VOM meters

The Mastech 8226T is true RMS model, from what I researched.  I wound up ordering the UNI-T UT61E that'll show up Tuesday.  I don't know what to say about the variance between your DMMs.

I suspect that the percentage of difference would be closer if you were using a postwar ZW, with a pure sine wave. 

 

I also wonder which type is more suited for model RR work, and why?

There is nothing like an old Triplett VOM meter with a pointer

to measure the DC voltage from an old ZW when the whistle button

is operated.

 

If you're measuing a non-sinusoidal waveform, a true-RMS meter will give different readings than a meter that doesn't have true-RMS capability.

GRJ, are you saying that with a sinusoidal waveform, both RMS and non-RMS meters will read the same?

If they're quality meters, they should be a lot closer than the above samples.

 

 

Now if I use a full bridge rectifier, I am left with a sinusoidal wave with the lower portion reflected into the upper.  So a DC voltmeter won't be accurate either, just like readings on AC.

 

Seems to me, that in such a rectified DC circuit, the voltage thatg should be planned for in selecting components is not the reading on any voltmeter, but the peak voltage, for which I don't see any voltmeters.

 

From wikipedia: http://en.wikipedia.org/wiki/Root_mean_square

 

The root mean square (abbreviated RMS or rms), also known as the quadratic mean in statistics is a statistical measure defined as the square root of the mean of the squares of a sample.^[1]

In physics it is a value characteristic of a continuously varying quantity, such as a cyclically alternating electric current, obtained by taking the mean of the squares of the instantaneous values during a cycle. This is the effective value in the sense of the value of the direct current that would produce the same power dissipation in a resistive load.^[1] An electric current of given magnitude produces the same heating regardless of the direction of current flow; squaring the quantity measured ensures that alternation of sign does not invalidate the result.^[2]

It can be calculated for a sequence of discrete values, or for a continuously varying function. The name is simply a description: the square root of the [arithmetic mean]] of the squares of the samples. It is a particular case of the generalized mean, with exponent 2.

Hi Gunner and RJR, you guys are really heavy. From your earlier posts I am confused, I also may be a bit old, but I would like to ask a question?? What in the heck is the

"bee's knees"??? 

 

Tommy

Just because I quoted the description of RMS doesn't mean I understand it.

 

The "bee's knees" is a phrase from the 20's.  "It's the bee's knees" was an expression of approval.  My mother told me of it.  It is synonomous with "The cat's whiskers."

 

If you google the expression, there are many sites endeavoring to explain the origin of the expression.

I am curious.  I would ask someone who has an RMS voltmeter and a DCS system , to advise what voltage reading they get on the output side a a TIU Variable circuit, when applying full transformer voltage to the input of that circuit and using the remote to cut the voltage to around 5 or 6 volts on the remote readout.  This would give an indication of whay some conventional locos take off at such a setting.

GRJ, can you answer this?

Here you go, using my Radio Shack TRMS meter on Var in/out 1 on my TIU input power was from a PH-180 at 18.8 volts:

 

Remote - Track

Setting - Reading

5 volts = 8.5 volts

6 volts = 9.4 volts 

7 volts = 10.1 volts

 

I was going to do this for you the other day, but I forgot about it.

Thanks.  This explains why some conventional locos take off.  I don't have an rms meter and have no perceived use for one.  Yet.

I got my Radio Shack TRMS meter a few months ago when on sale and I had a $10 off coupon from a previous visit to go with it. That made it like $29.95 or something. It was regular $44.95 or so, still not a tremendous price. It isn't a Fluke, but for the price it isn't too bad. They may be cheaper now that they are in bankruptcy?

What in the heck is the"bee's knees"???

 

Wouldn't it great if there were a website where you could just plug in a word or expression and search for the meaning?

 

 

Arthur, to find the origin of the phrase, I googled it and a page of such locations popped up.

 

rtr, there are companies that but these dying outfits and raise the price so they can advertise a big discount sale.  I doubt they'll be any cheaper.  Also, any warranty on a R-S brand item is worthless.

Yes, Google will provide answers to most things like this, even acronyms.

 

As for the RS meter, the warranty was worthless to begin with. I think it was only 90 days which was up before they went bankrupt. But even if it does break, for $30 I won't be out all that much.

True.  I have a "Micronta" (an R-S brand) digital meter I must have bought 30 years ago, that I still use regularly. And an even much older R-S analog.

