Taking Measurements

Making Measurements with Electrical Testers and Digital Multimeters

Understanding Core Measurements:

Volts, Amps, Ohms

The Electricity - Water Analogy

The Electrical System is similar to a water system.

Voltage, measured in Volts AC or Volts DC, is Electrical Pressure supplied by the electrical company or a battery
- like water pressure supplied by the water company or your well.
Current, measured in Amps AC or DC, is Electrical Flow, is Electrical Flow
- like water flowing through a pipe when you turn it on.
Resistance, measured in Ohms, Restricts the Flow of electricity, from fully open (short) to fully closed (blocked)

We will use this analogy to explain core concepts that help understand what measurements to make when installing, maintaining, and troubleshooting electrical, electronic, motors, process controls, and many different systems.

Understanding Ohm's Law is essential for working with electricity.

It is very basic and makes sense.
It is how electrical and electronic circuits get powered.
It is also how measurement tools work internally to take measurements.

Voltage = Current x Resistance [V = I x R]

Volts = Amps x Ohms

Parameters are moved around to see the other relationships:

Current = Volts divided by Resistance

[I(amps)= V(volts) / R (ohms]

Resistance = Volts divided by Current

[R (ohms) = V (volts / I (amps)]

Meter Function Dials are Ohm's Law in Action

Volts, AC or DC
Amps (Current) , AC or DC
Resistance (Ohms), Continuity (shorts or opens)

Ohm's Law is also how your meter works inside.

Main A/D converter only converts DC volts
Amps and Ohms, plus AC volts converted inside meter
Helps you understand there are limitations on these other measurements.

Understanding Ohm's Law is essential for making measurements.

Measure one parameter (V, I, Ohms) to see what's going on with another parameter.
Critical for Electrical Measurement Safety

Making Measurements with Digital Multimeters and Electrical Testers

Voltage Measurements

How Meters and Electrical Testers Measure Voltage

Voltage is measured "across" wires or components, called "in parallel"

Why?

Because Voltage (volts) is the same in all branches of a parallel circuit.

A meter and test leads create an additional parallel circuit where it is connected
and measures the same voltage that is on the wires, outlet, or any device it is connected to.

How can we do this when the meter sort of looks like a short across the point we are measuring?

Digital meters and electrical testers in the Volts function usually have a very high input resistance, typically 1 million to 10 million ohms (1Mohm, 10Mohm).
Current (amps) is different in each branch of a parallel circuit, depending on the resistance of the individual branch circuit. (Ohm's Law: I = V/R; V is the same , so I (amps) changes according to R (ohms).

Note: This Is Only in the Volts Functions and an important Safety feature:

Check the specification on your particular meter

Placing the leads of a meter in a volts function across open wires, an outlet, legs of a 480V motor.... , effectively creates another parallel circuit that "looks like" a 1 million to 10 million ohm resistor.

By Ohm's Law: I = V/R :

Across 120V outlet at 1Mohms = 120/1,000,000 = .00012 amps = .12mA through the leads and the meter.
480V motor leads = 480V/1,000,000 ohms = .00048 amps = .48 mA through the leads and the meter.
1000V (max) meter = 1000V/1,000,000 ohms = .001 amps = 1mA through the leads and the meter.

Safety! Wear your insulating gloves!! Full PPE!

The resistance of your body (300-600 ohms, I think?) is way less that the resistance of the meter.

If you slip and touch the bare metal with the voltage on, your body becomes a parallel circuit with that full voltage across it.

120V / 300 ohms = .4 amps, or 400mA, possibly through your heart
480V / 300 ohms = 1.6 amps, possibly through your heart

It only takes around .02 amps (20mA) to .04 amps (40mA) to Stop Your Heart.

Measuring DC Volts

LEADS: + Red to Volts/Ohms Jack, - Black to common.

