It’s one of those functions we often hear about, without fully understanding it… so let's shed some light on it.

Firstly, we'll look at a typical situation where LPF might be used - with Variable Speed Drives.

In today’s industrial environment, variable speed drives are popping up everywhere. Variable speed drives (VFDs) are used to precisely control the speed of electrical motors. For process manufacturing, and mechanical system control, variable speed drives are a powerful way to operate electrical plant and machinery using exact motor speeds to gain precise timing and outcomes during a particular process. That’s probably a fancy way to say absolute speed control.

This is all very well, but often technicians have to measure what voltages are coming out of the controller, to determine if the signals being sent to the motor are correct. It is here that we run into some measuring difficulty. To understand why, here’s a little bit on how the variable speed drive works.

First, the drive takes in an AC supply from the mains. This can be single or three phase, and rectifies it to a DC supply. The variable speed driver then switches the DC on and off for variable lengths of time to create an average voltage output.

In order to create a simulated waveform output at a particular frequency, the signal changes the time it is on compared to the time it is off as well as changes the total time for the on and off cycle to simulate the average increasing and decreasing voltage of the wave.

I’ll attempt to describe this in words, then show a graph for visual assistance.

At zero on the wave, the signal is off, then the signal goes on for a short time, then off again, next it goes on for a bit longer then off again, then on a bit longer again, then off. What it is doing here is simulating the rising side of a wave form by turning the DC supply on for gradually increasing time intervals to gain an overall average of what would be the increasing positive side of the wave. When it gets to the wave peak, the DC supply is on. To simulate the decline of the positive side of the wave, the signal starts to turn on for consecutively decreasing time periods until it gets to being off just off where it started.

At this point, the voltage is reversed, and the same process is repeated, but now in the opposite direction. The total result of both processes is a simulated waveform.

The total output frequency is adjusted accordingly based on the input request. This can be via electronically controlled signals, or manually adjustable inputs or both.

For three phase VFD output, three of the above simulated wave forms are emitted simultaneously 120 degrees offset to each other, as would be on a traditional three phase sinusoidal output.

Here is a graph to show a typical 3 phase VFD output along with a basic schematic of the set up.

As you can imagine, there is a lot of switching going on with all these on and offs. Think about a normal AC wave form. This occurs at 50Hz in Australia. That’s 50 times per a second. Now chop that into heaps of little parts to simulate this wave form. All these little parts are actually a measurable on and off.

**This is where the problem with measuring comes in. These ons and offs create what is called noise on the wave form. This noise adds to the simulated wave form to give an incorrect measurement of the true average output. **

**This is where the Low Pass Filter comes in. ** The filter in this measuring circuit blocks out high frequency values, and only measures the simulated wave form. This filter is usually set at 100Hz. That is, it will filter out the high frequency switching used to create the simulated wave form, and only measure the simulated form itself.

As can be seen, this feature is very handy when you need to know the actual output voltage and frequency as simulated, so that the technician can adjust and troubleshoot accordingly.

Variable speed drives are an example here to illustrate the use of a low pass filter, but this function can be used with any output that has a noise placed on it due to high frequency switching. Switch mode transformers are another good example of noise producing electronics that can be hard to measure accurately.

]]>When testing insulation on electrical equipment, sometimes it is necessary to obtain a polarisation index (PI).

Polarisation index, put simply, is a ratio of the results from the insulation test over 10 minutes to that of 1 minute. It is a more definitive insulation test than a single insulation resistance (IR). It is usually applicable to testing insulation in generators, motors and transformers.

What does it mean?

When a large DC voltage is applied to an insulator, there will be four different currents that will flow. Two of these currents will decay over time, and the other two will be constant throughout the whole test.

In the Polarisation Index test, when the DC Voltage is first applied the insulation is in effect like a dielectric between two capacitor plates. Thus, capacitance will charge up over time and the resulting current will eventually decay to zero as the capacitance reaches maximum charge. This current is omitted from the results as it is merely capacitive, decays to zero well before the first minute, and gives no indication as to the condition of the insulation. *This is why the first reading is not taken until after the first minute.*

Many impurities (including water molecules) that occur in insulation are polar in nature, especially if the insulation medium is organic.* *As a result they align themselves to an electric field placed on them. This aligning of the molecules when the DC voltage is applied presents itself in the form of an electric current. This current is expected to decay before ten minutes has expired as all of the molecules have aligned by then. *This is why the test is done for ten minutes.*

At the end of the ten minutes there are only two currents left flowing in the insulator. A current that flows through the medium, and a current that flows on the outside of the insulator.