Now that my work tends to be more along the hobbyist line, I don't feel the need to buy industrial quality (and expensive) test equipment for the most part.  So far I've been very pleased with my Mastech true-RMS meter, it seems very accurate and does everything I need it to do.  For $43, if it breaks down the road, I'll just get another.  I must admit, my old Fluke 77 lasted about fifteen years before drifting out of cal sufficiently where I didn't trust it, not bad performance.

GRJ, around the average layout, is there any real need for a true-RMS meter rather than a non-RMS?

Probably not, but I figured since I have tons of little meters that don't do true-RMS, and the Mastech was only $43 and does everything but mix drinks, I'd have the capability.  Truthfully, for most stuff around the layout, a 5% meter is probably plenty of accuracy.

 

Now you've ruined my excuse for getting another gadget--no use on layout and it won't mix drinks.

Excuse?  Who needs an excuse?

 

+1 on the Simpson.  I have one and it works great for me as a bench meter.  I also have a small Radio Shack digital I use.  I have older Flyer, Marx and Lionel so I don't need a fancy meter.

GRJ, you have a point.

 

I also have an Ideal that I bought because I needed a meter that could measure frequency, so I could check the RPM on a marine generator.

I had an almost new Fluke 179 at work when I retired, I should have asked them for it as a going away retirement gift! Probably would have gotten a swift kick in the posterior after asking, and then shown the exit door.

 

Many years ago I had an old analog Simpson that my employer gave me when they went to all digital meters and the electronic stuff started taking over. This was around 1984-85, they got everyone Fluke 87's at the time. I have no idea where it went, but I no longer have it? Had an old analog Amprobe to go with it too, it's also gone, must have left with the Simpson?

 

I got the RS TRMS meter for the same reasons as GRJ stated. I have a drawer full of those el-cheapo Harbor Freight meters and just wanted one that did TRMS after reading all the posts on the AC and chopped sine waves for our trains. As I said the RS TRMS meter was cheap enough, so here it is. I would still like to have a Fluke though, maybe not a 179, but a 115, 116 or 117 would be nice.

 

RJR, you can always find some justification for ANY new electronic gadgets! Think a little harder! Has to be something, like you thought it WOULD mix drinks, better color, it has nice leads, the dial works better, it has a continuity beeper, it comes with a battery, it doesn't come with a battery, didn't know what to do with income tax refund, wallet was too heavy, etc...

You're right.  I'm really making an effort to follow Barry's sidebar advice to replace my Rev G's with a Rev L.  But the Rev G works.

 

Stop tempting me!

i was taught with old school meters and instruments the tech courses i attended in the Marines had radios that still had tubes in them, but one thing remains,i used an exceptional tough VOM it was a Simpson 260, in fact i still have and use mine

 

On the very low tech end there are indicators that can, at the least, provide a small degree of a safety feature.  They will work to some degree on 18 to 20 volts AC.

Tick tester.

 

Good indication voltage is present. Though you might pay more for this device than one of the give away meters available.

There is also this light indication device.

 

Either has the ability to indicate that something is there.   (Safety meeting of the day).

Just plug in a trouble light or transformer, you can see if there's voltage present.

 

It IS tempting, isn't it.

I got one simply because at a show I attended a fella had one at a price I couldn't reject. It's still in the box ! (Main layout in early design / construction stages.)

Den layout is using the old one (was 3.2 when I got it).

 

GRJ, when I'm working on a 120-volt circuit, I use a sound device similar in size and shape to the first Mike shows.  I do not trust a trouble light, because it is possible that there is sufficiently high resistance in a circuit that the light won't light due to insufficient ampacity, but voltage would still be present.  Electricity can take unexpected routes.

I guess I don't put my butt in that kind of danger.

 

I just use my meter if I need to check the 120 outlet.  High impedance and I should get a reading if anything is there.

 

 I was servicing a neon sign, in the bucket truck.  Sign wasn't lit.  Didn't think much about it being on.  I wasn't within 3 ft of the sign, tick tester in my tool belt started chirping.  Back down and turned the sign off.   You gotta love the world of electricity.

I don't use my butt either.  I use my hands, as I find the opposed thumb to be helpful.  A meter is also helpful, but if I'm in an electrical box with many wires, by simply touching this device to the wire, insulated or bare, I supposedly can tell if it's live.

 

 

And grabbing the side of the box with one hand and the wire with the other will tell you for sure!

 

RJR;

You ever try that around 400 cycle they will be prying you out of the far wall. (ask me how I know)

Use a Meter or a test gadget. MUCH safer.

At the very least, you're not destroying relatively expensive testing equipment.

Again Safety Meeting of the Day.