ESR meters are a 'one trick pony' and in recent years, have tended to be eclipsed by Chinese 'multi-testers' which are so much more versatile. ESR meters used to be a popular homebrew project, such as the Television Magazine one in the late 1980s and the one that I posted on the forum back on 2010 (which ran for four years with more than 60,000 views). But time moves on and Chinese Multi-testers and now so cheap and versatile, that it makes a homebrew one pointless, and buying a commercial ESR meter questionable, compared to a multi-tester.
Unlike Multi-meters, where there are safety issues to consider, and where some enthusiasts strive for a level of accuracy quite beyond what's called for by hobbyists, those issues don't really apply to ESR meters. It's unnecessary to know the ESR value to three decimal places, and it's easy enough to check the calibration of the meter using close tolerance resistors if in any doubt about the result of a reading.
As a rough rule of thumb, for capacitors in a Switch Mode Power Supply, (which few of us dabble with), up to 0.5 Ohm is considered good, for general use, from 0.5 to 3 Ohms, then from 3 to 10 Ohms the cap in question should be compared to a new good one. If the ESR of the new one is well below the one under test, it should be replaced. Anything over 10 Ohms, chuck it in the bin. I've attached below a pic of a typical analogue homebrew ESR meter dial to show what I mean.
I have two homebrew ESR meters which are now replicas of the past, even though they perform well - one from TV magazine, and the one I made back in 2010 at this (now closed) forum thread:
https://www.vintage-radio.net/forum/...ad.php?t=54367
If nothing else, that thread shows what a popular tool ESR meters are considered to be.
Coming up to date, I've got two Chinese Multi testers, which test capacitors including ESR, but also test resistors, inductors diodes and transistors, including FETs. I also have two Peak Atlas Transistor testers - an aging DCA50 and a more recent DCA55 - an impulse buy when Maplin had them of offer for about £20. The Peak ones work well enough on silicon and germanium transistors, but compare poorly with the Chinese testers for FETs. As seen in the second and third Pic below, when a P Channel FET is under test, the DCA50 thinks it's a diode, the DCA 55 can't identify the source, drain and gate, but the Chinese tester in Pic 4 does so, and gives some data too. (It as a cheap unboxed meter for under a tenner which I boxed up).
So, if you buy an ESR meter, that's what you get. (Depending on the meter, it may also give you the capacitance value). They all work on a similar principle - a low voltage at 100kHz or so applied to the cap under test.
On the other hand, you could buy a much more versatile (and probably much cheaper) multi-tester such as the one at this link below, which - in my experience - when testing transistors, does a better job than my Peak DCA55, and also tests inductors, resistors etc. Many come with an infuriating ZIF socket, but this one comes with test leads, but also has a plug-in ZIF socket to use instead of the three test leads, if that's what's preferred. It's just one of a plethora of offerings:
https://www.ebay.co.uk/itm/MK-328-TR...-/122500532594
Just a final point, many if not most electrolytics on transistor radios such as Roberts, Bush and Hacker from the late 50 - 60s, especially physically small ones, will have dried out long ago, and if you put a cap tester across them, will often look like low value resistors. If the radio seems to be working, there's a temptation to 'let sleeping dogs lie' and leave them in place, especially given that they're on SRBP printed circuit boards where it's easy to damage the tracks when de-soldering. However, in some situations, leaving duff caps in place won't just degrade the sets performance, it can destroy the output transistors and seriously shorten battery life.
So, for aging transistor sets, a multi tester will tell you if an electrolytic cap has a high ESR, high leakage, and if it's still functioning as a capacitor or has morphed into a low value resistor. If it's a blue Philips, you might just as well switch your soldering iron on before the test meter. To be fair, what else can we expect of components that are almost 60 years old?
The last picture shows a 680µF capacitor in the emitter circuit of a Roberts R505 output stage. As can be seen, neither ESR nor capacitance came into it - the tester saw it as a 0.53 Ohm resistor. In other words, a dead short from the emitter of the AC187 to ground. It, and it's companion AC188, were getting too hot to touch but survived the ordeal.
Hope that's of interest and use.