DPCNews 026 - MT/PT Units: Follow the rules; Stop the mess

Written by Administrator
Thursday, 01 July 2010 12:21

July 2010

By Patrick DUBOSC and Pierre CHEMIN

“Reproduced with permission, Materials Evaluation, Vol. 68, No. 5, 2010, ©American Society for Nondestructive Testing.”


ISO-3452 (1998-2008), ISO-9934 (2001-2002) and ISO-3059 (2001) standards, for liquid penetrant and magnetic particle testing are written using the International System of Units (SI), aka Le Système International D’Unités (BIPM, 2006). ISO 1000 (ICS 01 060) (1992) details the SI units and gives guidelines regarding their multiples.

In nondestructive testing (NDT), this system is far from perfect. Documents often take no account of it. For example in ASTM E- 1417-05e1: Standard Practice for Liquid Penetrant testing (2005a), data is given in inches and pounds and SI units are given in parentheses.

Additionally, the European EN-1330 (1997-2009) standards dealing with terminology do not have an ISO status. This leads to problems interpreting and understanding documents from different origins, which leads to technical and financial consequences.

Measurement Units


The SI volume measurement unit is the cubic meter (symbol: m3) or the liter.
The symbol for liter is "l" but it is recommended to use the "L" to avoid any confusion with the numeral 1.

Specific Gravity and Density

This is one of the best examples of a mess. ISO-3675 (1998b) is titled (in French) "Pétrole Brut et Produits Pétroliers Liquides - Détermination en Laboratoire de la Masse Volumique - Méthode à l'Aréomètre". In English it is titled, "Crude Petroleum and Liquid Petroleum Products - Laboratory Determination of Density - Hydrometer Method". This standard is the basis for the measurements of liquid penetrant materials.

The translation of "masse volumique" in English is "specific gravity". Furthermore the French word "densité" is not equivalent to the English word "density". Densité is translated as "relative density". A bit confusing indeed!
However, this is just the beginning of the mess.

According to the standards the masse volumique is the ratio between mass and volume, the SI unit is kilogram per cubic meter (kg/m3) while the densité is the ratio between the mass of a given volume and the mass of the same volume of water (or air, for gases).It is the ratio between two masses, hence, it is a dimensionless figure without units.

For example: the densité of water is 1, when its masse volumique is 1000 kg/m3 at +4°C.
In the French version of ISO 3452-2 (2006), it is the masse volumique that shall be recorded, and not the densité.

Here are some subtleties.

ISO-3675 (1998b) requires measurement be done at a temperature of + 15°C. This same standard is used on chemicals other than petroleum that are tested as liquids with a Reid vapor pressure less than or equal to 100 kPa. Nevertheless, the French version of ISO-3452-2 (2006) requires that the measurement of the masse volumique be done at 20°C, without asking for a specific method.

Now things get interesting. (In fact, it is all interesting! But this pushes the envelope.) Technical data sheets and material safety data sheets are issued by several penetrant materials suppliers.
• A French supplier states the densité in comparison to water without any mention of the temperature.
• Another French manufacturer states sometimes the densité at 20°C and the masse volumique in g/cm3 without any mention of the temperature.
• A third French manufacturer gives the masse volumique in grams per cubic centimeter (g/cm3) at 20°C.
A British manufacturer mentions the density which is the French masse volumique in grams per liter (g/L). So, one could read for a fluorescent penetrant that the density in g/L at 15°C is 0.98, indicating that the penetrant is about 1000 times lighter than water! (Note that 0.98 is not the correct way to express this.)
• In the meantime, an American manufacturer states the "density" in pounds per US gallon (lb/gal) and in grams per cubic centimeter (g/cc) without any reference to temperature. Note that the right symbol for cubic centimeter is "cm3" and not "cc": though it is often used interchangeably.
• While a French manufacturer states the masse volumique in kg/m3, as it should be, but at +15°C.

ASTM D 4052: Standard Test Method for Density and Relative Density of Liquids by Digital Density Meter (2009) is used to determine the specific gravity or the density of petroleum distillates and viscous oils that may be handled as liquids in the +15°C to +35°C temperature range. It is used only for products whose vapor pressure is lower than 80 kPa and kinematic viscosity lower than about 15000 mm²/s at the test temperature. This method shall not be used with samples whose color is so dark as to prevent seeing air bubbles.

ASTM D 5002: Standard Test Method for Density and Relative Density of Crude Oils by Digital density Analyzer (1999) is used to determine the specific gravity or the density of petroleum distillates that may be handled as liquids in the + 15°C and + 35°C temperature range. It is used for crude petroleum with high vapor pressure provide all the needed precautions are taken to avoid evaporation while the sample is moved to the analyzer.

