Measuring TT Speed

In measuring my tt speed I am seeing the lines on the strobe bounce back and forth, not holding steady. This is a belt tt with a dc motor setup with a closed loop controller, and ideas on what can cause this? I am thinking the belt may be to tight, it is a new belt BTW...
Get the app "RPM" on your phone. Not only will it give you the speed but also Wow and Flutter.
If the belt is too tight, it may be causing the motor bearing to wobble by applying a bias to one side. Try backing off on the motor mounting screws if possible.
Is the belt made by the original manufacturer? Treat the belt with baby powder and try it again. Use only original belts. RPM is much more accurate than the strobe routine. 
WoW is every rotation, fludder is intermittent? Or is that backwards..
Been a while.. If the belt is in a grove and the NEW one is just a hair to wide, as the belt comes off the pully and out of the grove, BOING! it snaps out..

Make sure there is not a wide spot. Mark the wide spot, trim if needed (with sandpaper), and re-powder the belt.. BUT wipe and clean the pullies.. DON’T forget.. A sticky spot on a pully, can drive you nuts...
Slipping belt, whole different thing, touch the platter it stops... Slow startup.. la te da.

Hi, jsman,
What are you using for a light source to check the strobe disc? If the light isn't stable you'll get strobe "bounce".
I bought years ago a Sutherland Time Line that checks TT speeds VERY accurately. If you can find an Agon member in your area to borrow one that would be what I suggest. If you live in the Boston or Southern NH area PM me. 
Mijostyn, With much respect, especially as I have never used the particular application you recommend, RPM, cell phones are notoriously inaccurate for measuring TT speed.  If the strobe is wavering back and forth, it means something. Sometimes that can be an optical trick due to flickering of the strobe light or see below. So long as the strobe markings do not move definitively in one direction or the other, the problem may lie with the strobe.  The OP does not say what strobe he or she is using.  For me, the only strobe worth having is the KAB product, which comes with a battery-powered strobe light that is constant in its pulse and a larger than average diameter strobe, so the markings are moving at a faster speed due to a further distance from the center of rotation, compared to the average strobe disc.  Any strobe light that plugs into the wall is subject to the minute variations of the AC frequency and could give the phenomenon the OP is talking about. Unfortunately, the KAB kit with strobe disc and battery-powered strobe light costs $100, but is well worth it in my opinion.
Reality check: no belt, motor, AC, or anything else gonna make the lines "bounce back and forth" the changes in platter speed for that to happen are just insane, beyond the torque of the motor, and the OP wouldn't even need a strobe the table would be totally unlistenable. 

Its just light hitting the strobe bars at the wrong time. Ignore those, concentrate on the average, you'll be fine.
I am using the KAB light and disc for those interested...
Lewm, as far as I can tell the RPM app works great. I have a record weight with a flat top so I just put the phone on top of that. The phone reports in hundredths of a revolution. When I first tried it the phone reported a wow and flutter of 1.78 % which sucks. I got a new belt and wow and flutter dropped to 0.07%....on a 40 year old turntable. 
For speed it is certainly more accurate than a strobe. If I adjust it with the phone to an exact 33.33 RPM the strobe is dead still. It is cheap. I suggest you try it and tell us what you think! 
On the other hand, if you can't adjust your turntables speed it might piss you off.
jsman, if you're using the KAB strobe light (and the batteries are good) you should be seeing a series of numbers that correspond with the speed the platter is spinning. So assuming you've got your turntable set for 33.33 RPM you should see a "33" displayed that is stationary (indicating steady speed) or floats slowly left or right depending on whether your turntable is running fast or slow. Not sure what you mean by " the lines on the strobe bounce back and forth."
Are you saying that the numbers should be stationary? That's what I have been doing wrong, I am looking at the bars. For whatever reason I thought the lines were to be stationary....
There are a lot of variables with measuring speed and W&F. That’s not to say you shouldn’t try to measure it, but take the values with a grain of salt. As an example, I get the following for the same belt driven TT:

RPM App/World TT Database: 33.33 RPM, +0.00%, 0.07 W&F
WFGUI: 0.0199% RMS W&F
Analog Magik: 0.0443% AES W&F
I prefer and feel more confident with the off board Strobe.
They usually cost the price of a Vinyl Album or Two, and don't get a lot of usage once utilised.
A Strobe built in to a TT can offer inaccuracies in showing the Speed,
it is worth reading up on some of the descriptions how this can be happening.

