Replace "Potentiometric Null Tester" with Fluke 87 or later model?
My first post, guys. Interested to see if anyone can speculate about my situation. Here goes: I have a piece of test equipment called a Pot Null Tester that of late has been very problematic. I am trying to understand it better so that I may fix it and be able to troubleshoot any other problems that may arise. It is used in testing Fuel Flow Transmitters (FFT, for JT8 aircraft engines). This tester interfaces between the FFT itself and a 115VAC 400Hz power supply, 60VAC 8Hz "power supply" (actually a transformer, this receives 115VAC 400 Hz and transforms it into 60VAC 8Hz to supply to the FFT (in order to run a motor with known 240 rpm near the FFT inlet, which as it spins imparts force to the fuel and directs it onto a turbine, which is attached to a spring, at a specific angle - as the flow increases, the turbine deflects more and sends a signal thru a pickoff coil out of the FFT, indicating position which in turn creates a certain voltage which is related to flow amount) with power.
The signal into the pot null is compared to the signal out of the FFT through a Phase Angle Voltmeter (wall power supplied to it, but input and reference signals obtained via Pot Null Tester). A decade voltage divider on the pot null tester apparently takes the input signal and when the test technician adjusts it to a proscribed amount (which increases for each fuel flow rate) compares this signal to the output signal from the FFT. If they are within allowable phase tolerance on the phase angle voltmeter then the actual fuel flow rate is close enough to the fuel flow rate read by the FFT.
Now that you understand the function of the pot null tester, here is the problem. The pot null has gone bad somehow. The decade voltage divider went bad so I had that fixed and reinstalled. One of the three transformers in the pot null, the variac which controls/provides 115VAC (of the 115VAC 400Hz voltage) went bad. I had this transformer replaced, and another one of the transformers went bad (it appeared this transformer is for isolation). Everytime I fix a component, something else goes bad. I want to better understand this pot null tester for two reasons, either so that I can fix what ails it, or so I can altogether replace it with a Fluke 87 multimeter (as I have been told is possible).
The pot null has three transformers in the setup (one variac and two others, not manual adjustable), several resistors (ranging from 1K to 50K), and a capacitance selector/switch to allow several different model FFTs to be tested - each one requires a different capacitance for testing.
What appears to be the function of the transformers (aside from the variac), capacitance, and resistors, and decade voltage divider? Does it seem that this setup could be replaced with a multimeter? I have been told that the purpose of the capacitance is to function as some sort of filter to essentially read the output signal without acting upon it and thereby increasing or decreasing it.
Long post. Thanks for any insight.
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