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What do all the units of measure mean?

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Part Number: Unit Conversions


Unit Conversion



AC Power

Real Power
is the power that is actually converted into useful work for creating heat, light and motion. Real power is measured in kilowatts (kW)and is totalized by the electric billing meter in kilowatt-hours (kWH). An example of real power is the useful work that directly turns the shaft of a motor.
Reactive Power
is the power used to sustain the electromagnetic field in inductive and capacitive equipment. It is the non- working power component. Reactive power is measured in kilovolt-amperes reactive (kVAR). Reactive power does not appear on the customer billing statement
Total Power or Apparent power
is the combination of real power and reactive power. Total power is measured in kilovolt-amperes (kVA) and is totalized by the electric billing meter in kilovolt-ampere-hours (kVAH). Your power company must provide generation, transmission and distribution capacity to supply both real and reactive power to all its customers in the system.
Maximum Demand
Also known as demand. The greatest average value of the power, apparent power, or current consumed by a customer of an electric power system. Usually the averages being taken over successive time periods of 15 or 30 minutes in length.

Kilowatt Calculation


Three Phase Delta

Delta Power formula for a balanced, 3-phase load (delta):

PTOTAL = 2 x Vba x ib x cos(30°) x PF 
PTOTAL is power in watts. Vba is the voltage between any two phases of the three phase circuit for example measure AC-volts between phase B and phase C. The term ib means the AC-amps measured on any one leg (wire) of the three phase circuit (phase A, or, phase B, or, phase C).
My calculator says the cos(30°) = 0.866
PF is the power factor.

For an example balanced load assuming a power factor of 1. If the voltage were about 208 volts between any two phases, and the phase current was about 28 amps on any phase leg then:

2 x 208(volts) x 28(amps) x 0.866 x 1(pf) = 10088 Watts (roughly 10 kW)

If the load is a motor, the power factor (PF) might be as bad as 0.5 instead of PF of 1. Therefore the watt or Kilowatt values might be about half for a worst case motor load.


Three Phase Wye

PTOTAL = (Va-n x Ia x * PF)+(Vb-n x Ib x * PF)+(Vc-n x Ic x * PF)


Va-n is the voltage between Phase A to neutral
Vb-n is the voltage between Phase B to neutral
Vc-n is the voltage between Phase C to neutral
Ia is the current on Phase A
Ib is the current on Phase B
Ic is the current on Phase C
PF is the power factor if known. For some motors this might be as bad as 0.5 instead of the 1.0 ideal.


Power Measurement

The end user has a purpose for monitoring Power.

  • Moment in time - volts amps instantaneous reading at regular intervals kW
  • total over time - Volts times AMPS totalized kWh
  • time when change of state occurs - State or Event logging want to determine when equipment is operating


KW demand, real power, true (rms)

Voltage and current values of the monitored conductors are continuously measured and calculations are updated to provide highly accurate true RMS power readings. The T-VER-8044-100 KW transducer measures true (rms) power demand (kW) expressed as a 4 to 20 mA signal.


T-VER-8044-100 Veris 480 V, 100 Amp Kilowatt Transducer



Veris Industries model H8040 Enercept® series are:
  • Self-contained Split-core kW Transducers
  • 4-20m output
  • Nearly instantaneous response-time of real power.
  • Harmonics can be a problem
  • Bulky with built in fuses
  • 1% total system accuracy, (10% to 100% of CT rating)

APPLICATIONS for KW transducers

  • Real-time power monitoring
  • Process control
  • Energy management & performance contracting
  • Optimization of chillers, pumps & cooling towers


kWh consumption, real power, true (rms)

Voltage and current are continuously measured and updated to provide highly accurate, true RMS power information. kWh transducers measure true (rms) power consumption (kW) expressed by contact closures (pulse output).


