A question from a member on meter measurements

See my previous post as I edited it a little. And as Frank said even using different test gear than what was specified required the Test to have a fudge factor like paralleling a TVM or VTVM with a 1 Kohm resistor to bring the readings to the same loading requirement of older test specified equipment. So as not to be an influence on inductance and capacitance or changing voltage loads in a circuit.

Excellent explanation Cliff!!

I would like to add that I have tried to do repairs and alignment with newer meters on some equipment, and I was unable to get the correct specified readings, due to the higher ohms per volt of the newer test equipment. Using the wrong meter can throw WAY off readings in oscillator circuits too.

That being said, with experience, typically you can use the newer meters, but you will have to add a "fudge factor" in to allow for the different readings.

meerkat1952 wrote:

Thank You Cliff, I do understand that using the various ohm per volt meters,  is going to give you different readings and in some cases actually change timing and waveform. Perhaps I will ask it this way. If you were going to calibrate and align an older piece of equipment that was originally done with a 20k/per/volt meter.  Would you use the calibrated 20K.... or a 11 meg calibrated> It is my understanding besides the obvious circuit loading, there are other properties that come into play, the actual, inductance, capacitance, etcetera. Thank You for your previous response. Peter

EDIT 10-01-2018 8:50pm
You need to go with the calibration instructions. 

As time went on there was found a need to get more sensitive instruments, notice I didn't include the word accurate, because they were not required to be Lab quality instruments on the Radio Shop Bench Till FM and TV and Radar along with Microwave measurements were required.
Most test equipment evolved around sensitive meter movements such as the electroscope to measure static electricity, The Hotwire amp meter to measure RF currents and AC small power, then along came Galvanometers to measure minute voltages, then D'Arsonval, Weston, and iron-vane designs. and taut-band meter movements eventually leading up to Oscilloscopes, VTVMs, and more sensitive and accurate test equipment. 


On the other hand, the Military Needed More accurate Test equipment along with Commercial Broadcast Stations, Radio and TV Manufacturers and the Scientific Research and Test equipment Manufacturers. And the Electronics consortium decided that they needed to set standards for all (Science measurements included) Electronic and Electrical Standards. So they set up International Standards. That itself is an enjoyable History to find out about.

VTVMs and TVMs are used today in Solid state equipment with no adverse effects.
Again if it mentions a particular tester, Philco and RCA and others Wanted the user to use their specific test equipment. But you can always look up the specs.

The accuracy of High input impedance or Volts/per /Ohm will of course effect into a higher reading. If the specifications of calibration require a 20,000 v/p/Ohm then you can expect to get the correct readings, the only time you would need to know if the inductance or capacitance is etc., causes adverse effects is if the values of the inductance or capacitance are not intolerance

The engineers that designed electronics don't just use any old piece of equipment that they happen to have. If they did without declaring the equipment used then the repairing Technician wouldn't know the results accurately.

In electronic Labs they found that more precise and accurate testers were required with sophisticated electronics because the voltages were becoming so low the current VOMs either couldn't read micro-or nano- or Pico- Volts so test equipment became Active Equipment rather than Passive. In other words, Amplified to compensate for correct readings.

If you are that concerned about the influence that test equipment has on a circuit, then if you have issues with capacitance changes in the circuit then you need to use specialized test equipment.

All test equipment that is used in labs and repair stations, to bring electronics to its original state of operation Follow standard by that company. And most electronic Companies specify Traceable standards. So the product has consistent results or they would be pulling their hair out during tests.

And yes all test equipment has an effect on all other components. That's why standards were developed.

If you can recognize the effect of and the difference of test equipment used and do a comparison of the differences in the readings, just realize battery operated multi-meters like the Simpson 260, is a standard in most repair shops, because most radios that didn't have high-frequency reception in or above the FM bands require the accuracy. 

The only concern is loading down of a grid circuit with a lower ohm's per-volt meter. Some tests could use even a 1000 ohms per volt meter. They were very common in the 20-40s.
I don't know how to explain it any better. If you took an Electronics class in college you would have your answers at your level of understanding answered.
I took over 4 years in college and 31 years Using calibrated test equipment in bench work and calibration Lab work while working for the Military and even a stint working in test equipment sections in Tektronix. And that is not to say I know everything.

If you have further explanations needed then go to some online Manufactures and electronic forums to download PDF files addressing this issue. A good place to Start would be FLUKE.

Or ask what are the effects on Circuits with various test equipment used online.
click Here

Thank You Cliff, I do understand that using the various ohm per volt meters,  is going to give you different readings and in some cases actually change timing and waveform. Perhaps I will ask it this way. If you were going to calibrate and align an older piece of equipment that was originally done with a 20k/per/volt meter.  Would you use the calibrated 20K.... or a 11 meg calibrated> It is my understanding besides the obvious circuit loading, there are other properties that come into play, the actual, inductance, capacitance, etcetera. Thank You for your previous response. Peter

meerkat1952


I have been told the most accurate way to trouble shoot old electronic gear is to use the same ohms per volt meter that was originally used to establish the voltages on the schematic. I understand you can take a 11 megaohm meter can compensate by putting a resistor across the leads, but this is not as accurate and considering all other factors, capacitance, inductance, etcetera of the 11 meg meter. Does anyone know the answer. Thank You

There is a definite Yes to your Question, in this respect. When Voltages are taken in Vintage gear, there is usually an understanding that there were no VTVMs that  were even available to the bench Mechanic of that era. Most VOMs and Voltage meters were either 1Kohm, 5Kohm or 20Kohm or some where in in that ball park.

Schematics did include voltage measurements in a lot of instances, that referred to meters that the readings were taken with in Ohms/per/volt.  One example is the Military Tube testers that specified all measurements of voltage were taken with a standard ohms/per/volt meter.
The reason was to get consistent readings and allow for specific meter loads that would not change. The reason is that all meters were referenced to ground and to measure the voltage the meter leads would be in parallel with the component that has to have a reference to ground.

Using ohms Law of a voltage may have many paths to to ground. So when you have a test point that your meter is to measure it is another path to ground. so measuring that voltage, is decreased because there is more than one resistance present. The Meter plus the floating voltage.

Paralleling a known load is accomplished by using the equivalent  meter resistance. So if you were to measure a referenced voltage point, then the voltage would be draw only so much current because of the Meters Load. So the voltage drop would be consistent with the load applied. Voltage drops with lower resistance because more current is used.

So measuring with a VTVM of 11megohms is not the same as measuring with a 1Kohm Meter. So to compensate for reading expeted voltages at that test point an external load is added to the circuit to simulate the original meter load.

100v/1komh = 0.1 A
100v/ 11Mohms = 0.0000090909 A
So the voltage would remain Higher at the test point with the VTVM
"...all other factors, capacitance, inductance, etcetera of the 11 meg meter... "

These were not considered the same, as the meter load was different with all other factors, capacitance, inductance, etcetera of the 1Kohm meter were taken into account at only the used method of measurement at the time.

I hope this answers your question from my perspective.