Analog meters output more current to measure resistance at low currents? Vacuum Tube

[Reloadron]

my manager said that the simpson can measure 10 amps of current , dvm can only measure 2 or 3 amps. also the simpson has a faster response time than a fluke 8842 when measuring time in circuits

I have to disagree here. The Simpson 260 I pictured, is as I mentioned in detail a Simpson 260 Series 6 XLP and it has a 5 amp range not a 10 amp range. Some Simpson 260 Series meters do have a 10 Amp range, I have one of those too. All Simpson 260 Series meters are not the same. Next I have my Fluke 87 sitting here, it does in fact have a 10 Amp DC range. My belief here is that the Fluke 87 DMM is hands down overall a much better meter for measuring DC / AC Volts, DC / AC Current and Resistance. The older analog Simpson meters do have their place for certain measurements as we have discussed but they certainly do not come close in offering up the uncertainty of the newer DMMs like the Fluke 87 as an example. Doesn't take much to read and interpret the specifications. Therefore I respectfully disagree with your manager.

Ron

 

[Billy Mayo]

what about the response time? a simpson response faster than a dvm meter

 

[chemelec]

Digital Meters have Update Times.
Some better than others.

Analogue meters have a Needle that Swings.
This takes TIME to go both UP and Down.

So You Can't accurately compare the two.

 

[Reloadron]

Billy, work with me here... This is what was said:

also the simpson has a faster response time than a fluke 8842 when measuring time in circuits

First and most important I haven't a clue what circuit or what "time" is being measured? That is why I left that part alone. Never worked with the Fluke 8842 but worked extensively years ago with the good old Fluke 8840 which was a real nice bench type DMM. The 8842 was a top end 8840 as far as I know. You mention "when measuring time" in circuits. Why the reference to time? Now if you mean sample time of the meter the meter, depending on range and parameter can run 100 samples per second if I am running it over a bus. That is pretty quick and I see no way to compare a meter like this to a Simpson and here I assume your reference to Simpson is like the Simpson 260? Summary is I have no idea what you or your manager are talking about. Billy, I am not going to sit here and download data sheet after data sheet when you can do that as well as I can.

Ron

 

[rumpfy]

This thread started with queries about ohm measurement and how it is different between analog and digital meters.
There is a thread referenced below here 'Of the analog and digital meters, which one......'; this seems to discuss many of the points now being raised.
It is going to be difficult to maintain a line of thought if we are going to be asked about different aspects all the time.
The question about response time is simple on the surface, but the DVM typically gives 3 readings per second; so that makes it slow as a wet week; but the reading has a better resolution(not necessarily accurate). The analog meter gives an indication of the reading by the speed of the needle movement, so for some checks (not tests), the analog gives an indication in a fraction of the time of a DVM. A DVM needs about 3 seconds for you to know that the reading has finished and the result stable. Then you know the reading.
Reloadrs remarks are spot on, and obviously come from extensive practical experience using meters of all sorts. With this experience comes the ability to read a specification and comprehend it, and to value the instrument manual as an important document, and to understand the principles of operation and limitations of the instrument.

 

[ChrisP58]

my manager said that the simpson can measure 10 amps of current , dvm can only measure 2 or 3 amps. also the simpson has a faster response time than a fluke 8842 when measuring time in circuits

From the Fluke 87 users manual, "10A continuous, 20A for 30 seconds maximum"

 

[cowboybob]

Not at all sure what Billy is getting on about, but...

I know that on occasion I will use an old analog (the venerable Simpson 260, borrowed from the USN back in 1974) meter to "detect' the presence of very short (100ms or less) single pulses from, say, an LM567 to see if I've hit the center frequency I hope the RC on it is set at. Just the flick of the needle off the stop post is enough for this purpose. I know a scope would do just as well, but a quicky with the meter is easier.

A DVM can not respond fast enough (AD conversion times and all that) to show anything on its screen in this case.

<EDIT>Just read rumpfy's post. Just confirming his observations.

 

[Billy Mayo]

I haven't a clue what circuit or what "time" is being measured?