+ or - .5% + 2 basic accuracy

Five Measurement Ranges:

6.000 V AC resolution .001V

600.0 V AC resolution .1V

<span style=">1000 V AC resolution 1V

Measuring AC Volts

Fluke 87V Volts AC Measurement Function Example:

LEADS: + Red to Volts/Ohms Jack, - Black to common.

Input impedance (resistance): typically 1MOhms to 10MOhms (10 million ohms), no load on circuit under test.

+ or - .5% + 2 basic accuracy

6000 count display:

Five Measurement Ranges:

600.0mV AC resolution .0001V

6.000 V AC resolution .001V

60.00 V AC resolution .01V

600.0 V AC resolution .1V

1000 V AC resolution 1V

Yellow Function: Low Pass Filter for reading PWM (pulse-width modulated) signals off motor side of VFD

NOTE: True-RMS ohms converters are important if you are working on any type of electronic or "energy efficient" equipment.

See "Basics of DMMS" tutorial, section 5
See application note: "Why True RMS?"

Measuring milliVolts (mV)

milliVolts (mV) is a special function not found on all meters.

This is where learning to read the meter dial can help you determine that quickly

(See "Choosing" page)

Typically a separate function due to high sensitivity. Used for low level measurements on electronics, checking sensors in HVAC and control systems, and adding external accessories to measure temperature, pressure, and other physical parameters

DC mV does not mean the meter can also measure AC mV

Fluke 87V millivolts (.001V) DC Measurement Function Example:

Special high resolution/low voltage function for measuring the output of sensors and other low voltage output devices

LEADS: + Red to Volts/Ohms Jack, - Black to common.

Input impedance (resistance): typically 1MOhms to 10MOhms (10 million ohms), no load on circuit under test.

+ or - .05% basic accuracy

Range: 600.0mV DC resolution .1mV (.0001V, or 100uV) - Single range in this function

Temperature is a secondary function: Range changes to degress (DEG), thermocouple junction and conversion is built into this meter.

Measuring Ohms (Resistance)

Safety Note for Resistance Measurements!

Meters use different electronics to measure Ohms and Voltage, even though the leads are connected in the same input jacks.

In Ohms functions, the input resistance/impedance changes to a very low value (almost a short circuit)

even though the leads are plugged into the same jacks that have a very high resistance for Voltage functions.

Resistance and Continuity are measured only on disconnected, de-energized wires , resistors, motors, or any other devices.

Meters should have safety protection built-in for the Ohms function, equal or greater than the highest voltage rating (check the specs, don't assume).

Accidentally connecting to a live circuit when checking for shorts and opens (continuity) happens often in electrical work.
Ohms function input protection opens the meter measurement circuit when voltage is detected on the leads, displays "OL", and closes back up when the leads are disconnected from the applied voltage - meter continues to work fine.
Meters without Ohms function safety protection can blow up, short out the connected circuit, or hurt you if accidentally connected to a live circuit.

Fluke 87V Ohms Measurement Function Example:

LEADS: + Red to Volts/Ohms Jack, - Black to common.

Special Functions:

Continuity Mode: press ))) button. Continuity makes it easy to test for shorts (beep) or opens (no beep), if a switch is closed (beep) or open (silent)

Capacitance: press the YELLOW button. Measure capacitors in ranges of 10.00nf, 100.0nf, 1.000uf, 10.00uf, 100.0uF, 9999uf

+ or - .05% basic accuracy

6000 count display:

Seven Measurement Ranges:

600.0 Ohms resolution .1 ohm

6.000 KiloOhms resolution 1 ohm (.001Kohms on display)

60.00 KiloOhms resolution 10 ohms (.01Kohms on display)

600.0 KiloOhms resolution 100 ohms (.1Kohms on display)

6.000 MegaOhms resolution 1000 ohms (.001 Mohm on display)

50.00 MegaOhms resolution 10,000 ohms (.01 Mohm on display)

60.00 nS nanoseimens

Amps (Current) Meaurements

Amps Measurements with DMMs and Current Clampmeters

Current Clampmeters measure amps more safely, inductively or with an electronic sensor, using the magnetic field generated by electricity when it runs through a wire.

