For Density Measurement, Precision is Key Reply

Density is a measure of the mass to volume or weight to volume ratio of a material. This is an important property for systems that relate weight to volume for mixing or dispensing purposes. Volume measurements using traditional methods such as measuring cups or spoons, or even measuring pipettes, are imprecise. Knowing the density of a liquid or coating allows us to instead measure out precise volumes by weight or mass using a high resolution scale. In order to do this, the density of the material must be known. The question is, how can we precisely measure the volume of a material in order to calculate its density in the first place? The answer is density cups.

Density cups are manufactured to a very precise volume. By filling this container to that precise volume and then weighing it, we can calculate the density of any liquid. One of the major problems with filling a container to a precise volume is the meniscus. Liquids tend to be cohesive, meaning the individual molecules attract each other. This is why when we fill a container to the very, very top, we can often see a small mound of liquid that extends above the rim of the container. This is called the meniscus:

meniscus on a glass of water
meniscus on a glass of water

The meniscus means that the volume of the liquid in the container is actually slightly larger than the volume of the container itself. To solve this, density cups have a precisely fitted lid with a small hole in the center and a rim that extends slightly into the cup. The cup is filled with more liquid than is needed, and when the lid is placed on top the excess material will flow out of the hole. Wipe away the excess and voila – the cup is filled precisely with the appropriate amount of liquid.

image to show removing of meniscus

The liquid is then weighed in grams, which can be achieved either by zeroing the scale before filling the cup or by using a tare weight, which ensures that only the mass of the liquid is measured. One thing to note is that bubbles in the liquid can throw off a density measurement. If a material must be shaken or mixed vigorously, which can cause bubbles in the liquid, it is often a good idea to do the mixing well in advance of the density measurement so the bubbles have time to settle out.

Once the mass in grams is known it’s a simple calculation to arrive at the density. For US density cups, which have a volume of 83.2 ml, simply divide the mass of the liquid in grams by 10 to achieve the density in pounds per gallon (lbs/gal). US Midget Cups have one tenth the volume (8.32 ml); with these cups the mass of the liquid in grams is exactly equal to the density in lbs/gal, and they also have the benefit of requiring less material for measurement. For imperial or ISO density cups which have a volume of either 50 or 100 ml, simply divide the mass of the liquid in grams by the volume of the cup to achieve the density in grams per ml (g/ml, or g/cm^3). Once the density of the liquid is known, any desired volume can easily be converted to a mass or weight and measured out with a scale.

Density and Specific Gravity table
Table courtesy of BYK-Gardner

Dry Film Thickness Measurement On Non-Metal Substrates Reply

Film thickness is an important type of measurement for many manufacturing and research facilities. Variations in the thickness of a paint or coating can influence a multitude of properties affecting the final product including color, gloss, hardness, adhesion, scratch resistance, and a host of others. In order to attain the desired properties of a coating, the correct film thickness must be achieved. There are several ways to measure film thickness, both in the wet and dry phase of application. Wet film thickness gages such as interchemical and comb type gages can be used to measure the thickness of a coating before it has been cured. More often however, research and quality control departments want to know the thickness of a coating after it has cured.

Instruments for measuring the thickness of a dry coating can be split into two categories; destructive and non-destructive. Destructive film thickness tests involve cutting through the coating down to the substrate, often with the help of a specialized blade, and then looking at the layers under a microscope to determine the thickness. The drawback of this method is obvious: the product must be destroyed in order to take the measurement. In addition, destructive film thickness measurements are usually more time consuming than other types of thickness measurements. Generally preferred is a non-destructive method using what is typically called a dry film thickness or DFT gage. Most DFT gages operate using one of two measurement principles that can measure the thickness of a film applied to a metal substrate. The measurement principle used depends on whether the substrate is “ferrous”, meaning it contains iron and is typically magnetic, like steel, or “non-ferrous” meaning the substrate does not contain iron and is not magnetic, like aluminum. A dry film thickness gage is generally selected based on whether the substrate is ferrous or non-ferrous, and there are many gages available that contain both measurement principles for measuring on any type of metal substrate.

Much trickier is measuring dry film thickness on a non-metal substrate such as plastic. For non-destructive film thickness tests on these types of substrates, a different type of gage is needed. The PosiTector 200 uses a sonic principle to measure dry film thickness. This operates similar to sonar; sound waves are sent through the material, and the reflected sound waves are measured. Whenever a material of a different density is encountered the reflection will change, telling the gage it has reached the substrate or a different type of coating. By using this measurement principle the PosiTector 200 can measure film thickness on a wide range of non-metal substrates, and unlike typical DFT gages it can even differentiate between different layers of coatings, measuring the thickness of up to three layers at once.

Dry film thickness measurement of a clear coat on
plastic headlamp covers using the PosiTector 200.

One such application involves automotive headlamps. A hard protective clear coat is applied to the clear plastic of the headlamp in order to protect it from weathering and abrasion. It is crucial that the clear plastic remains clear so as to not obstruct the light beams. In order to achieve this, the clear coat must be applied at a specified thickness; thick enough that it retains the protective qualities of the coating, but thin enough that the coating remains smooth and clear. Since this coating is applied on clear plastic rather than metal, a typical DFT gage will not work for this application. However, tests have shown that the PosiTector 200 is very effective at measuring the thickness of the clear coat, alleviating the necessity of destroying the product in order to measure it. This instrument can save not only time by taking quicker measurements, but money as well by not wasting product. If your company has a need to measure dry film thickness on a non-metal substrate, be sure to talk with your BYK-Gardner representative about free sample testing today.