When a hydraulic "repairman" comes to the critical point in his career, where he begins to consider the use of a flow meter as something useful and necessary rather than a back-breaking burden, he automatically evolves from a "grease monkey" to a "technician". It is from that point on that he starts to carry that turbine and the needle valve everywhere he goes, and pump tests make more and more sense.
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 this thrilling operation would give 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 complex hydraulic systems 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 not be considered as something "extra".
Most of the times flowmeters are used to field-test hydraulic pumps, as the pump is usually the suspect number one when loss of pressure/force/speed is detected, and the best way to take it out of the troubleshooting equation is through flow-testing 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 oil super-heating can be, generally, overlooked when you test an open loop pump, especially in systems with large oil tanks, as normally the time necessary to test the pump and make all the necessary adjustments is 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 presure inducing needle valve.
Just as an example, imagine that you have 100 lpm (around 25 gallons per minute) flow in the loop, and the pump test pressure is, say, 400 bars. 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 over-heats lightning fast! We are talking about several Kelvins per second increase rates here!
I personally have witnessed water boil on test fittings after only a minute long tests (well, not exactly boil, but rather making a "sh-sh-sh" sound upon a "licked finger contact")... And although it is possible to test closed circuit pumps in this simplified manner, it is far from perfect, as you are forced to make tests and adjustments in "dragster race" fasion, and have to wait for some time in between the "test 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 just one example of such a test I made a few years ago. It went well, but I had to wait for fifteen minutes in between the tests for the hoses to cool down. Let me know if "you're alive" if you read this, James (James is the rig's owner I lost contact with).
Now it is time for me to tell you that there is a safer way to field-test closed loop pumps, and it basically boils down to connecting a calibrated pressure compensated flow control valve downstream the needle valve, diverting a controlled amount of the loop oil to tank (see the schematics). There are two main advantages in this layout. 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.
As you know, in a "healthy" closed loop hydrostatic transmisison 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 loop, you take the motor leakage out of the equation, which, in cases of "marginal efficiencies", can make all the difference between stamping the pump as "good" or "bad". Using a calibrated flow control valve to vent controlled part of the loop oil to tank is the perfect way to simulate the 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 "stealing" from the loop. The one I use the most has almost linear characteristics, giving approximately 3 liters per minute per turn, 30 liters per minute maximum flow. All I have to do is count the turns and multiply them by three lpms.
This technique also allows you to evaluate roughly, when needed, the maximum amount of added leakage a given pump system can take, as 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 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 rpms, 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.
This is a picture of a closed loop pump field test I made just a few days ago. It was a large and old A4V250, working at 350 bars and a thousand and something rpms, I still used my old 30 liter regulator, but I had to open it all the way. You can see from the picture that the loop hoses I used were quite short, yet I was able to test and adjust the pump (both pressure limiters and the cut off) without stopping, and the temperature inside the loop never rose over 70 C!
If you understand how it works, this technique is very easy to apply and doesn't require much investment, providing more complete test results and facilitating the adjustments. And by the way - it can also be applied in shop tests as well! If you didn't know about it - feel free to tuck it under your "test" belt, and do let me know if you liked it should you ever chance to use it.
P.S. Please do not laugh at the "vintage" data logger from the first picture, despite the "scars" and the age it is still quite accurate and can tell lots and lots of stories...