Using Current Clamps is safer because they isolate you from the circuit, and do not need fuses to protect you from arc flash.

Digital Multimeter can measure amps directly or with a current clamp accessory.

Safety Note:

Direct amps measurement with a meter requires wiring the meter into the circuit.
The meter imitates a peice of wire in the circuit, so as not to change the current it is measuring
Another word for a "piece of wire" is a Short Circuit
When the meter leads are in the A or mA jack, the meter leads are a direct short.

Amps (A) and mA meter input jacks must be protected by fuses

Some older meters are not! Check your meter!

Never replace these fuses with a low-energy fuse-only use the high energy fuses recommended by the manufacturer.

Try not to leave the leads plugged into the A or mA jacks

Please see Electrical Measurement Safety and Digital Multimeter Basics Section 5 for more information on this.

Direct In-Line Amps Measurement

Meters are wired into the circuit in Series for Direct Amps measurement

Fluke 87V Amps/milli-amps AC or DC Measurement Function Example:

Leads: Black lead to COM

Red Lead:

Use A (amps) input jack for up to 10 amps
Use mA/uA lead input jack for special high resolution/low current mA function. This is for measuring 4-20mA control loops, and other low current output sensors

Using Current Clamp accessories is safer because they isolate you from the circuit, and do not need fuses to protect you from arc flash.

YELLOW button selects DC mode, and goes back to AC mode (toggles back and forth)

(1% + 2) AC basic accuracy , (2% + 2) DC basic accuracy

mA range: Switch RED lead to mA/uA input jack

60.00 mA resolution .01mA (10uA, or .00001 A)

A Ranges:

400.0 mA resolution .1mA (100uA, .0001 A)

6.000A resolution .001A (1mA)

10.00A resolution .01A (10mA) please note: on Fluke meters, this range and input can typically measure up to 20 amps for 30 seconds.

If no measurement, check fuses - last person to use your meter may have connected to voltage with leads in these jacks

Input impedance (resistance): typically 1 ohm or less = like connected a wire across your measurement points. Fuses for input protection are a MUST for your safety.

Never replace the fuse with the wrong or underrated fuse. Always use the manufacturer's recommended fuse value and rating.

Measuring Amps with Current Clamps

A Safer Way to Measure Amps (Current)

Use a Current Clamp Accessory or a Clamp-On Current Meter

A Current Clamp is a coil of wire or an electronic sensor that picks up the magnetic field of the electricity flowing thorugh the wire.

No metal connections = greatly reduced hazard from accidental short circuits.

Two Types:

(1) AC Amps only (coil of wire inside-mA input jacks)

(2) AC or DC Amps (electronic Hall effect sensor-mV input jacks)

For low currents, direct measurement with the meter in-line, connected in Series, is needed where high measurement accuracies are required.

Current clamps increase the error/ decrease the accuracy of the measurement.

AC Current Clamps

AC Amps Current Clamp Accessories (for DMMs)

Require a mA (milliamps) function on meter

Leads: Red lead to mA; Black lead to COM

Reading displayed Amps with an AC Clampmeter Accessory:

Place meter in mA (milliamp) range

Typically sensitivity 1mA (.001A) per amp

Example: For 8 amps, displays "8.0 mA" (milliamp range) or "0.008 A" if no milliamp range on meter

With sensitivity switch on clamp, can change from 1mA per amp to 10mA per amp or 100mA per amp.

AC/DC Current Clamps

DC/AC Amps Current Clamp Accessories (for DMMs)

Typically measure both AC and DC using a Hall Effect sensor that senses the wire's magnetic field and converts this into a mV (millivolt) output proportional to the amps flowing in the wire.

Require a mV (millivolt) function or range on meter.