How they work — what to look for

Aircraft Maintenance Technology - May, 2000

There are a variety of differential compression testers on the marketplace today. For the most part, the devices are fairly simple. The specifications for how they are suppose to be manufactured are called out in AC 43.13-1B, so it's not a secret as to what it takes to put one of these together. Yet, if you begin to shop around, you find a wide variety of prices (from $35 to over $100). So what do you purchase and what's the difference?

But first, a bit of history
For over 35 years, Eastern Technology Corporation has manufactured differential compression testers. According to Aaron Friedman, the company's CEO, "The Military developed differential testers during World War II. At that time, the government was ordering compression testers as a standard tool with every Pratt & Whitney radial engine. Because the engines had such large bores to them, someone did a calculation and said that you need an orifice of a particular size in order for the test to be fair.

"This was when the specifications for the testers were developed."

Friedman says that 100 psi is common to pressure test engines in the automotive field, but speculates that the pressure was probably reduced for safety reasons. It's a pressure that can be easily handled while holding the propeller in position.

Friedman says it was decided that you would place 80 psi air in a cylinder with an air source capable of something greater than 80 lbs. You regulate the air pressure using a simple regulator with a gauge that will hold within 1 percent of what it is indicating (which means that you can actually be reading + or minus 8/10ths of a pound. An orifice is placed in the path and a gauge is placed between the orifice and the cylinder to monitor the cylinder. With no leaks, you would read 80 psi on both gauges.

The orifice that is placed between the gauges was calculated to provide a sufficient volume of air yet restrict the air to allow pressure to build. More air flow is required for larger cylinders, so two different orifices are recommended depending on engine size.

AC 43.13-1B specifications say that engines up to 1,000 cubic inch displacement require a 0.040-inch orifice diameter, 0.250 inch long, 60-degree approach angle.

Engines in excess of 1,000 cubic inch displacement require a 0.060-inch orifice diameter, 0.250 inch long, 60-degree approach angle.

There is a proposal in to the FAA to change these specifications a bit, however. The proposal states that instead of using 1,000 cubic inches as the criteria for using the smaller diameter orifice, the criteria would be a cylinder bore of 5 inches. According to the FAA, this change should take place by September 2000.

The reason for this change, according to Friedman, is that the 1,000 cubic engine standard didn't make sense. "You can have an engine with a large cubic inch displacement that has many small cylinders, or vice versa. So it's more relevant to know how big the cylinder bore is instead," he says.

"In any event," Friedman explains, "the tester with the .040-inch orifice is used on most of the engines in the marketplace. It's rare to need it unless you're working on large radials," he says.

What to look for
Friedman continues, "In terms of the gauges that you use on your compression tester, the important thing is that they are matched and are calibrated to read the same pressure. We buy about 5,000 gauges at a time and we place them all onto a master gauge. We tolerate only one pound in either direction from the master. The regulation says that you can be plus or minus 2 psi, but we only tolerate 1 psi. Additionally, we match all gauges that read exactly the same so that the compression tester has two matched gauges on it."

Friedman says that when considering a compression tester, look for one that uses quality components and comes with a certificate of certification. "Be aware of vendors selling inexpensive units manufactured with low quality parts that are not precision made. The best units are made of brass with high pressure hoses and everything machined and plated properly."

Friedman continues, "It's not just the appearance you've got to be aware of. The orifice that is used in the unit must be manufactured according to specifications or it won't give you the proper pressure readings. For instance, we've inspected some orifices and found them to be under or over the .040 inches that they are required to be. Or the lead-in angles on the units are not proper. We really have a problem with units that are manufactured with the incorrectly sized orifice. Some of the units that we've checked from other manufacturers have orifices that are drilled closer to .045 inches. This means that a unit that reads that it's time for a major overhaul with our units, would continue reading good by almost 10 more psi on the gauge of the bad tester!"

If you suspect that your gauge is not accurate or you don't have any documentation that it has been calibrated, Friedman says that you can send it to Eastern for calibration. "We charge $30 per unit to test a differential pressure tester, and this includes replacing both gauges with new and shipping it back to you.

"You can also purchase two new calibrated gauges if you can't afford the time to send your unit in for testing, but if you do send it in, we will flow test the orifice and verify the proper operation of the entire unit."