Uses and Working Principles of a Dilatometer (Video)
A dilatometer is an instrument for rapidly but roughly measuring the expansion of a liquid. It consists of a glass bulb B, with a graduated stem S. the volume Vb of the bulb, up to the zero of S, is known, and S is graduated in cubic millimeters or other small units. The volume of the bulb, and the value of one scale division, varies with temperature; the dilatometer therefore measures apparent expansion. The dilatometer is filled with the liquid under test to a point just above the zero of the stem, at a temperature t1. The volume V1 of the liquid is found by adding Vb to the stem-reading V1. Next the dilatometer is warmed to t2 , and the liquid rises to V2. Then (V2 – V1) is the apparent expansion of the liquid, and hence
Ya = V2 – V1 / V1(t2 – t1)
A dilatometer can be compensated so that it shows roughly the true expansion of a liquid. Mercury is introduced into the bulb, until it occupies 1/7th of the bulb’s volume. The expansion of mercury is about equal to the expansion of the glass, so that the free volume in the bulb is almost constant. The cubic expansivity of mercury is yhg= 18.1 x 10-5K-1
while that of glass is
yg= 3 x 8.5 x10-6 = 2.55×105K-1
Therefore,
yhg/yg = 18.1/2.55 = 7.1
Thus, the expansion of the mercury offsets that of the glass, within about 1.5%. The space above the mercury, whose volume is constant, is filled with the liquid to be examined. When the bulb is warmed, the movement of the liquid up the stem shows the liquid’s true expansion. The dilatometer can also be used to show the anomalous expansion of water.
A dilatometer is also a device that measures the dimensional change in a material(not just a liquid) as a function of temperature. These measurements can help researchers and manufacturers understand the underlying characteristics of their materials.
An optical dilatometer is a type of dilatometer that uses a digital camera to take pictures of the sample as it expands. This non-contact measurement method allows for the analysis of a wide range of specimens.
What is Dilatometry?
Dilatometry is a material testing technique that measures the dimensional changes of a sample as a function of temperature. It is a simple method that can be used to measure a wide range of parameters, including coefficient of thermal expansion, glass transition point and softening point. It can also be used to monitor the kinetics of a sintering process.
To carry out a dilatometry test, the specimen is placed in a holder and then subjected to a pre-programmed cycle of temperatures. As the temperature changes, the dilatometer measures the displacement of the sample holder using a set of pushrods. The resulting data can then be plotted to produce a linear temperature vs. length curve.
This data can be used to calculate a material’s coefficient of thermal expansion. It can also be used to determine the glass transition temperature of a sample, or the softening point of a metal. The dilatometer can also be used to monitor the kinetics and rate controlled sintering of a sample.
This method is particularly useful for measuring the allotropic transformation behaviour of steels, such as their conversion from body-centred cubic (BCC) ferrite to face-centred cubic (FCC) austenite. It is a complementary technique to other methods such as optical microscopy and scanning electron microscopy, although these can only provide one-dimensional information. However, the high-speed strobe lighting and video imaging used in this type of experiment can provide very high resolution images at elevated temperatures.
How Does a Dilatometer Work?
Dilatometry is a thermal measurement technique that measures the expansion of a material when its temperature changes. Typically, dilatometry is used to measure the change in length of a sample, but other measurements can also be made (e.g., sintering dilatometry). Dilatometry is typically carried out on cylindrical specimens that match the diameter of the pushrods used in the instrument.
The dilatometer is controlled by a computer programme that heats and cools the sample in a specified sequence. The temperature is controlled very accurately – for example, by using induction to heat the sample and pressurised inert gas to cool it. During each segment of the temperature programme, the change in length of the sample is measured.
Modern optical dilatometers can be very sensitive, measuring even the smallest changes in volume of the test item. These devices can be used for a variety of items, including powders and liquids.
The most common use of dilatometry is to measure the coefficients of thermal expansion of solid materials. However, it can also be used to study the phase transformations of steels and other metals. These allotropic transformations can involve large changes in the material’s volume, for instance the transition from body-centred cubic (BCC) ferrite to face-centred cubic (FCC) austenite. In these cases, dilatometer data can be used to plot the ferrite formation rate curve, and can provide valuable information about the degree of transformation and the kinetics of the process.
Uses of a Dilatometer
A dilatometer can be used for many different applications in the glass, foundry, metal and other industries. The most common use is determining the thermal expansion of a material. However, it can also be used to determine the softening point and glass transition temperature as well as other properties such as vacancy formation and dislocation density.
There are several types of dilatometers available, each with their own advantages and disadvantages. The most common is the push rod dilatometer, which uses a small ceramic pushrod to measure length changes. This type of dilatometer is relatively simple to operate and can provide accurate results. However, it does not offer the same precision as a high resolution laser dilatometer.
A capacitance dilatometer is another popular type of dilatometer. It consists of two parallel plates, with one plate being stationary and the other being movable. The sample is placed between the plates, and the change in volume causes a change in the capacitance. This allows for very precise measurements, with a change of only 10th of a picometer.
Optical dilatometers are also available, which utilize cameras to measure the length of a sample. This method is non-contact and can be used for items or substances that are too fragile to be handled manually. The camera will record a shadowed image of the sample and then compare it to a reference. The computer will then calculate the amount of dimensional change that has occurred.
Repair and Servicing of a Dilatometer
The dilatometer is an important analytical tool for the characterization of solids. It allows a wide range of thermophysical effects to be analyzed that impact the volumetric expansion of materials including; the coefficient of linear thermal expansion, annealing studies, glass transition point, softening point, densification and sintering studies.
However, the high complexity and cost of most commercial dilatometer systems makes them difficult to use and maintain. They also require the support of the manufacturer for routine/incidental repair/maintenance.
A number of dilatometers have been developed to address these issues by simplifying the design, fabrication and operation of the system. These dilatometers typically utilize either high resolution cameras that compare the length of a sample to a reference and hence measure sample dimensional changes (Linseis dilatometer) or a LVDT-based method which provides absolute measurement of linear displacement (TA Instruments dilatometer).
Whether your dilatometer needs repairs or you would like to upgrade your equipment, our qualified engineers can assist. Our field service technicians will come to you or you can take your dilatometer to one of our Service Centers for evaluation and repair. Our goal is to minimize the downtime of your equipment and get you back to production as quickly as possible. Our onsite and offsite repair services are supported by a network of qualified service engineers and technicians to ensure you can keep your process running at all times.
