When a "hydraulic repairman" comes to a point, where he starts to consider the use of a flow meter as something useful and necessary rather than annoying, he is no longer a "grease monkey" - he's a "full-blown technician". It is from that point on that he starts to carry that turbine and the needle valve everywhere he goes, and pump tests finally begin to make sense. I salute you, brother!
In my early "hydraulic" years, every time I had to apply the "stupid turbine", as I used to call it, I was imagining all the hard work and all the oil my poor self would have to soak. But with practice came experience, and now I know for a fact that "avoiding" the use of a flow meter when troubleshooting a hydraulic system is like avoiding the use of an X-ray machine when treating a broken leg. I am not saying it's an easy or pleasant procedure, I'm saying that it should never be considered "optional".
Most of the time flowmeters are used to field-test pumps, because the pump is usually the suspect number one when a loss of pressure/force/speed is detected, and the best way to take it out of the troubleshooting equation is to flow-test it under load, typically with the help of a needle valve to induce pressure. In most cases, the time you spend to install the test gear is less than the time you would waste wandering around the machine and guessing what might be wrong without being sure the main pump is OK.
Since the pressure is induced by flow restriction, all of the input energy is transformed into heat during such tests. This "super-heating" of oil can be, generally, overlooked when you test an open-loop pump, especially in systems with large oil tanks, because the time necessary to test the pump and make all the necessary adjustments is usually not enough to raise the oil temperature to unsafe levels. But what about closed loops?
When you need to field-test a closed-loop pump you, obviously, need to create a closed loop. In most circuits the loop flushing system will be placed inside the motor you've just disconnected, which means that when you start restricting the flow in this new (most likely very compact) closed loop, it will be, basically the same very reduced amount of oil (minus the pump's internal leakage, of course) going over and over around the loop, and which you will be super-heating with your pressure-inducing needle valve.
Just as an example, imagine that you have a 100 lpm (around 25 gallons per minute) pump, and the test pressure is, say, 400 bar. The power transformed into heat will be 400*100/600= 66 kW! Let us say you've used around 4 meters of a one-inch hose to create the loop. Along with the flow meter, the whole loop will hold just around five liters of oil. Can you imagine how fast the temperature can climb when you are heating five liters of oil with a 60 kW heater? Even with the heat taken away with leakage and dissipated through loop components the loop overheats lightning fast! We are talking about temperature increase rates in the order of several Kelvins per second here!
I have seen water boil on test fittings after only minute-long tests (well, not exactly boil, but rather making a "sh-sh-sh" sound upon a "licked finger contact"). It is possible to test closed-loop pumps in this simplified manner, but it is far from perfect. You are forced to conduct your tests and adjustments in "dragster race mode" and wait in between the "bursts" for the loop to cool down (unless, of course, your goal is to deliberately scrap the pump and go home early). The risk of (severe) oil overheating with all of the consequences is always present.
This is an example of such a test I made a few years ago on a classic series 20. It went well, but I had to wait for fifteen minutes in between the tests for the hoses to cool down:
There is, however, a "cooler" way to field-test closed loop pumps, and it boils down to adding a calibrated pressure-compensated flow control valve downstream of the needle valve, diverting a controlled amount of the loop oil to the tank:
There are two main advantages to this layout. The first one, of course, is the fact that the oil will not overheat as fast (or at all), allowing you to make more extensive tests and adjustments. The second advantage is less obvious but is equally big (if not bigger)! Let me explain what I mean by that.
As you know, in a "healthy" closed-loop hydrostatic transmission the charge pump must provide enough flow to compensate for the internal leakage of both the pump and the motor. When you test a closed-loop pump with a simple (motor-less) loop, you take the motor's leakage out of the equation, which, in cases of "marginal efficiency", can make all the difference between stamping the pump as "good" or "bad". Using a compensated flow control valve to divert a controlled portion of the loop oil to the tank is the perfect way to simulate a motor's internal leakage, thus making the test much more informative and reliable.
Note that it is important to use a well-calibrated flow controller to have a good idea about the flow you are draining from the loop. The one I use the most has an almost linear characteristic, with approximately 3 liters per minute per turn and 30 liters per minute maximum flow. All I have to do is count the turns and multiply them by three lpm.
The ability to precisely control the amount of oil that you are "stealing" from the loop allows you to evaluate the maximum amount of added leakage a given pump system can take because you can easily detect the moment when the charge pressure starts to drop. Or, if you know the charge pump flow, you can estimate the pump's internal leakage, again by noticing the exact "stolen" flow rate at the moment when the charge pressure drops, and subtracting it from the charge pump flow. If you don't know the charge pump flow, you can roughly measure it by reading the "leak" flow when the charge pressure drops, with the main pump at zero displacement. If you know the rpm, you can estimate the charge pump size, the list goes on... This is why using a flow regulator is better than using a flushing relief valve.
Here's another closed-loop pump field test I did. It was an old A4V250, working at 350 bar and a thousand-plus rpm. I used my trusty 30-liter flow regulator for the "loop-oil-stealing" (open all the way). You can see how small the test loop is, and yet I was able to test the pump and adjust both of the the pressure relieves and the cut-off without ever stopping, and the temperature in the loop never rose over 70 Cº!
This technique is easy to apply and doesn't require much investment. It enhances the test and facilitates pump adjustments. If you didn't know about it - feel free to tuck it under your "test" belt!
Please do not laugh at the vintage data logger from the first picture (the rounded box under the earmuffs), despite the "scars" and the age it is still quite accurate and can tell lots and lots of stories...