Some current will flow through the insulation regardless of what the insulator is made of. However, if the insulation has cracks, or has begun to break down, this current can become larger than desirable. That is, the insulation is no longer at it’s optimum.

The current that flows on the outside of the insulation is due to moisture, dirt, grime, carbon, etc that builds up on the surface and allows for some degree of conductivity.

Here’s where the PI comes in.

From ohm’s law, the total current and supply voltage are used to determine the resistance of the insulation.

After one minute, the capacitive current has decayed away, the polarisation current is still near its highest value, and the internal and external currents are also flowing. By ten minutes, the only currents flowing are internal and external currents.

If the one minute current is high, compared to the ten minute current, then the resistance value at one minute will be low compared to the ten minute value. This is because the majority of initial current is polarisation and the other two currents are comparatively minimal, indicating that the insulation is in good condition.

If the one minute current is NOT high compared to the ten minute current, then the value of resistance taken at 1 minute will be closer to the ten minute resistance value. This indicates that more of the current is due to the internal and external currents, rather than polarisation, and suggests that the insulation is beginning break down.

A polarisation index less than 2 indicates that the insulation is becoming questionable to unreliable. Greater than 4 is excellent. Most companies will have specific information that indicate what test results are acceptable. Be sure to find out, and if not available on a company level, then the information should be sourced through the equipment manufacturer.

]]>Duty cycle is the ratio of how long something is turned on for, in relation to total time. It describes the relationship between **total time on, compared to time off, and is expressed as a percentage of total time**.

For example, if a cycle of a particular on/off function took 10 seconds, and the duty cycle was measured at 30%, then the total time the test subject was on for would be 3 seconds. (30% of 10 seconds)

This may be useful if measuring a pulse wave from a digital source, or measuring how often a machine is doing a particular task, or how often a signal is received from a particular input.

Most good digital multimeters have a duty cycle function as duty cycle is closely related to frequency. In fact, it is usually found in the same settings that the frequency is located.

]]>However, sometimes it is necessary to measure a wave form that is not DC or a pure AC sine wave, for example, a pulse width wave output from a digital source, or a wave from an inductor that is cycling a charging and collapsing magnetic field.

Crest factor is the ratio between the RMS (Root mean square) value and the peak value of a wave form. It is calculated by dividing the peak value by the RMS value. *If you are not familiar with RMS, see our guide on RMS. *

The graph below shows a pure sign wave with the RMS at 0.707 and the peak at 1. The crest factor here would be 1 divided by 0.707. That is 1.414

If a pulse width digital wave output is on for a short time, then off for an extended time in its cycle, the RMS (equivalent DC effect) will be low compared to the peak, hence the ratio between the RMS and the peak will be high. That is, the crest factor will be high. The graph below shows this. Here the crest factor would be about 3 (Peak/RMS).

If the opposite is occurring and the output is on for a longer time and off for a shorter time, then the RMS value will be closer to the peak value hence the ratio will be lower. That is, the crest factor will be lower. In the example below the crest factor would be a little bit more than 1.

A collapsing magnetic field on an inductor can cause large spikes in the wave peaks, but the RMS value can be rather low in comparison to these peaks. If you have cause to be measuring such wave forms, you need to choose a DMM that is designed to measure a wave form with the high expected crest factor.

**If the crest factor is outside the specifications of the multimeter, then the measurements will become inaccurate.**

In conclusion, if the type of measuring you are doing only involves DC or pure sine wave AC, then crest factor will not be a major consideration in regards to the multimeter specifications.

If however you are measuring other wave forms, and expecting larger variables in crest factor, then you need to choose a multimeter with appropriate crest factor specifications.

]]>IP rating is a scale of how well a multimeter or clamp meter stands up to dust and moisture.

Most people understand that the higher the IP rating, the better the DMM can withstand a dusty, moist or even a wet environment. However, at best they may be guessing based on the number, but may not really understand how the rating works and whether the equipment will actually survive a particular working condition.

To begin with, the initials IP stand for “Ingress Protection”. What can and can’t get in.

This is followed by two more numbers. The first number is a rating of particle and dust ingress protection. The second is a rating of moisture and water ingress protection. Each number represents a specific correlation to the maximum exposure the equipment can handle.

For example...

An IP rating of IP68 on your multimeter, would mean dust tight (6) and continuous immersion (8). Be careful with this rating as some manufacturers have a limited immersion depth and time frame. Don’t assume with an IP68 you can throw your meter in the deep end of the pool and just leave it there.

An IP67 would be dust tight and could stand a temporary immersion in water, but get it out quick, or it will die.

An IP43 would handle particles greater than 1mm in diameter, and survive water spaying on it, but a jet of water, or a drop into a bucket of water would likely mean the end!

**So when choosing a digital multimeter, take into account the environment which it will likely be exposed to, and select one with the appropriate IP rating. **

*Note: Other special environments (such as explosive environments) are different again, and require specialist equipment for such situations.*

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The CAT rating refers to what installation type the multimeter is safe to work on. Each category has been chosen based on **expected transient over-voltages** that may occur for each category.

The higher the expected transient over-voltage, the higher the category rating required of the multimeter.

*The information below has been sourced from “Electrotechnology Practice” by Jeffery Hampson.*

**CAT I -** Category I testing equipment is used on electronic circuits (signal level) and other low-energy equipment which has transient protection (isolation transformer). The testing equipment rated as Cat I must be marked with its actual voltage protection level.

**CAT II -** Category II testing equipment is used on energy consuming equipment such as portable appliances and power tools that are connected to a socket outlet.

**CAT III -** Category III testing equipment is used on an installation's distribution circuit and includes all mains circuit board equipment and devices connected after the kWh meters. Installation Category III also includes all fixed wiring circuits and permanently connected devices such as air conditioners, space heaters, hot water heaters and ranges.

**CAT IV –** Category IV testing equipment is for the highest level of electrical power and is suitable for measurements with respect to consumer's mains, power consumption meters and off-peak switching devices.

**A point on CAT ratings; **

Although the equipment may be rated to a particular category, it should be thoroughly inspected before use to ensure no damage exists that may compromise its insulation qualities.

Further to this, swapping leads on multimeters can be hazardous as the leads for a multimeter are specific to the CAT rating of the meter. By swapping leads between meters, you could inadvertently downgrade the category rating and be susceptible to transient voltages.

]]>Accuracy refers to how accurate the ** displayed figure** is compared to the

The accuracy value given in the specifications of a digital multimeter will look something like,

“Basic accuracy ±0.5% ± 2 dgt”

*The term accuracy is often confused with count/digits, and resolution. **Y**ou can read our blog on Understanding Count, Resolution and Range for a full description of count/digits and resolution.*

*“Basic accuracy ±0.5% ± 2 dgt”*

The first figure will give the **percentage accuracy** for a given range. In the above example 0.5%. It means the displayed result could be 0.5% higher or 0.5% lower than what the real result should be.

The second figure refers to the last, or least significant figure, on the multimeter display. This **last digit could be incorrect to plus or minus** “that many digits”. In this case 2 digits. This error is in addition to the above mentioned percentage error.

Putting it together with an example - a multimeter with a 2000 count display is showing a result of 2.005V and has a basic accuracy of ±0.5% ±2 dgt.

The percentage error is 2.005 x 0.5% = 0.01. Thus the real value of what’s being measured could be anywhere between 1.995V and 2.015V *(2.005 - 0.5% = 1.995V and 2.005 + 0.05% = 2.015V)*.

However, an included error of ±2 digits on the display means we have to push the last significant figure of these values out by another 2 digits. Thus the real value of what’s being measured could be anywhere between 1.992V and 2.017V *(1.995V – 0.002 = 1.992V and 2.015 + 0.002 = 2.017V).*

*For a lot of users, such minor errors would be insignificant to the values they are looking for. However, for some, small errors could increase fault finding difficulties dramatically. Eg. when measuring outputs on sensors where the control circuitry is looking for very small changes.*

*As such, if accuracy is imperative to your measurements, be sure to check the specifications on the multimeter to ensure that the accuracy is suitable for your requirements.*

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The example above uses voltage values. Delta Pro includes voltage basic accuracy in the "Quick Stats" for most products. However, any function that a meter has will have a basic accuracy associated with a measurement. These basic accuracies can be found in the product specification sheets.

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The terms Count and Digits are used interchangeably, and refer to the way a digital multimeter can display a result. They describe the maximum displayable value in each measuring range, and by default the resolution displayable for each range. When understood, it can help significantly in choosing the correct DMM for your needs.

Before we go into this, we’ll have a look at the terms ‘range’ and ‘resolution’. These two terms need to be understood, as they refer to digital multimeters, in order to get a good grasp of count and digit.

**Range. ** Depending on how big or small a quantity that you are measuring, the multimeter needs to use the circuitry that is best suited to that particular quantity. This is known as range. Each range is described on the meter in terms of the maximum value measurable for that range.

For example, if you are measuring between 0V and 10V the multimeter should be set on a range that uses circuitry best suited for these values, eg 20V range. Here, 20V is the maximum measurable value on that range, and 0V -10V falls comfortably in that range.

However, if you are measuring 50V - 100V, then the circuitry that is suitable to measure the 0V-10V range would be unsuitable. Another range, say 200V should be selected.

With most digital multimeters these days, automatic ranging is performed by the meter.

**Resolution.** The term resolution refers to how many significant figures can be displayed by the DMM. That is, the smallest fraction of the measurement that can be shown. A meter displaying a value of 1.0198V has better resolution than one that can only display 1.020V on the same test. Here, the resolution would be better by a factor of 10.

Count is a figure used to describe how well a digital read out can display the measurement abilities of a digital multimeter, and gives an indication of the likely limit of each range.

The count number refers to the limit of the highest value the DMM will display for each range, and the number of digits displayed for these values. As soon as a measured value reaches this limit, the range will move up and the resolution will be downgraded by a factor of 10, that is, the decimal place will be moved to the right one spot and the last digit on the right will drop off.

Count becomes important if high resolution results are required when measuring.

***

**Looking at an example –** if we have a multimeter with a count of 40000, then the highest digit readout for each range would be 39999 (not withstanding the decimal point). Note that the display shows one less than the count specification. This is because the number 40000 is easier to work with!

If we measure 1V the screen will show 1.0000V. However, if we increase the voltage being measured until it goes above 3.9999V to 4V, then the screen will display 4.000V (which is actually 04.000V in the count). It cannot display 4.0000V because that is the cut off for that range.

Now that the range has changed you can see that the resolution displayed has been downgraded by a factor of 10. The decimal place has moved to the right one spot causing the far left digit to be dropped.

Now we can move up in this range from 4.000V until we reach 39.999V, at which time we have again reached the limit for this range. At 40V the display will move to 40.00V (Which is actually 040.00V in the count).

Again the range has changed and the resolution has been downgraded by another factor of 10. This change in range when the measurement passes each range limit, will continue to occur until the measurement extends beyond the testing abilities of the multimeter.

If we had a multimeter that was designed to measure up to 1000V with a 40000 count, then, depending on the actual circuitry, the ranges and resolution could go;

0 to 3.9999 with resolution of 0.0001

4 to 39.999 with resolution of 0.001

40 to 399.99 with resolution of 0.01

400 to 1000 with resolution of 0.1

When buying a digital multimeter, it is important to understand what you are testing, the range you will be testing in, and whether or not minor values in regards to resolution are important.

For example, if you are testing a car battery, then two decimal places at 12V will more than likely be ample resolution, so a 2000 count display will be plenty. This will give a resolution to two decimal places up to 19.99V. The same multimeter is capable of displaying up to 199.9V (1 decimal place) and 1999V (0 decimal places).

However, if you require more resolution in your results, eg. down to three decimal places for the same 12V supply, then a 2000 count will not suffice because it needs an extra digit. A 20,000 count will work here.

