Measurement Glossary

Basic knowledge in a nutshell

In our measurement glossary you will find all the related terms from A to Z. If anything is missing, simply notify us via the contact form.

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Air-to-fuel ratio Lambda

In heating plants, the oxygen required for combustion is supplied through combustion air. Optimal combustion requires the correct ratio of fuel and air. The air volume supplied for combustion should be higher than the theoretical volume. The ratio between excess combustion air and the theoretic air requirement is referred to as the mass ratio or the air to fuel ratio Lambda.

Annular gap measurement

Annular gap measurement checks if flue gases enter the combustion air, or if the flue gas system is tight. Here the O2 concentrations (oxygen) in the annular gap are measured in airtight gas firing plants. The combustion air suctioned through the annular gap must be free from flue gas components. Flue gasses in the combustion air cause a considerably increase in the CO concentration.

Averaging measurement

The 1st BImSchV requires QA averaging measurement (averaging the flue gas loss) to achieve greater measurement accuracy. Here the measured values are measured continuously and averaged over a period of 30 seconds. The flue gas loss limit value varies by useful heat rating of the oil- or gas fired furnace.


Carbon dioxide (CO2)

Carbon dioxide is a chemical compound of carbon and oxygen. It is a non-combustible, acidic, colour- and odourless gas which soluble in water (colloquially “carbonation” in connection with beverages containing carbon dioxide). Plants convert carbon dioxide (CO2) into oxygen (O2). Human and animals in turn convert oxygen into carbon dioxide again by breathing. Combustion flue gasses create an imbalance since it promotes the greenhouse effect.

Carbon monoxide (CO)

Carbon monoxide is a chemical compound of carbon and oxygen with the chemical formula CO. Carbon monoxide is a colourless, odourless and flavourless, toxic gas. In high concentrations is prevents blood to uptake oxygen, which can result in death. The limit in the workplace is 30 ppm. It is in part produced through incomplete combustion of carbonic substances.

Combustion air temperature

The combustion air temperature is measured at the burner intake. In airtight combustion plants the temperature must be measured at an appropriate point in the feed pipe.


The better the coefficient, the lower the flue gas loss. An optimal coefficient can be achieved by preheating the combustion air and the fuel. The coefficient measures the efficiency of energy conversion and energy transfer and defines the ratio of the effective power to the output conducted or the ratio between the effective energy determined within a defined period and the energy delivered during the same period.

Core stream

The flue gas temperature is highest and the oxygen content (O2) lowest at the core of the flue gas stream.



Dust refers to tiny particles in the air which may be present in any form and density. It is created by ash and minerals in solid fuels.


Flue gas loss limits

Oil and gas combustion plants must not exceed the following percentages (§ 10 of the Small and Medium Combustion Plant Directive dated 1/26/2010 – 1st BImSchV):

Furnace useful heat
rating in kilowatt

Flue gas loss limit

over 4 to 25

over 25 to 50

over 50




Flue gas measurement/flue gas loss measurement

The oil, gas and/or solid fuel combustion system is adjusted to its optimum in line with flue gas measurement or flue gas loss measurement. The goal is to achieve the lowest possible energy loss. Flue gas loss refers to the unused energy which escapes from the flue gas system. Legal bases such as the 1st Ordinance of the Federal Immission Control Ordinance (BImSchV) ensure the energy loss is as small as possible . This regulation is intended to provide a considerable reduction in the emissions from small combustion plants.

Flue gas temperature

Low flue gas temperatures impair chimney draught and, when below the dew-point temperature, result in water vapour in the flue gas condensing. This can result in flue sootiness or corrosion of the flue gas plant. High flue gas temperatures can indicate poor heat transfer in the heat exchangers. This can result in soot deposit. The flue gas temperature is measured at the core of the stream of flue gas (core stream), where the temperature is highest.


A fuel is a chemical substance where the stored energy can be converted into useful energy through combustion.

Fuels are used in solid, liquid and gas form.

Solid fuels are e.g. hard coal, brown coal, combustible peat, straw and similar vegetable substances. They are primarily used to generate electricity in steam power plants, but are gaining popularity in small and medium size combustion plants. The drawback of solid fuels is that they produce large amounts of ash, dust and soot.
Solid fuels consist of carbon (C), hydrogen (H2), oxygen (O2) and small amounts of sulphur (S) and water (H2O).

Liquid fuels are petroleum products (petrol, diesel, heating fuel) and biofuels. Refining creates extra-light (EL), light (L), medium (M) and high viscosity (S) heating fuels. Heating fuel (EL) is primarily used in small combustion plants.
Gaseous fuels are a blend of flammable and non-flammable gasses, e.g. natural gas and biogas. They’re primarily used for heating buildings and to generate electricity. Heating now primarily uses natural gas (main component methane CH4).