the RPM app for cell phones is accurate and a lot easier to use than a strobe. Check it out !
In my phone LG v30+ RPM metter shows 32.83 instead of 33.33
Both my turntables has strobe's and one of the even has custom built. And if I do measure friends machines I am getting the same error always so for me it's usable but it's not measurement tool. It's phone with gyroscope which is not calibrated.
its a bit extreme but the proper way to measure is with a calibrated test tone disk and a frequency counter. You can also send the output to a scope and visually watch the deviations.
yes, it's the illuminated number icons that count. If there's drift, and you have a way to adjust platter speed, you can usually get the illuminated numbers to hold steady, plus or minus. I always make speed adjustments with the tonearm lowered and playing the first song of a LP sitting under the strobe disc. It's surprising how much stylus drag there is.
If you don't have a way to adjust speed you can get a close estimate of actual speed and error using this technique posted at the KAB website:




Mijo, You are certainly OK to prefer and use the RPM app on your cell phone, but your little experiment doesn't prove anything about its accuracy relative to an un-named strobe to which you compared it.  I am only speaking for the KAB strobe, not strobes in general. I got the idea not to trust cell phone apps based on numerous posts to this forum and the Vinyl Asylum website where users complain about their lack of accuracy, or they had a problem with tt speed that ultimately was proven to be related to their reliance on a cell phone app.  Like I said, I don't recall whether anyone specifically named the RPM app.
@johnss:  Correct.  And that is what you get with AnalogMagik.  But still there are potential variables, including proper center hole on test LP, warp, etc.

@ lewm.  Some RPM apps are better than others.  The preferred ones have a way to be calibrated.

@edwyun .... re. “Some RPM apps are better than others. The preferred ones have a way to be calibrated“

Please name some of the apps that have a way to be calibrated.

+1 on the Timeline
Lewm, You have to place the phone directly over the spindle for it to measure accurately. Most just lean it against the spindle so the phone is anything but horizontal. I use a record weight with a flat top so that the phone is right over the spindle and horizontal. If it is not dead center the wow and flutter readings will be off. Readings are repeatable and ease of use relative to a strobe is superior. How long do you want to stare at a strobe to see if it is drifting. You also do not get wow and flutter readings off a strobe. I use an iPhone by the way in spite of hating Apple. I have used several strobes. Why don't you give this a spin and see for yourself. If you can't get it to read accurately and repeatedly stick with your strobe. No big loss.  
The phone does not have to be calibrated. It already has an accurate built in clock and this is a measurement against time, Revolutions Per Minute. Counting against the clock is an extremely simple function. 
Your way is fine, and so is the KAB strobe.  It is quite a precise instrument in use, and a speed error is immediately detectable.  This is really not worth further discussion, in my opinion.  I learned something from your last post; maybe failure to center the cell phone is a reason why others have reported problems with such apps, and maybe the RPM app is superior to some other cell phone apps, as well.  I am OK with that.
The RPM Turntable Speed Accuracy/World TT Database app for iPhone has a way to calibrate the app and then applies a multiplier for better precision.

The RPM Speed and Wow app and the RPM Calculator app both for android, which are popular, dont have any way to calibrate the app, last time I checked.

Have you ever been aware of a speed change when listening? If not, try to ignore it.

Wonderful Thorens TD 124, idler wheel drive, mirror constantly reflecting strobe on underside of platter: each session the motor had to warm up before you adjusted it to get the built in strobe lines steady. then after a while, check, might be a speck slow or a speck fast.

Not that I heard it, just when changing lp's I saw it. And that's a 7lb or 9lb platter to behave like a flywheel, with the most precise bearing I have ever seen.

Direct Drive, not quartz, similar, warm up needed, but steady after that.

Direct Drive, Quartz Locked, ON, walk away.

The speed controls are nice, but wait 10 minutes for warm up, and then only a minor adjustment should be needed, their larger range is for purposeful audible changes.
I have yet to see actual W&F measurements for a DD table that is any better than most other tables. Published W&F measurements from the manufacturer, sure.
I replied to the OP already but I thought I'd add a comment about the calculations for platter speed from the KAB USA website. Please correct me if I have this wrong, but the platter speed calculation didn't mention that when the platter is running slow you have to account for it by either using a negative error, i.e., [1 + (-count/360)] x RPM, or, subtracting the error, i.e., [1 - (count/3600)] x RPM.
Doh! That should be [1 + (-count/3600)] x RPM.
Lewm, I used a strobe for decades and whatever error is involved is is meaningless sonically. Watching your telephone spin has a hypnotic effect particularly after a few gummies.

Well guys thanks for all the replies with some help from above and you guys I have it sorted out now, thanks....
The RPM app does have a calibration feature that is very easy to do , but you have to get the premium or paying version.  You also get the option of entering your tables in the database and compare it with others from around the globe. Is around 15 dlls ( i paid in pesos so i can,t remember for sure) but is well worth it.