This method has been validated by round robin tests between laboratories with samples of crude petroleum whose specific gravities were in the range 0.75 to 0.95 g/ml (it is better to write: 750 to 950 kg/m3). A lighter crude petroleum sample may need specific precautions to avoid evaporation. Heavier petroleum may require measurement at higher temperatures to avoid air bubbles in the sample.

ASTM D 5002 (1999) states that SI units shall be the units to use. Accepted units are grams per milliliter and kilograms per cubic meter.

Kinematic Viscosity

ISO 3104: Petroleum products - Transparent and opaque liquids - Determination of kinematic viscosity and calculation of dynamic viscosity (1998a) is commonly used for penetrants and emulsifiers. The SI unit is the square millimeter per second (mm²/s). As for mineral oils and other chemicals, the test is carried out at +40°C. ISO 3452-2 (2006) does not specify any test method or temperature.

American and UK suppliers give a viscosity figure in their technical data sheets and their material safety data sheets but do not refer to the kinematic viscosity. Furthermore, they use the centistoke (cSt), a unit that is now obsolete. The figure when using the mm²/s is ... exactly the same: 5 cSt = 5mm²/s. So why not use the standardised unit? Some think it much easier to pronounce "centistokes" than "square millimeters per second". We all agree, but the correct unit is the mm²/s.


While many in the US and UK continue to use the pound per square inch (psi), the bar (symbol: bar), roughly equivalent to 1 atmosphere (atm), is still widely used in France. The SI unit is the pascal (Pa). It is easy to remember that 1 bar equals 0.1 MPa.

Electric Current Intensity

The SI unit for electric current intensity is the ampere (A). The abbreviation "amps" is often used, which is not acceptable.


For illuminance, the SI unit is the lux (lx). Foot-candle, as well as lumen per square foot, shall no longer be used.

Magnetic Field

Though the SI unit for magnetic field measurement is the ampere per meter (A/m), "amp/inch" (which should be A/in.) is commonly written.


Even for such a common unit as time measurement, the abbreviation "sec" is often used, when the SI unit, the second, should be written "s". Also, the abbreviation of "minute" is "min" and not "mn" as is often seen.


Kelvin (K), the unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water (Ttpw) (BIPM, 2006). It follows that the thermodynamic temperature of the triple point of water is exactly 273.16 kelvins, Ttpw = 273.16 K.

At its 2005 meeting, the International Committee for Weights and Measures (CIPM) affirmed that this definition refers to water having the isotopic composition defined exactly by the following amount of substance ratios: 0.000 155 76 mole of 2H per mole of 1H, 0.000 379 9 mole of 170 per mole of 16O, and 0.002 005 2 mole of 18O per mole of 16O.
Because of the manner in which temperature scales used to be defined, it remains common practice to express a thermodynamic temperature, symbol T, in terms of its difference from the reference temperature T0 = 273.15 K, the ice point. This difference is called the Celsius temperature, symbol t, which is defined by the quantity equation t = T- T0.

The unit of Celsius temperature is the degree Celsius, symbol °C, which is by definition equal in magnitude to the kelvin. A difference or interval of temperature may be expressed in kelvins or in degrees Celsius (BIPM, 1969), the numerical value of the temperature difference being the same. However, the numerical value of a Celsius temperature expressed in degrees Celsius is related to the numerical value of the thermodynamic temperature expressed in kelvins by the relation t/oC = T/K - 273.15.

The kelvin and the degree Celsius are units of the International Temperature Scale of 1990 (Preston-Thomas, 1990) adopted by the CIPM in 1989.

Symbols of Multiples and Units

The multiple "kilo" (symbol: k) is often displayed as "K". In SI, "K" is the symbol of the kelvin (unit of thermodynamic temperature). Hence a "Kg" would mean a "kelvingram"!

In many American documents, there is a common confusion between the symbol "m" which means "milli" (10-3) and the symbol "M" which means "Mega" (106). If "M" is written in lieu of ‘‘m’’ this comes to a factor of 109 or a billion times. Also the abbreviation "mcg" is used for microgram instead of the standardized "µg".

Submultiples should not be used simultaneously in the numerator and in the denominator, as well as the multiples. For example, measuring ultraviolet-A irradiance should give data in W/m² instead of µW/cm².

Often, computer programs edit submultiples incorrectly. For example, if the submultiple "m" (a small letter) is written followed immediately, without any space, by the symbol of a unit beginning with a capital letter like A for ampere, the computer program modifies the "m" (10-3), to a capital letter. In this case, "m" becomes "M" (mega). One way to avoid this trouble is to first write the "m", add a space, then type the "A". Afterward, go back and finally delete the space between the "m" and the "A".

