Deutsch English Francais Espanol Portugues

Mercury Stack Gas Monitor


Continuous monitoring of total mercury emissions (CEM)
in stack gas


  • Continuous operation
  • Sample dilution: system works with virtually any sample matrix
  • Maintenance free converter directly at stack:
    no transport of ionic mercury thus minimizing mercury adsorption
  • Thermo-catalytic converter principle operating at low temperature
  • Detects elemental, ionic and bound mercury
  • Mercury speciation available (as an option)
  • Automatic calibration systems (oxidized and elemental) available
  • Automatic QAL3 as an option
  • Approved and certified by TÜV Rheinland (QAL1)
    Energie und Umwelt GmbH, Köln
    Report-Nr.: 936/21213740/A
    of March 26, 2012




Mercury Instruments has specially designed the Mercury Stack Gas Monitor SM-4 for precise and reliable measurement of very low Hg concentrations in stack gases with a complex matrix containing SO2, NOx, HCl and others.


Typical fields of applications include:


  • Emissions monitoring for facilities of the 13. and 17. BImSchV
  • Coal fired power plants
  • Waste incineration plants
  • Sewage incineration plants
  • Cement kilns
  • Survey of general Hg emissions as well as efficiency monitoring of mercury reduction techniques
  • Thermal treatment of contaminated soils, hazardous waste, etc.
  • Metallurgical facilities with potential Hg emissions

An example:

link zu MI-News Coal fired power plant,
Valmy Sation, Nevada, USA


Operating principle


The heated sampling probe is mounted at the stack. A maintenance-free ejector pump continuously draws sample gas into a dilution unit and past a heated particle filter. A critical orifice stabilizes the sample flow, thus making it independent of the pressure in the stack. The suction vacuum is continuously monitored by a pressure sensor to make operation reliable. A partial flow of the diluted sample is passed through a catalyst where the different forms of mercury contained in the sample are converted to the elemental state. This treated sample is then transported to the mercury detector through slightly heated PFA tubing , which may be tens or even some hundreds of feet away from the sampling probe. The detector (a Mercury Ultratracer UT-3000), one of the most sensitive and selective mercury detection devices in the market, uses a pre-concentration technique featuring the unique Mercury Instruments GoldTrap.

A speciation function to determine the different proportions of elementary and ionic mercury is available as an option)


SM-3 Flow diagram


Thermocatalytic reactor


The mercury present in stack gases is often elemental, but ionic and bound mercury also occurs . To measure total mercury, all these different speciations must be successfully detected. To this end the SM-4 uses a thermocatalytic method to convert all mercury to its elemental form. The reactor works without wet chemistry at low temperature (little above 200 °C) and therefore assures high reliability with low demand for maintenance.

All surfaces in contact with sample gas are heated to temperatures >200 °C to avoid adsorption of mercury.

The sample line consist of Perfluoralkoxy (PFA), a material that stands out for its extremely low interaction with mercury vapor.

The reduction of Hg-compounds takes place at temperatures well above the dew-point. Therefore no condensate occurs which would lead to an increased background signal noise and nonspecific peaks.




The SM-4 uses a highly sensitive detector based on the atomic absorption principle and features an amalgamation unit (GoldTrap) connected upstream. The ultrapure gold surface captures the mercury which is then abruptly released when the gold trap is rapidly heated to approximately 700° C.

Purified air is used as a carrier and there is no need for special carrier gases. The use of air has a self-cleaning effect on the GoldTrap, extending its lifespan.




Sampling probe


Sampling Probe SM-4


The sampling probe plays a key role: to extract a representative sample of gas and supply it to the sample conditioning system. All parts that come into contact with the sample are made of special material, guaranteeing a minimum memory effect. All parts are heated so no mercury is retained in the probe. An automatically backflushed filter keeps particles away from the analytical pathway of the sample. Calibration gas is introduced upstream of the sample filter. Both the dilution unit and the catalyst are integral to the sampling probe, thus ensuring minimum length for their connecting lines.


Prevention of interferences


SO2, NOx, HCl, and VOCs all interfere with standard mercury measurement techniques (AAS; AFS).

Modern mercury CEMs try to minimize this negative effect either with preconcentration by amalgamation or by sample dilution. Amalgamation alone, however, reaches its limit when the gold surface is affected by matrix constituents, whereas sample dilution alone may not reduce interference to tolerable levels.

That is why the SM-4 uses both sample dilution and gold trap amalgamation to completely eliminate interference.


