---
title: "How thermogravimetry separates mass loss from the gas around it"
description: "A hot specimen and its balance both respond to the gas. Empty-pan runs, heating rates and atmosphere changes help explain a thermogravimetric mass-loss curve."
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language: "en"
page_type: "story"
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  zh-CN: "https://sandboxphysics.com/zh/stories/instruments/thermogravimetric-analyzer/"
last_modified: "2026-11-03"
---

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Inside the instrument

# How thermogravimetry separates mass loss from the gas around it

A little powder hangs inside a furnace while a balance follows its mass. Heating reveals when material begins to escape. Yet the surrounding gas also pushes on the pan. Measuring what an empty pan does under the same conditions is an essential step toward understanding the specimen’s loss.

Sandbox Physics · 2026-11-03

## Keep the specimen hot and the balance cooler

A thermogravimetric analyzer, usually called a TGA, weighs continuously as a specimen follows a temperature program in a controlled gas. A fine suspension can hold the pan in the furnace and leave the delicate balance mechanism above it. Flow carries escaped material toward an outlet; a separate protective purge helps keep that material out of the balance.

When the specimen loses weight, the balance beam moves slightly. A position sensor detects the change, and feedback adjusts an electromagnetic coil to restore the beam to its reference position. The required coil current provides the weighing signal after calibration with known masses.

Sources: [1]

## An empty pan can appear to gain mass

Gas provides buoyancy, just as water does. Heating lowers its density and weakens its upward push on the pan. The balance can therefore report a small apparent gain even though no material has been added. The displaced volume includes the holder and the specimen.

An independent empty-pan run under matching temperature, gas and flow conditions removes much of the holder contribution and reproducible drift. The specimen’s own displaced volume still needs a correction. Gas released by a reacting specimen can also alter local density, linking a precise correction to how efficiently the chamber removes that gas.

Sources: [2] [3]

## A loss and its fastest moment are different observations

Suppose ten milligrams settles at seven milligrams after heating. The two plateaus show that three milligrams left. Calculating the loss per minute reveals when the escape was fastest. This derivative curve often has a peak, marking the greatest rate during that particular experiment.

The temperature at the peak must also be specified. Heat takes time to reach the specimen, so the programmed temperature, furnace temperature and specimen temperature can differ. Fast heating often makes that separation more noticeable. Recording the actual specimen channel keeps the rate attached to the temperature the material experienced.

Sources: [1]

## Time and atmosphere reshape the loss

A reaction generally speeds up as temperature rises, but it also needs elapsed time. A slow ramp gives it time to proceed at lower temperatures. A fast ramp carries unreacted material farther before it can escape. The final lost mass may agree even though the rate peak shifts.

Some decompositions release carbon dioxide that can participate in the reverse reaction. Increasing its partial pressure can suppress the net decomposition and move the loss toward higher temperature. Flow matters because it removes product gas. Comparing atmospheres is therefore a way to probe the process, alongside changing the heating rate.

A specified single-step model can be fitted to several heating histories and tested against a separate history excluded from the fit. At only one constant temperature, many activation-energy and prefactor pairs give the same rate. Temperature variation helps distinguish how strongly the rate responds to heating.

Sources: [4]

## The missing mass needs a chemical explanation

Evaporation, drying and solid decomposition can leave similar steps. Overlapping processes can even merge into one derivative peak. Heating-rate and atmosphere comparisons narrow the possibilities. Routing the outlet to infrared or mass spectroscopy can identify the escaping molecules. The balance supplies the mass change; the gas analyzer supplies another part of its explanation.

Sources: [1]

## [Compare a hot specimen with an empty pan](https://sandboxphysics.com/labs/thermogravimetric-analyzer/)

Turn the gas valve, replace nitrogen with product gas and test a withheld heating rate.

A synthetic single-step release in a hang-down thermobalance with finite thermal response, buoyancy, matched blanks and conditional kinetics.

## Sources and further reading

1. [TA Instruments, TGA 2950 Thermogravimetric Analyzer, TA031B](https://www.tainstruments.com/pdf/literature/TGA_2950.pdf)

   Technique, Principles of Operation and purge-system pages: null balance, force-compensation current, controlled atmosphere and adjacent thermocouple. The illustrated apparatus is a synthetic hang-down design, not a reproduction or performance claim for this product.

2. [Setaram, TN745 Hang-down thermogravimetric analysis technology](https://setaramsolutions.com/app/uploads/sites/2/2026/01/TN745-Hang-down-thermogravimetric-analysis-TGA-technology.pdf)

   Buoyancy effect, thin suspension and blank-subtraction sections. Gas density, displaced holder/specimen volume and repeatability enter the weighing correction. Comparative marketing claims are not adopted.

3. [Füglein and Schmölzer, TGA Measurements By Means of the BeFlat Correction, NETZSCH (2012)](https://analyzing-testing.netzsch.com/en-US/application-literature/tga-measurements-by-means-of-the-beflat-correction/)

   Introduction and blank determination: separate empty-crucible measurements under matching temperature, flow and gas conditions. Proprietary automatic correction and numerical performance are not simulated.

4. [Moukhina et al., Kinetic Analysis of Pressure-Dependent Reactions in Solids, NETZSCH OnSet 29 (2025), pp. 18–20](https://kinetics.netzsch.com/_Resources/Persistent/6/4/a/a/64aa07e8813a8fc6f61aecf0397583111fae8136/Kinetic%20Analysis%20of%20Pressure-Dependent%20Reactions%20in%20Solids%20OnSet_29.pdf)

   Carbon dioxide effects on reversible decomposition and the heating-rate comparison. The teaching example uses a declared forward inhibition law, not the published calcium-carbonate data or a full reversible-reaction fit.

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