The Veris Kilowatt Hour (kWh) Transducers for the HOBO® Energy Logger Pro™ Data Logger System Require S-UCC-M006 Pulse Input Adapter or
S-UCD-M006 Pulse Input Adapter

T-VER-8051-300 Veris Kilowatt Hour Transducer 1-Phase, 300 Amp

T-VER-8053-800 Veris Kilowatt Hour Transducer 3-Phase, 800 Amp

The WattNode Kilowatt Hour (kWh) Transducers for the HOBO® Energy Logger Pro™ Data Logger System Require Magnelab AC CTs and S-UCA-M006 Pulse Input Adapter


T-WNB-3Y-208 WattNode 208/240 VAC 3-Phase Wye kWh Transducer

T-WNB-3D-240 WattNode 208/240 VAC 3-phase Delta kWh Transducer

T-WNB-3D-480 WattNode 480 VAC 3-phase Delta kWh Transducer


Power Factor

Power factor is an energy concept that is related to power flow in electrical systems. To understand power factor , it is helpful to understand three different types of power in electrical systems.

Power factor (PF)
is defined as the ratio of real power to total power, and is expressed as a percentage (%).

     Real Power (kWH)  PF = ------------------ X 100  Total Power (kVAH)  

In general, electrical systems are made up of three components:

  • resistors
  • inductors
  • capacitors.

Inductive equipment requires an electromagnetic field to operate. Because of this, inductive loads require both real and reactive power to operate. The power factor of inductive loads is referred to as lagging, or less than 100%, based upon our power factor ratio.

In most commercial and industrial facilities, a majority of the electrical equipment acts as a resistor or an inductor. Resistive loads include incandescent lights, baseboard heaters and cooking ovens. Inductive loads include fluorescent lights, AC induction motors, arc welders and transformers.


Barometric MSL to Local Pressure

If you want to compare (or use) a non-local source of barometric information, such as the NOAA website, you must adjust for the altitude of your Onset Computer transducer by converting the NOAA barometric pressure back into the local "station" pressure. Any formula for conversion will be an approximation.

1 PSI = 68.948 mbar

NOAA reports barometric pressure as "Mean Sea Level" pressure (MSLP or SLP). This sea level pressure is derived by correcting the station pressure where NOAA's barometric transducer is located to a sea level equivalent pressure. This correction takes into account the standard variation of pressure with height and the influence of temperature variations with height on the pressure. The temperature used in NOAA's sea level correction is a twelve hour mean, eliminating diurnal effects. The calculation of T (the mean temperature of the layer between the station height and sea level) is the most significant parameter in the estimation of SLP. The a white-paper describing difficulty of conversion between SLP and station pressure. Surface Pressure Analyses...Clearing the Confusion In the Presence of Differing Solutions - A brief essay and training exercise on the various types of sea level reduction of pressure in models and analyses Revised April, 2001 Convert "weather report" pressure (corrected to sea level) to station pressure.

General barometrics

Standard pressure is 1013.25 mbar (millibar). Rates of change less than 2 millibars per three hours (mb/3hr) are within expected ranges associated with barometric "station" pressure variation due to diurnal oscillation in temperate climates. Vertical variations of "station" pressure range up to 150 mb per mile, whereas horizontal variations are usually less than .1 mb per mile. The barometric pressure in the tropics (with the exception of tropical storms and hurricanes), simply doesn't change a lot from day to day. In the tropics the barometric pressure (corrected to sea level) should vary from about 1008.7 mbar - 1018.9 mbar. Miami's pressure might change from 1002.0 mbar - 1032.4 mbar. In higher latitudes, variations from 981.65 mbar - 1049.4 mbar are not uncommon.


Difference between PAR and Pyranometer sensors

There is no direct conversion between solar radiation(W/m2) values, and PAR sensor values. You would use a "Silicon Pyranometer" to measure total irradiance. There are some approximations that can be made for known light source types, however for agricultural measurements irradiance measurements are not made using a PAR sensor, the reasoning is outlined below:

Color Wavelength (nm)
Red 780 - 622
Orange 622 - 597
Yellow 597 - 577
Green 577 - 492
Blue 492 - 455
Violet 455 - 390

Not all light is useful for photosynthesis, only light in the wavelength of 400-700 nm (blue-green to red-orange). All light is useful for heat content. One type of measurement is needed to determine how fast a plant will be growing, the other type of measurement is needed to determine how fast water is transpiring and evaporating.