Automatic calibration


The SM-4 can be equipped with an automatic calibrator (IAS HovaCAL for ionic Hg or Mercury Instruments MC-3000 for Hg°). It can perform an automatomatic baseline check as well as automatic calibration at predefined time intervals.

To meet the latest demands for quality assurance QAL3 (European Regulation EN 14181) can be performed automatically by using the integrated HovaCAL calibrator.


Calibrator MC-3000


Special benefits of the SM-4


  • Dry thermocatalytic method: no need for reagents, water refills or solid reagent cartridge replacements.
  • Sample dilution eliminates interference from matrix gases such as SO2, NOx, and HCl and
    produces a large measuring range.
  • Approved according to European regulations, for USA: RATA tests passed with excellent result
  • No liquid waste or condensate is generated
  • Extreme high sensitivity: low measuring range is 0.05 µg/m³
  • Automatic cleaning of particulate filter
  • Fast response: t(90) time typically 180 sec
  • Sample line length up to 300 feet
  • ionic calibration gas generator as an option allows
    automatic quality assurance (QAL3)
  • Option to install an elemental mercury calibration gas generator,
    NIST traceable, inside the analyzer cabinet
  • Very low maintenance needed
    (service interval according to the evaluation of suitability: 3 months)


System components


  • Sample conditioning system with detachable sampling probe
  • Heated bundle containing sample line, air supply line, calibration gas feed line
  • Probe controller module
  • Analyzer cabinet with detector and calibrator (option)


SM-4 Setup


Mercury Stack Gas Monitor SM-4:
Technical Specifications


Measured component:

Total mercury (oxidised and elemental speciation as option)

Measurement principle:

Dilution probe with thermocatalytic converter at stack,
Amalgamation (MI GoldTrap),
Cold Vapor Atom Absorption Spectrometry (CVAAS),
Wavelength = 253,7 nm


Low temperature type (200 °C);
directly at stack

Measuring range:

0,05 μg/m³ - 500 μg/m³ Hg
(up to 1000 µg/m³ as an option)

Detection limit:

0,0001 μg/m³ (Detector);
< 0,01 μg/m³ (System)

Response time:

180 – 360 sec t[90]

Max. Sample gas temperature:


Ambient temperature range allowed:

-5°C to 40°C

Sample line length:

up to 30 m (100 feet);
up to 100 m (300 feet) option

Signal outputs:

  • analogue: 4-20 mA (500 Ω max.)
  • seriall: RS 232
  • Modbus RTU/RS485 (option)
  • Ethernet (option)

Status output:

3 x pairs of relay contacts (dry contacts)

Remote access:

Modem and SM-4 communication software (option)

Power supply:

230 V / 50 Hz;
(110 V / 60 Hz option)

Electrical power consumption:

Analyzer cabinet: 450 VA;
Probe-Converter: 1000 VA;
heated sample line: 30 VA / meter


Analyzer cabinet: 161 x 80 x 60 cm
(63.6”x 31.5”x 23.6”) H x W x D
Probe: 36 x 36 x 66 cm
(14.2”x 14.2”x 26”) H x W x D
Probe controller: 48 x 44 x 27 cm
(19”x 17” x 11”) H x W x D


Analyzer (including automatic calibrator):
130 kg (285 lbs)
Probe controller:
14 kg (30 lbs)
Converter unit:
28 kg (62 lbs)

Air consumption:

6 bar (85 PSI); 16 l/min approx.


  • automatic, with built in calibration gas generator for elementary Hg (option)
  • automatic, with built in calibration gas generator for ionic Hg (option)
  • manual, with built in heated fitting for feed of externally provided calibration gas (e.g. HOVACAL®)


Conforms to US EPA 40CFR60 PS-12A Mercury CEMS;
conforms to EN 15267 (European regulation);
Approved and certified by TÜV Rheinland Energie und Umwelt GmbH, Köln,
Report-Nr.: 936/21213740/A of March 26, 2012

As a leading supplier of high precision analytical equipment, we strive at all times to offer top quality solutions. Our quality management system is certified according to ISO 9001.

Funded by the Ministry for Economics and Technology on grounds of a resolution by the German Bundestag


SM-4 cover

Download Brochure:



Download additional information concerning the SM-4 !

Facts contra Fairy Tales:
SM-4 Marcury Stack Monitor:



SM-4 Fact sheet


SM-4: Mercury CEMs:
Results of verification test studies at cement plants



Leaflet RATAs


Measuring of mercury in raw gas for quantity control of sorbent





Last Update: