What 1 mg/kg Actually Means On A Cleaning Specification

Overview

Cleaning specifications use three different mass units that look similar and are not interchangeable. A machine described as achieving one milligram per kilogram, another quoted at two milligrams per part, and a third at ten milligrams per thousand square centimetres are not describing the same thing, and comparing them without conversion is a common source of disagreement at acceptance.

The unit matters most when part mass varies. One milligram per kilogram on a one-kilogram component is one milligram of residue; on a three-hundred-kilogram housing it is three hundred milligrams. The same headline figure describes a requirement that is three hundred times looser on the larger part, which is either exactly right or exactly wrong depending on what the requirement is protecting.

This article explains the three units, when each is appropriate, how to convert between them, and where conversion stops being valid. Big Bird Industrial (formerly Shimada Big Bird Industrial) has been building cleaning and deburring systems for automotive powertrain components since 2001, and the figures used here come from specifications on production lines.

1. Three Units That Are Not The Same

Each unit answers a different question, and the question determines which one belongs in a specification.

Milligrams Per Part

This is an absolute quantity: the total mass extracted from the whole component. It is the most direct statement for a part of consistent size, and it is what most automotive specifications use, with figures ranging from two milligrams on a housing to two hundred and twenty on a large casting. Its weakness is that it does not scale: the same figure applied to a part twice the size is twice as demanding per unit area.

Milligrams Per Kilogram

This normalises by part mass, which makes it useful where a family of parts spans a wide range of sizes and the requirement is meant to follow the part. A specification of one milligram per kilogram is a statement about the ratio of contamination to part, which is a sensible basis where the risk scales with the amount of material rather than with a particular surface.

Milligrams Per Unit Area

This normalises by the area sampled, and it appears where a specific surface is controlled rather than the whole part. A figure such as ten milligrams per thousand square centimetres on a threaded hole is an area-based limit on a named region, and it cannot be converted to a part figure without knowing how much of that area the part contains.

2. Why The Same Figure Means Different Things

The arithmetic is trivial and the consequence is not.

Working It Through

One milligram per kilogram applied to a one-kilogram component is one milligram. The same specification applied to a ten-kilogram housing is ten milligrams, to a hundred-kilogram block a hundred milligrams, and to a three-hundred-kilogram marine component three hundred milligrams. A figure that looks like a precision claim is, on the largest parts, a very loose requirement expressed in a way that sounds tight.

Which Reading Is Correct

It depends on what the residue threatens. Where the risk is contamination of a fluid circuit, the quantity that matters is what reaches the circuit, and that does not grow with the mass of the casting around it. Where the risk is a coating or an adhesion failure, the area that matters is the surface being coated. In neither case is part mass the natural quantity, which is why a per-kilogram figure is usually a convenience rather than a requirement.

3. Conversion, And Where It Stops

Converting between per-part and per-kilogram is multiplication. Converting area-based limits is not possible without the area.

The Two Valid Conversions

Milligrams per part to milligrams per kilogram is a division by the part mass in kilograms. The reverse is a multiplication. Both are exact, and both require the part mass to be stated, which is why a specification that uses a per-kilogram figure should state the mass range it applies to.

When Conversion Misleads

Converting a per-kilogram figure across parts of very different size produces a requirement that may be unachievable at one end and meaningless at the other. A figure that is reasonable for a one-kilogram part becomes an extremely tight requirement on a fifty-gram part and a very loose one on a three-hundred-kilogram casting. The unit did not change; the meaning did.

4. Particle Size Is A Separate Axis

A mass figure and a particle figure describe different risks, and a specification that states only one of them leaves the other uncontrolled.

Mass Without Size

A part can hold a large number of small particles and meet a mass limit, or hold a single particle just under the mass limit that is large enough to block a passage. Mass measures quantity; size measures the ability to cause a blockage or a scoring event.

The Pair That Works

Production specifications usually pair them: a mass limit per region and a maximum particle dimension, often with a percentage-of-samples rule. Both belong in the same table, and the region they apply to must be named for either to be measurable.

5. What A Supplier Needs To Quote

A supplier can price a requirement that states the unit, the region, the mass limit, the particle limit and the extraction method. Missing any one of those turns a specification into an assumption, and the assumption is usually the most expensive reading.

The extraction method is the item most often left out and most often disputed, because different methods extract different amounts from the same part. Naming the method, the solvent and the sampling location is what makes two quotations comparable.

6. Reviewing A Claim

A cleaning accuracy quoted as a single number should be read with three questions: on what part mass, on which region, and with what method.

A figure of one milligram per kilogram quoted without a part mass is incomplete rather than impressive. On a small component it is a demanding claim; on a large casting it is a routine one. Asking for the mass and the method takes one sentence and settles which it is.

7. Where The Confusion Costs Money

The cost appears at acceptance, when a machine is measured against one reading of a specification that was written with another in mind. It also appears at comparison, when two quotations using different units are placed side by side and the lower number is assumed to be the better machine.

Both are avoidable by writing the unit, the region and the method into the requirement, which costs a paragraph and removes the ambiguity that a purchase order cannot resolve later.

8. A Worked Comparison

Take two parts: a small housing of one kilogram specified at one milligram per kilogram, and a large casting of three hundred kilograms specified at seventy milligrams per part. The first allows one milligram of residue; the second allows seventy. Read as a percentage of part mass, the second is far looser; read as a risk to a fluid circuit, the second is the more realistic requirement because a circuit fed by a large casting carries a larger volume of oil.

What The Comparison Shows

Neither figure is wrong. They are answering different questions, and the mistake is assuming that the one with the smaller number is the stricter requirement. On a per-kilogram basis the large casting is allowed roughly a quarter of the residue that the small housing is, which reverses the impression the raw numbers give.

That reversal is the reason a specification should state the unit and the region together, and the reason a comparison table should never place figures from different units in the same column.

Frequently Asked Questions

Is 1 mg/kg Stricter Than 1 mg Per Part?

It depends on the part mass. On a one-kilogram part they are identical; below one kilogram the per-kilogram figure is looser, and above it the per-kilogram figure is stricter.

Why Do Specifications Use Different Units At All?

Because different requirements protect different things. A fluid circuit cares about absolute quantity, a coating cares about area, and a part family spanning sizes finds a normalised figure easier to write.

Can I Compare A Per-Part Figure With A Per-Area Figure?

Only if you know the area that produced the per-part figure. Without it the comparison is between a total and a rate, which has no meaning.

Does The Unit Affect The Machine Design?

It affects how the requirement is read, and therefore whether the stage list matches it. A per-area limit on a thread implies a targeted stage; a per-part limit on a housing implies a bulk stage plus a rinse.

Conclusion

Milligrams per part, per kilogram and per unit area are three different statements, and the same number in each describes a different requirement. The conversion between the first two is arithmetic; the third cannot be converted without the area.

A specification that states the unit, the region, the mass limit, the particle limit and the method can be quoted and accepted. One that states a number alone will be interpreted, and the interpretation will be the one that favours whoever is reading it.