The circuit has a input threshold at .300amps of current
Once the input has .300amps it will take 3 seconds to SWITCH the output from 0 to +25 volts

A DVM meter response time takes awhile for it to switch from 0 to +25 volts

When measuring this time with a stop watch from the inputs threshold is tripped to when the output switches from 0 to +25 the DVM meter also adds in time to response tim

The Stop watch is 4 seconds because of the DVM meter response time was added to the circuits switch time

the DVM typically gives 3 readings per second; so that makes it slow as a wet week; but the reading has a better resolution

So My Manger is right about the Simpson 260 , it has a faster response time than a DVM meter right?

A DVM can not respond fast enough (AD conversion times and all that) to show anything on its screen in this case.

And My manger is right again about this?

DVM can not respond fast enough? so a simpson 260 is has a fast response time than a DVM meter?

 

[Billy Mayo]

They use the Simpson 260 at my work to test for Switches , AC and DC switches

What's the difference between an AC switch and an DC switch? the power rating is different on them on what the switch poles can handle without melting or getting damaged

Now when the switch is damaged or causing intermittent problems , a DVM fluke 87 can't tell the problem of a Damaged or intermittent problem with a SWITCH but a Simpson 260 can because the needle will move and show you the fluctuating in ohms or in voltage when applied to the AC switch or DC switch

Is this true or not?

 

[MrAl]

Hi,

First, thanks for posting the meter faces for the different meters. It's interesting to look at these pics and it explains something about what is being measured with the different meters.

Ron:
With the meter face you posted we can see that the internal resistance is just a tiny tiny bit over 6 ohms, but i would call it 6 ohms until we can get a measurement of that resistance, and they may have tried to include some other series resistance in with that. But for now, 1.5 volts divided by 6 ohms of course equals 250ma, and with a small drop in voltage that could easily drop to 240ma which is what you measured. 0.240 times 6 gives us 1.44 volts and that is not unreasonable for the 1.5v battery loading.
Due to the schematic that was posted which showed 11.5 ohms we can see that the meters were made a little differently for the two "versions". So that explains a lot about your measurement vs what we found elsewhere. Thanks for posting that pic, and it was certainly a very nice and clear picture so we can examine all the details.
About the "time" factor that has been talked about, that's the response time of the meter. Most digital meters are very poor when it comes to this so that's when an analog meter is usually recommended. A digital meter could take a full second to respond, while the tiny mass and tiny friction in a typical analog meter means it only takes milliseconds for it to rotate.
For example, measuring the voltage of a power supply or line voltage with little load, then suddenly applying a large load and then suddenly removing that large load. The digital meter might not even budge a bit from it's reading of say 120.00 volts because it just takes too much time to make the measurement, but the analog meter would dip down just a little bit (maybe 10 percent) to show that something changed fast. So we would not see anything on the digital but on the analog we would see that something did in fact happen, even though we could not get a super accurate measurement. So analog meters are sometimes favored over digital when there may be fast transients that we dont need to measure accurately but we still want to know that they are there.
That meter is pretty awesome as we can see the Ohms scale down near 1 Ohm is graduated into tenths (0.1 Ohms) which is pretty good for a general purpose analog meter. If we are careful in reading we might even be able to distinguish between 0.9 Ohms and 0.95 Ohms for example.

Billy:
Your manager is recommending things that are mostly based on fact, but unfortunately this last one about the current measuring capabilities of the different meters is not exactly true. Different digital meters can measure different max current levels. I have meters that can only go up to 200ma, but others that can do 10 amps, and another that can do 20 amps. You can also add a current shunt to go up to 200 amps but that's another piece of test equipment we'd have to discuss.

Rumpfy:
I think what you are talking about is to estimate the diode curve using the Ohms scale alone combined with the DC scale. This might work but you have to know the internal battery voltage too, and that can vary a little so you have to be careful. To get the reversed scale, just subtract the reading from the max of that scale...so if you read "4v" on the "10v" scale just subtract 10-4=6 so that would be 60 percent of the voltage.

 

[rumpfy]

Mr Al,
I may have been vague with my story in post#79.
Firstly, I have sometimes trouble with loading pictures off a posting. This is because I use an old version of Safari 4.1.3. with a Apple OSX 10.4.9
I have OSX 10.5.8 on a laptop which runs Safari 5.0.6 and this always shows posted pictures.
Mr Al I think you said once before that there are some vagaries when you try to get photos and other '.jpg' type attachments. Anyway, not to worry.
In post 79 there IS a picture of a meter scale. The lowest calibration scale is a reverse voltage scale which is used for forward voltage measurements on diodes etc using the ohm range. Of course too, if the meter one is using doesnt have that scale, then it is necessary to 'derive' the actual forward voltage as you say.

 

[MrAl]

Hi there rumpfy,

Oh yes i checked that post and was able to see the pic in another way and i also see now the scale on the bottom which is a reverse voltage scale that goes from 0v to 3v DC so it is linear.

Now let me recap the measurement procedure for the benefit of other readers as well as for myself and you can verify if this matches your procedure...

First we'll start with the bottom reverse voltage scale on your meter which appears in post #79 and that is the lowermost scale on the face, and then i'll move to using the non reversed scale and repeat the procedure for a simple example of a "diode" that just happens to drop 1.00 volts at 100ma exactly.

The meter we are looking at in post #79 must have a 3v internal battery so that is our battery voltage, and it must have a 20 Ohm series resistor because "20" appears exactly mid scale on the Ohms scale for that meter.

Starting with the reverse voltage scale, when we connect the diode lets say the needle jumps up to "10" on the Ohms scale, indicating that we are measuring 10 Ohms. Looking now also at the lower reverse scale we see the needle hovers over the "1" and that indicates we are seeing a 1 volt DC voltage across the diode at the same time. Now since we know the voltage AND the resistance, we can use Ohm's Law to calculate the current, and thus we have 1 volt divided by 10 Ohms and that means we have 100ma (0.1 amps). So we have our first data point, 1 volt and 0.1 amps. If that's not enough we can switch ranges and get another data point.

Next using the DC scale that goes from 0 to 10, we also see the needle hover over the point that is roughly 6.6, and since 10 minus 6.6 equals 3.3, we have about 33 percent of the total voltage of 3v across the diode which is of course again 1 volt. And again now knowing the 1v and 10 Ohms we divided and get 100ma, so again we have our first data point which is 1 volt and 0.1 amps.

This doesnt seem too hard to do i guess so it is worth looking at.

 

[picbits]

Never mind - I really can't be bothered.

 

[MrAl]

Never mind - I really can't be bothered.

Hi,

Oh ok well no problem, most people dont care about the actual forward diode curve anyway but we started talking about it just in case anyone does want to look at that too sometime.

 

[picbits]

I started to get involved but having read through his other threads decided against it.

 

[rumpfy]

Mr Al,
thats about right. The only thing about my meter that you missed was that the ohm selector switch has a current value in 'blue' on the scale. This is equal to 3 / 20 x ohms multiplier. So for Rx1 it is 150 mA; Rx10 is 15 mA; and so on for Rx100 and Rx 1000.
So, because diode forward drop is around 0.2 to 1 volt, then the forward current is around 70 to 90 % of the full scale current.
I dont use this feature all that often, but I did need it once when I was designing around getting rid of the tunnel diodes in my TEK 3B3 timebase. I needed specific forward voltages according to the forward current. It was very handy, particularly in characterising some germanium diodes I had.
This thread is/was about the test current of analog vs digital voltmeters, and my postings are really addressed to this feature of ohms measurement, and the general utility of the analog meter.

 

[Billy Mayo]

I needed specific forward voltages according to the forward current. It was very handy, particularly in characterising some germanium diodes I had

.

And how did the Simpson 260 , give you those specific forward voltages and the forward currents?

What range and settings did u use?

 

[4pyros]

Wow Billy; Did you go to school for Electronics?

 

[chemelec]

This IS: Billy's School.

 

[Billy Mayo]

This was not made clear on how to do this

And how did the Simpson 260 , give you those specific forward voltages and the forward currents?

What range and settings did u use?