Irradiance is a measure of radiometric flux per unit area, or flux density. Irradiance is typically expressed in W/cm2 (watts per square centimeter) or W/m2 (watts per square meter).

Illuminance is a measure of photometric flux per unit area, or visible flux density. Illuminance is typically expressed in lux (lumens per square meter) or foot-candles (lumens per square foot).

PAR (Photosynthetically Active Radiation) is the quantum measurement of Photosynthetically Active Radiation per unit area, or the number of photons in the 400-700 nm from green to range collected per unit area, during a unit of time. PAR is typically expressed in Microeinstein per second and square meter (µE m2 s1).

A Silicon Pyranometer is instrument most often used to measure irradiance over a defined wide range (300 to 1100 nm) of spectra (including infra-red and ultra-violet). Irradiance as measured by a silicon pyranometer is typically expressed in W/m2 (watts per square meter).


Light values in log Lum/sqf

Lumens and LUX are actually a measure of energy (power) per unit area. Decibels are the logarithmic units commonly used to express power gain or loss.

Gain in Bels = log10(A)
where A = Power amplification factor

Is 100 lumens a small change or a big change of intensity?

The reason for expressing Light values in a logarithm with base 10 is to give more realistic step value changes to your data. A 100-LUX (or lumens) change in light intensity does not represent the same power gain every where along the LUX scale. Eyes would sense a relatively small change of intensity (power gain) if the light level changed from 901 to 1000 lumens, the same eyes would detect a larger intensity change (power gain) if the light level changed from 1 to 100 lumens.

  • Excel function =LOG( ) will work for converting "log Lum/sqm " back to LUX, and will convert "log Lum/sqf " back to Lumens.
example 0.0013 Lumens =LOG( 0.0013 ) will result -2.86 log Lum/sqf
example -2.86 log Lum/sqf =10^ -2.86 will result 0.0013 Lumens
example -2.86 log Lum/sqf =POWER(10, -2.86) will result 0.0013 Lumens

Intensity in (log Lum/sqf) is closest to the native raw storage format of the logger. Yes, there are slight differences between values that you can obtain with Excel LOG, and raising 10 to the power of, functions versus the values given by Boxcar. The limitations of integer math and log conversions cause the slight differences.


lumens/sq. ft and LUX

Units of illumination
  • The lux is a unit of illumination equal to 1 lumen per square meter.
  • The foot-candle is a unit of illumination equal to 1 lumen per square foot.
The foot-candle (fc) is also called lumens/sq. ft
  • To convert from lux to lumens/sq. ft, multiply by .0929.
  • To convert from lumens/sq. ft to lumen/sq meter, multiply by 10.764.

Light intensity measurements are scaled relative to the output of a candle (lumen). To give you an idea of light levels, at noon on a overcast winter day, the light level could be less than 500 fc however on a clear summer day at noon direct sunlight would be over 10,000 fc.

LUX is the equivalent lumens per square meter.


mA to Engineering Units

Media:Formula.xls You have to hit enter after changing the value in a cell

MIN Units and Max Units

In the green cells under ‎MIN Units and Max Units fill in your transducer's range, for example if you had a pressure transducer that read from 0 to 100 PSI you would put 0 under MIN Units and 100 under Max Units. (actually already like that by default.

Out Min and Out Max

If the output of your transducer is 4 to 20 mA you can leave Out Min at 4 and Out Max at 20.

If you have a CONLAB CON-ACT transducer you would need to change Out Min to 0. You would also have to change the Max units to match your particular CONLAB transducer.

You have to hit enter after changing the value in a cell

Test Out and Sensor Out

You can play with Test Out value to see what the Sensor Out represents. Handy during testing.