The symbol of any unit named after a person begins with a capital letter. Therefore, the symbol for joule is "J" while the symbol for candela is "cd".
There is one exception: at the beginning of this paper it was stated that the symbol for "liter" is "l" but that it is better to use "L" to prevent any confusion with the numeral "1". "L" was the symbol of the obsolete luminous luminance unit lambert. This SI unit is the candela per square meter: cd/m².

Parts per Million (ppm)

The term parts per million, meaning 10−6 relative value, or 1 in 106, or ppm, is also used (BIPM, 2006). This is analogous to the meaning of percent as parts per hundred. The terms “parts per billion”, and “parts per trillion”, and their respective abbreviations “ppb”, and “ppt”, are also used, but their meanings are language dependent. For this reason the terms ppb and ppt are best avoided. (In English-speaking countries, a billion is now generally taken to be 109 and a trillion to be 1012; however, a billion may still sometimes be interpreted as 1012 and a trillion as 1018).
The abbreviation ppt is also sometimes read as parts per thousand, adding further confusion.

The ppm symbol is a way to express percentages. This is widely used in Europe but is often confused in different parts of the world. The ppm allows for simplifying writing, preventing confusion between units. The abbreviation ppm could be a comparison of volumes (sometimes referred to as ppmv), masses (just ppm) or of a mass with a volume (once again, just ppm).
Here are some examples to clarify this. The atmosphere contains 0.038% of carbon dioxide volume/volume, that is, 380 ppm. One million ppm is 100%. Some specifications for Liquid Penetrant Testing and Magnetic Particle Testing require that the products show less than 200 ppm of sulfur: this is equivalent to 0.02%, or 200 mg per liter, or 200 mg per kilogram (specifications often ask for mass/volume).

Writing with ppm avoids the many "zeros". 0.001% and 10 ppm are the same figure; 1 ppm (1 part per million, a very low figure) is equivalent to 0.0001%. It is easier to say, write, read: 1ppm. It also gives less opportunity to make a mistake in the number of zeros.

Powers of Ten

A useful reminder:
• 100 = 1.
• 101 = 10.
• 102 = 100.
• 10-1 = 0.1.
• 10-2 = 0.01, or 1/100.

Comma, Dot and Plurals

In the US and UK, documents often use a comma to show thousands. For instance 123,456 means one hundred twenty-three thousand four hundred fifty-six. SI uses spaces instead of coma. So 123,456 should be written 123 456.

The European metric system uses a comma for the decimal part of a number. For instance: 104,7 means 104 and 700/1000. This is a very important difference that may lead to accidents! At a minimum, it is confusing.

In the US and UK, 104.7 means one hundred and four and 7/10. In the European metric system, this writing has no meaning at all.

Another interesting point: the plural. American documents show 1 mile and 1.5 miles (with a dot and plural). As per the SI documents (should the mile be accepted), it would be written as 1 mile and 1,5 mile (with a comma and singular).

Physical Quantities

Magnetic Particle Testing is the NDT method in which we see most confusion between imperial and SI units. For instance magnetic field and magnetic induction are very often used as if equivalent. Europeans are surprised to read that magnetic particle inspection shall be carried out when there is a magnetic induction of at least 30 Gauss on the part. If it were for an induction it would be advisable to write 3 millitesla (mT), and far better to write the requirement of a tangential magnetic field of 2400 amperes/meter (A/m). Gauss is a unit that has been obsolete for at least 3 decades, and will no longer be used. We also recently read in an American document: 1 T= 10 Gauss. This is another example of a problem of powers of ten.

ASTM E 1444 :Standard Practice for Magnetic Particle Testing (2005b) states: "Except with CEO approval, formulas may only be used if the amperages are confirmed with known or artificial defects (QQIs) or with the Hall effect probe gaussmeter".

The word "gaussmeter" is improperly used, as the gauss is an obsolete unit for magnetic induction (or magnetic flux density). The meter called a "Hall effect probe Gaussmeter" in fact measures a tangential magnetic field (sometimes an axial magnetic field when used to measure residual magnetic fields) and not a magnetic induction.

Annex XI of ASTM E 1444 (2005b) explains how to use the Ketos ring to check the performance of the magnetic bench. Tables X 1.1 and X.1.2 detail the electric current forms to use and their intensities. Results are given depending on the number of holes detected using the figures in the tables. Unfortunately the document does not identify whether the intensities are given as RMS or peak values.

