibrator in a number of ways: clogged valves, worn out parts, difficulty maintaining pressure, and worse. If the contamination gets into the sensor, it can actually change the behavior and throw off your readings.
If a contaminated standard is used to calibrate a gauge, transmitter or other device, it may pass on the contamination to the device under test, which in turn causes even more problems. Devices like these can be damaged easily by rust, corrosion, damaged “O” rings, or other problems that affect performance. As the contamination gets passed to more devices, things can get nasty very fast.
What causes this type of damage? A common problem is when the wrong substance gets into the calibration system through connections to the device under test. For example, if you use a gas calibration system to calibrate a gauge that has just a little water or oil in it, the calibrator can get contaminated.
Another way to introduce contamination is by putting the wrong oil, or oil with foreign particles in it, into a hydraulic calibration system.
Yet another way contamination gets into a system is through the vacuum pump. Most vacuum pumps have oil in them. If you turn off the vacuum pump while you still have the vacuum on the system, the oil from the pump will move to fill the vacuum inside of the unit, causing a lot of expensive damage.
How do you avoid problems like these? Most importantly,always inspect the device under test before you connect it to the calibration system. In general, if you are calibrating something that is just coming off the manufacturing production line, you can be fairly confident that it is clean. But if you are servicing devices that come in from the field, you won’t know they’re clean unless you check. During a busy day, it’s easy to skip the extra time it takes to check and clean a device that looks like it’s okay. After all, 95% of the time it probably is ok. But it only takes one gauge with water in it to create a mess that you don’t want to have to deal with.
Today’s digital instruments and electronic measuring devices make this sort of inspection easy to do, because you can see the test ports and use a cotton swab and a little alcohol to clean out any contamination. If the device is a bourdon-tube-style gauge with a large internal volume, you can use a purging process to get any fluid out of it.
Of course, some devices are hard to clean, and missing just one will cause problems. To supplement your inspection and cleaning routine, you can purchase an external device to help trap or separate the contaminated fluids from the standard.
Dirt/moisture traps provide an effective barrier against the transfer of moisture and dirt from an instrument under test to the sensitive piston/cylinder assembly of a pneumatic deadweight tester. Unexpected particle contamination or fluid inside the device under test will be prevented from entering the deadweight system.
A self-purging liquid trap communicates with the pressure controller to purge liquid from the device under test at the appropriate time, without requiring manual intervention. It uses gravity to make any liquid fall out of suspension with the gas as it goes through the trap. A purge valve at the bottom of the trap lets you remove the liquid.
A separator, as its name suggests, separates the fluid in the device under test from the fluid in the calibrator. If you’re using a deadweight tester containing mineral oil to calibrate a device containing a phosphate ester fluid, for example, the separator would keep the two fluids apart during the calibration. This method adds some error or uncertainty to your calibration because of the head height difference and the diaphragm’s location, but that is usually something you can anticipate and compensate for in your calculations. (It’s certainly better than having phosphate ester fluid in your deadweight tester!)
If you’re concerned about vacuum pump contamination, you can use one that’s equipped with an auto vent valve. When you turn off this type of pump, it vents the vacuum, or lets the atmosphere into the system and releases the vacuum, preventing the oil from the vacuum pump from getting sucked back into the controller.
Using a filter does have its potential downside. A filter can add error to your calibration because the filters restrict flow and, if you have a flow restriction with a flow across it, you can have a pressure drop across the restriction. Be extra careful with systems that have built-in filters. Look for one designed to get minimal or no flow restriction across the filter so that it is appropriate for the accuracy of the measurement you need to make.
Sometimes it’s the little things that make or break a calibration. One way to be on the safe side is to buy your pressure calibrator and accessories from someone who understands pressure calibration and makes instruments specifically for that application. At Fluke Calibration you will find sales and support representatives who are experts in the field and who can help you to select the best solution for the work you do. Speak with one of our product sales experts.