Loading test (previously design inspection 1 bar)

Pursuant to TRGI 2018, the loading test is performed for new or modified circuits (without fittings, gas pressure regulators, gas meter and gas appliances as well as related control and safety installations) before the lines or sections of lines are plastered or clad and their joints are coated or encased. The circuits can be tested as a whole or in sections.
The loading test is performed with 1 bar air/inert gas. The testing period is 10 minutes. A drop in pressure is not permitted.


Measuring CO

Measuring the CO in gas-fired furnaces is part of the Sweeping and Inspection Ordinance (KÜO). It is performed to assess the quality of combustion and safety of the gas-fired furnace. CO is produced by incomplete combustion. The CO content is specified in ppm. According to the 1st BImSchV and the Federal KÜO, CO is measured in oil-fired furnaces. The units of measurement are mg/kWh or ppm depending on the legal basis. Pursuant to 1st BImSchV the limit is 1,300 mg/kWh (approx. 1.040 ppm). CO is measured to better assess the quality of combustion.


Here the standard volume (standard cubic metre, Nm3) is specified as a reference value, the mass of the gas is specified in milligrams (mg).


mg/kWh means “milligram per kilowatt hour of energy used”. The gas concentration in the unit used for energy, mg/kWh, are determined through calculations using fuel-specific data. This results in different conversion factors depending on the fuel.


Nitric oxides (NOx)

Nitric oxide is a collective term for the gaseous oxides of nitrogen. They’re also abbreviated as NOx. During combustion the fuel and the nitrogen (N2) present in the air bond with the atmospheric oxygen (O2) for form nitrogen oxide (NO). It’s a colourless gas and when combined with oxygen (O2) oxides to nitrogen dioxide (NO2) after some time. Nitrogen dioxide is a water soluble lung toxin and can cause severe lung damage when inhaled.

Nitrogen (N2)

Nitrogen is a gas and makes up 79% of our inhaled air. It is therefore a principal element. Oxygen only makes up 21% of our inhaled air. Nitrogen is colourless, odourless and tasteless and not involved in combustion.



Oxygen is a chemical element with the chemical symbol O and the atomic number 8. Some oxygen (O2) dissolved in air fuses with the hydrogen (H2) in fuel to form water (H2O). Depending on the flue gas temperature, this water is present in form of flue-gas moisture or flue gas condensation. The remaining gaseous ratio of oxygen is a gauge for the coefficient of combustion and is used to determine flue gas loss and the carbon dioxide content (CO2).


Rated useful heat

Rated useful heat is the maximum output and refers to the maximum heat emitted by a heater which is actually present in the respective heating system.


Smoke number

To determine the smoke number, a specific amount of flue gas is suctioned through a filter paper. The degree of blackening is compared to a scale of grey shades with different numbers. The smoke number determined ranges from 0 to 9 (according to Bacharach).


Soot is a black powdered solid which almost exclusively consists of carbon (C) and produced by incomplete combustion.

Suitability testing

Suitability testing is part of leak rate measurement. Pursuant to TRGI 2018, circuits operating at pressures up to 100 mbar must be handled according to the degree of suitability for use. The gas leak volume must be determined with a leak rate tester (preferably certified to DVGW test basis G 5952). Leak rate measurement in 15-20 minutes! Without removal, without determining volumes, and without pressure boosting, using the Dräger P7 series.

Suitability criteria (operating plants):
Suitability testing and the gas check must be performed every 12 years. This is the responsibility of the operator (NDAV§13).


Unlimited suitability

Reduced suitability

Not suitable

Loss in the line:
1 L/h, no further defect.
Plant operation may continue.

Loss in the line:
equal to or more than 1 but less than 5 L/h.
Repair required within 4 weeks.

Gas line loss:
equal to or greater than 5 L/h.
Shut down immediately!

Sulphur dioxide (SO2)

Sulphur dioxide is a colourless, pungent, sour, toxic gas which irritates mucous membranes. It is water soluble and produces very small amounts of sulphurous acid in water. Among other things, it’s produced by combustion of sulphurous fossil fuels such as coal or petroleum products and pollutes the air (e.g. acid rain).


Tightness testing

Stabilisation time and measuring time table for tightness tests according to DVGW TRGI G600:

Line volume

Adaptation time

Min. test duration

< 100 litres

< 200 litres

≥ 200 litres

10 min

30 min

60 min

10 min

20 min

30 min


Units of measurement

ppm (parts per million):

ppm means “parts per million” and represents a ratio. This unit is irrespective of pressure and temperature and is used for low concentration.

For example Higher concentrations are specified in percent (%).


10 000 ppm = 1%

1 000 ppm = 0,1%

100 ppm = 0,01%

10 ppm = 0,001%

1 ppm = 0,0001%