CAAP 42V-3(0) Magnetic Particle Inspection - Use and Implementation of ASTM E 1444 (CASA, 2006), issued by the Australian Civil Aviation Safety Board states that the "Black Light Intensity" should be replaced by "UV-A radiation irradiance". In a similar manner "white light intensity" should be replaced by "white light illuminance". However, in another paragraph that lists quality control equipment "black and white light meter" is used, when a "dual purpose UV-A radiometer/visible light luxmeter" is better wording.


It would be useful if the US truly adopted the SI system. It would be a step farther to globalisation of the system: a small step for everybody every day, a huge step for the world. Technical and commercial exchanges would be eased, and very costly mistakes could be avoided.


Patrick DUBOSC
124 Allée des Jonquilles, F-62780 CUCQ LE TOUQUET, France.
e-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it


Résidence Diderot, 294 Rue de Charenton, F-75012 PARIS, France.
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• ASTM, D 5002: Standard Test Method for Density and Relative Density of Crude Oils by Digital Density Analyzer, West Conshohocken, Pennsylvania, American Society for Testing and Materials, 1999.

• ASTM, E 1417 - 05e1: Standard Practice for Liquid Penetrant Testing, West Conshohocken, Pennsylvania, American Society for Testing and Materials, 2005a.

• ASTM, E 1444 – 05: Standard Practice for Magnetic Particle Testing, West Conshohocken, Pennsylvania, American Society for Testing and Materials, 2005.

• ASTM, D 4052: Standard Test Method for Density, Relative Density, and API Gravity of Liquids by Digital Density Meter, West Conshohocken, Pennsylvania, American Society for Testing and Materials, 2009.

• BIPM, Resolution 3: SI unit of thermodynamic temperature (kelvin), Resolution of the 13th Conférence Générale des Poids et Mesures (CGPM), Sèvres, France. Bureau International des Poids et Mesures, 1969. http://www.bipm.org/en/CGPM/db/13/3.

• BIPM, The International system of units (SI), aka Le Système International D’Unités, 8th edition,Sèvres, France, Bureau International des Poids et Mesures, 2006.

• CASA, Draft CAAP 42V-3(0) Magnetic Particle Inspection- Use and Implementation of ASTM-E-1444, Canberra, Australia, Australian Civil Aviation Authority, 2006;

• CEN, EN 1330 : Non-destructive testing. Terminology, Brussels, Belgium, European Committee for Standardization, 1997-2009.

• ISO, ISO 1000: SI units and recommendations for the use of their multiples and of certain other units, Geneva, Switzerland, International Organization for Standardization, 1992.

• ISO, ISO 3104: Petroleum products - Transparent and opaque liquids - Determination of kinematic viscosity and calculation of dynamic viscosity Geneva, Switzerland, International Organization for Standardization, 1998a.

• ISO, ISO 3675: Crude petroleum and liquid petroleum products - Laboratory determination of density - Hydrometer method, Geneva, Switzerland, International Organization for Standardization, 1998b.

• ISO, ISO 3059: Non-destructive testing - Penetrant testing and magnetic particle testing - Viewing conditions, Geneva, Switzerland, International Organization for Standardization, 2001.

• ISO, ISO 9934: Non-destructive testing - Magnetic particle testing, Geneva, Switzerland, International Organization for Standardization, 2001-2002.

• ISO, ISO 3452-2: Non-destructive testing - Penetrant testing - Part 2: Testing of penetrant materials, Geneva, Switzerland, International Organization for Standardization, 2006.

• ISO, ISO 3452: Non-destructive testing-- Penetrant testing Geneva, Switzerland, International Organization for Standardization, 1998-2008.

• Preston-Thomas H., ‘‘The International Temperature Scale of 1990’’, Metrologia, Vol.27, pp.3-10.

We, Pierre CHEMIN and Patrick DUBOSC, welcome any comment, any idea. If you have some examples you would like to see discussed here, please give us all the useful indications. If you require confidentially, we would modify locations, names and some parameters to prevent any traceability.
Nevertheless, we are convinced that our site may be a kind of surge-valve: the topic is NOT to target this company, or that auditor; but it is always to make users think, to make them ask themselves, or others, the right questions.
We may also give advice, once again on a confidential basis if needed: please, feel free to ask questions, to document our data basis: about Material Safety Data Sheets (MSDS), about environment, a chemical name you don't understand, a Penetrant process you have heard about, etc.
We have plenty of examples, some being out of all the specifications/standards, which led to the discontinuities detection, when the "current, normal, processes" prevented discontinuity finding.

Last Updated ( Saturday, 21 May 2011 13:01 )