I'm been re-reading your first sentence for the past hour.

I live in CT so I recommend you perform every check the vendor recommends at least twice and see if there are any other checks you have not been doing.

As for a pinball coil, the resistance check is not done for the strength of the coil, it is usually done when a driver transistor fails or a fuse blows and you want to make sure there isn't a short.

Now stop thinking about pinball during work hours and please make sure those control rods can be inserted.

I use the following charts to verify resistance of coils. While not complete, they are a start. I've never used inductance to determine if a coil is good or bad, just resistance.

https://www.flippers.com/coil-resistance.html
http://www.pinballmedic.net/coil_chart.html

In your case you have a vendor who provided you an inductance spec for your control rods coils which you can verify conformance to. In the pinball world, I'm not aware of an inductance spec for coils. All I can think is to do a test and see what you get. You would need 2 coils which both have resistance that checks out but one acts weaker or worse than the other in some way. See what the difference in inductance is.

Not to get off on too much of a tangent here, but in a nuclear emergency scenario is there some alternate means to Scram the control rods? i.e. will failure of the coils prevent the rods from Scraming or is there some alternate method that can assist in the event of coil failure (gravity, actuator, etc.?)

I picture Spock with his LCR meter trying to get a measurement and growing weak as greenmachine looks up the specs.

Interesting calculator for establishing an expected value. I wonder what effect the multiple layers of wire will have since they don't all have the same loop diameter.
https://www.eeweb.com/toolbox/coil-inductance

I just looked at the wiki page for inductance a few days ago. I don't fully grasp the concept, but I do recall that JJP went to larger inductors on one of the boards for WOZ. So it is pinball related for sure.

I'd like to know more about everything related, so I'll be watching this thread.

Yes, inductors used on circuit boards (the JJP one you reference, for example) have a spec for the inductance. These are chosen by the circuit designer based on their inductance value so they better have a spec and be within it.

Coils on the playfield on the other hand, are not considered inductors and do not have an inductance spec. They are not chosen based on any inductance value.

I work with an EE, and not long ago I was discussing this topic with him. I was simply inquiring about coils, diodes and SCRs and by the time he was done, he had fully explained how I should just make my own coils... and how to do it.
After that hour long tangent, I went to PBL and Marco and spent about $50 to just buy the few I needed, I didn't really care how to make 'em, nor could I ever get that long hour back.

Further to check the coils in my machines, all I use is my DMM's resistance measurment setting. Guss my pin's aren't worthy of a nuclear scientist.

Inductance would be a good indicator if you had a reference and easy way to measure. However, inductance is not as easy to measure as resistance.

In general a pinball coil goes bad because it has overheated and the insulation on the wires melt, shorting a portion of the coil. A partially shorted coil shows a significant change in resistance so that is an easy check WITHOUT removing it. If the actual wire is corroding, unraveled, etc then the inductance may be affected without changing the resistance much on a DMM.

To measure inductance the coil has to be removed from any circuit, a current applied (and measured) and the time between voltage pulses measured using an oscilliscope. Then run it through an equation. There are a few different methods, but all generally require a scope and removal. Simply not worth the trouble for our cheap coils. At the point you remove it, it's easier to just put in another coil and see if it works better. If it does, throw the old one away. If not keep the old one as a backup.

We do have some custom test stations we made for testing extremely expensive coils (specialized uses). On those stations, you can just plug the coil in and get all of the info about it. They are several thousand dollars each because they have an integrated scope, pc, etc (about the size of a dorm fridge too).

Magnetic force F = −(1/2)*(i^2)*(dL/dx)
Where i is current, L is inductance in micro henries and x is plunger displacement, d is the diameter of the core in inches. So inductance is extremely important. BTW I looked up the formula, don't have that memorized.

But I thought Resistance is futile?

I'm surprised the NRC doesn't want you to write up a dissertation on how reliable gravity is.

Resistance is the "distance" of the wire wound around the core.
Inductance is the resistance to changing currents in the core.

Both will change if shorted...
Almost all DMMs only measure R and maybe C(apacitance). You need an LCR meter or a scope to measure Inductance.
In this case; I'm not sure Inductance is a critical as Resistance because for most pre 2000 pinball machines; the coils are driven by DC.
For modern machines (stern, jjp, etc) the coils are driven by PWM for which Inductance may be a factor.

Simple but effective explanation, thanks.

Greg, stop being a smartass.

Just busting stones. But I have never seen a coil issue due to an inductance degradation. We play many games with coils a lot older than us (by decades) that function fine as long as the rest of the assembly is taken care of properly. Sleeve and stop first. Linkage, plunger and bushing next.

As for those control rods. Gravity sucks, and I hope it continues to do so.

If you've never measured the inductance how did you determine it isn't an indicator of a coil issue?

The OP had a great question. I think responses have shown it's impractical to measure pinball coils on a routine basis given the cost of replacement. But I don't see anything to suggest an inductance measurement wouldn't be an indicator of coil degradation. We simply don't have the data.