Handheld density meters can provide accurate results in places where benchtop density meters cannot go. However, because density can change with temperature, these units need to have a way to compensate for changes in the surrounding temperature. Temperature coefficients provide an easy way to maintain accuracy in fluctuating conditions.
Density is a physical property of all materials. It is defined as mass per unit volume. The density of a material changes as its temperature changes due to a principle called thermal expansion. Thermal expansion is the tendency of matter to change in shape, area, and volume in response to a change in temperature through the transfer of heat. This means that the same mass of a sample will occupy a greater volume when warm than it will occupy when it is cool.
Because of the tendency of density to change with temperature, it is very important to stabilize temperature when taking a density measurement. Benchtop density meters have sensitive temperature probes which allow the instrument to precisely control the temperature inside the measuring cell. This allows for the most accurate density measurement possible. Handheld density meters do not have temperature control, and instead measure the density at ambient temperature.
How then are these handheld density meters able to provide an accurate result when density needs to be measured at a fixed temperature and ambient temperature is anything but? The answer comes to us in the form of a temperature coefficient.
The temperature coefficient is a sample-specific value with units of grams per cubic centimeter per Kelvin (g/cm3/K). Breaking this down, we can see the temperature coefficient contains units of density (g/cm3) and temperature (Kelvin). Looking at it generally, we see density per unit temperature. A temperature coefficient allows us to plot out how the density of a sample changes as temperature changes.
Determining a temperature coefficient for a sample is easy. First, the density of the sample (in g/cm3) has to be measured at two different temperatures (Kelvin). The resulting difference in the density values is divided by the difference in the temperature values. The absolute value of the result, in g/cm3/K, can then be applied directly to a measurement.
Certain handheld density meters like the DMA 35 use the temperature coefficient to provide the ability to measure samples at ambient temperature and report a value at a defined reference temperature. In addition, certain measuring units in a DMA 35 do not require a temperature coefficient even though they are reporting a value at a reference temperature that may or may not be the measured temperature. These particular measuring units are usually sample-specific, and depend on the sample being correct for the measuring unit to be accurate. For instance, a water sample measured on an API measuring unit will not read correctly, because the API measuring unit is expecting a crude oil, not water.
While benchtop density meters are defined by their accuracy, the handheld density meter is much more flexible. Handheld density meters can be used in more places and in more varied environments due to their slim profile and robust design. The use of a temperature coefficient in the density measurement allows for these handheld units to provide satisfactory results in these environments in a very short period of time.