Do not get confused between a larger count for display resolution, and the accuracy of the multimeter. They are two different characteristics. One gives the measurement accuracy of the multimeter (see Understanding Terms - Accuracy for more info). The other (count) is the ability to display this accuracy to the user.

Digits is just another way to describe the count, but in my book, a little less user friendly.

If the specification of the multimeter gives digits rather that count, it will be the number of digits that can be displayed from 0-9 plus the most significant digit, which cannot be displayed right through to 9.

**The format is given as a whole number followed by a fraction**, usually a 1/2 or a 3/4.

The whole number is used to represent how many digits displayed from 0-9.

The fraction is used to describe how high the most significant digit in each range can go. A 1/2 usually means the most significant digit for each range goes to 1. A 3/4 means the most significant digit for each range is greater than 1. *This actual figure will change between manufacturers.*

For example. A 3 1/2 digit can display up to 1999. That’s 3 digits from 0-9 AND a most significant digit from 0-1. Note that this is the same as a 2000 count.

A 4 3/4 digit may display up to 39999, 49999, 59999 or even 69999. That’s 4 digits from 0-9 and the 3/4 meaning a most significant figure is greater than one. The 3/4 fraction given leaves room for ambiguity, which is why I prefer the count.

What’s wrong with the most significant figure? Why doesn’t the multimeter display the most significant figure in digits up to 9 like the rest of the display? Common question!

It’s got nothing to do with the readout not being able to show higher digits at the most significant place. It’s actually to do with the circuitry in the multimeter and when the circuitry gets to it’s limit in each range. So, with a 2000 count the circuitry in each range gets to its limit at 1999 and has to change range.

Sometimes it is important to know when the multimeter actually changes range. If you are measuring a quantity that requires a certain amount of significant figures, but what you are measuring is on the border of when the multimeter changes range, then the resolution required could down grade by a factor of 10 right near the point of measurement and you lose the required lowest significant figure. Thus, this multimeter may actually not be suitable. Again, it is important to understand what you are testing, and the range you will be testing in.

]]>AC in electrical terms means ‘alternating current’. This means the current has a cycle that goes from zero to a peak value in one direction, back to zero, then to a peak value in the opposite direction, returning to zero. It does this repeatedly.

Depending on what point in time during the cycle you look at, you could measure a value anywhere between these two positive and negative peaks.

So how can we take a measurement of an AC current that makes sense?

Well, let’s first get some sort of comparison to the AC current. Let’s compare it with a DC current which is unchanging and continuous in one direction. What values in the AC current can we find that will match what the equivalent DC current can do?

By doing a little bit of fancy maths (which we won’t) on the AC signal you can calculate a special type of average value, called a **root mean square (RMS)**.

It turns out that this calculated current represents a figure from the AC supply that allows us to directly compare the AC and DC currents. That is, a value where the same AC current will cause the same dissipated power in a resistor as the equivalent DC current.

** **

Some digital multimeters do not use a true RMS calculation to obtain a value, instead they take an average value and multiply it by a special factor that gives the equivalent RMS value. This is known as the averaging or mean method. (Here the negative values have to be flipped to positives or the average would be zero.)

With a pure sine wave, the two calculation methods, whether TRMS or averaging, will give the same result. However, if there is any impurity or ‘noise’ in the sine wave, or the wave you are measuring is not a sine wave, then the averaging calculation method will have errors. The True RMS method however, will still be accurate.

This is why DMM brands make sure they advertise TRMS in their specifications if they use that method of calculation.

**Although current is used to explain the RMS value above, RMS value is also used to measure the alternating voltage. **

Note: a grid connected supply voltage is given as the RMS value of the supply, and a quoted current on apparatus designed for this supply is deemed to be an RMS value.

**AC+DC**

It is possible to have a wave form that is not symmetrical about the x axis (zero point). It may have more of the wave in the positive or negative region. This is known as a DC off set.

It turns out that some digital multimeters that advertise True RMS cannot give accurate values for this situation because of the way RMS is derived.

There is great YouTube video here that explains True RMS and AC+DC in detail.

If any of your measuring is likely to fall into this category, then ensure that the meter has AC+DC function if accuracy is critical. The function will be mentioned in the specifications sheet and will include the accuracy margins.

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