Engineering Units = (Output x #) - #

Here is your formula in plain text. Output of your transducer times some value, then you may need to subtract some value offset.


PAR micro-moles into watts per meter squared -ROT

Converting from PAR to watts is not simple because you must account for the characteristic spectral bandwidth for different light sources. To make an approximate conversion from µmol·s-1·m-2 to lm/ft2 (footcandles) see the link below: FAQ0017-0800.PDF Photosynthetically Active Radiation (PAR) Units search for - Light Measurement Handbook

Then (after you understand why this is a bad idea) you can use this ROT (rule of thumb): convert from Lumens to Watts multiply by ...0.0015.


PAR term

How the term PAR (µE) came into existence

The PAR unit "einstein" is used to refer to one mole per square meter per second. It means that each second, a 1 square meter of surface has 6 x 1023 photons falling on it. Irradiance levels for plant growth can therefore be measured in micro-einsteins or in PAR watts/sq. meter.

Because jargon peculiar to a field of study is a necessary evil, therefore jargon-busting is needed occasionally for clarification.

Yes, HOBOware and BoxCar PRo software labels PAR units as PAR (µE) not as (µE m² s).

No, we do not mean (µE) as a raw quanta of photons.

We expect (µE) to be understood as quanta in the range of 0 to 2500 µmol/m²/sec, wavelengths 400 to 700 nm.

The manual specifies the PAR Smart Sensor measurement range as 0 to 2500 µmol/m²/sec, wavelengths 400 to 700 nm

Traditionally the quantum flux is measured in micro-moles per second per square meter. By yet aother tradition (eponym), moles of photons are called Einsteins. A crop science (agricultural) jargon term for PAR quanta units is "micro-Einsteins." The conversion factor is: 1µE/sec/m² PAR= 1µmole/sec/m² PAR = 6.02*1017 quanta/sec/m² PAR

Scientific terminology is nothing more than a confabulation of jargon, eponyms and abbreviations. Instead of saying "the-force-that-accelerates-a-mass-of-one-kilogram-at-the-rate-of-one-meter-per-second-per second, " we just say Newton and be done with it.

Few people can say
more than once without causing injury to their tongue.


Volts to Engineeering Units

Please see MA to Engineering Units

µmol/m2/sec conversion to W/m2

The answer is obtuse because there is no real conversion, a formula derived from texts by McCluney and Glover help a little.


The PAR sensor counts the number of photons

  • equally counts low energy photons (infra red) and high energy photons (UV)

The Pyranometer (solar radiation) sensor measures total energy

  • weights on energy (photons).

On a cloudy day, there may be tons of radiation (W/m2), but no PAR (µmol/m2/sec).

  • The Pyranometer measurement of solar radiation is from 300-1200 nanometers .
  • The PAR measurement is from 400 - 700 nanometers.

Here are the conversion equations. Remember that these are very approximate. The two sensors (Solar Radiation and PAR) measure different wavelengths and meaure different things (energy and photons respectively). I hope this helps.

Required Parameter(s): PAR (umol/m2/s) or PAR (uE)
Conversion Equations:

Watts/m2 = 0.21*L
Lumens/m2 = 140.2*L
Lux = 140.2 *L
Lumens/ft2 = 13.03*L


Note: These conversions are only approximations and assume that sunlight is being measured in the 400 nm to 700 nm range.

Source: Calculations by Stalcup based upon texts by McCluney and Glover

Photosynthetic Light (PAR) Smart Sensor S-LIA-M003:

Silicon Pyranometer Smart Sensor S-LIB-M003:

HOBO Weather Station H21-001

HOBO Micro Station H21-002


Quantum Response Definition

Quantum efficiency: In an optical source or detector, the ratio of the number of output quanta to the number of input quanta. Note: Input and output quanta need not both be photons. explains the difference between Lumens (H8, U12 measurements) and PAR (PAR sensor for the weather station)

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