Rosemount® 400A


400A General

Total Hydrocarbon (THC) is a measurement of the hydrocarbon content (aggregated, not specific to an individual HC compound) concentration in a measured sample.

A Flame Ionization Detector (FID) analyzer tests for the presence and concentration of select compounds in a gaseous test sample by ionizing the compound of interest with a controlled combustion reaction.

The 400A Flame Ionization Detector (FID) THC analyzer tests for the presence and concentration of HC compounds in a continuous gaseous sample by combining the sample with a fuel gas stream, ionizing the present hydrocarbons through combustion, and then measuring proportional charge produced by the presence of the ionized hydrocarbons in the sample.

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(Video Link – short operational overview video)
(Video Link – in-depth overview video)

Overview of the 400A (PDF)                   <<(Not the same as the above PDF?)
Need drawings?                                       <<(See Drawings tab?)
Need assistance?                                      <<(See Problem/Solution tab?)


First, there was the Beckman 400. This unit dates back to the 1970’s; it sported a heavy duty metal case, an analog meter, a bull’s eye flame status indicator, and the necessary electronics.

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Second, came the Beckman 400A with new electronics, a better preamplifier board, and a notched case (the fuel solenoid is exposed and hangs out the back). It was quickly renamed Rosemount 400A as this was during the transition of when Emerson/Rosemount acquired Beckman Industrial.

The third revision is our favorite, the Rosemount 400A (square case; no notch). This has the standard Main Electronics Board (one of two possible versions), the better Preamplifier Board (only one tried and true version that we know of), the better case heater assembly (a fan, heater, and dedicated proportional controller circuit board, and one of three versions of the Flame Safety Board.

The last version, the fourth and present version, sports miniaturization of the Main Electronics Board, 4-20 mADC daughter board, and case heater controller board.  Everything is driven by a 24 vDC off-the-shelf DIN mounted power supply. The result was the Rosemount 400A . . . We know, it’s confusing. That’s why we call it the “Rosemount 400A-circa 2009 analyzer.” Here’s the link to that specific page.

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What’s next? How about the Rosemount 400R? It’s the 400A but enhanced and built by RIGAS, but sold through the vast Rosemount sales channels. You’ll have to ask your local Rosemount Sales Rep to ping on his upper management in order to make the 400R something to purchase. You’ll like it – it will have the same great little burner, same Brooks regulators, same Siemens-Moore Products backpressure regulator, similar pneumatics, and the same Rosemount Preamplifier Board.  But beyond that, RIGAS is going to fully digitize the Main Electronics Board so that it can read out in percent of scale (as it presently does) or part per million (ppm) of any hydrocarbon molecule that you can think of. It will also have diagnostics to give the user some insight as to what’s occurring within the analyzer (temperatures mainly but pressures and key voltages too).

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Click here for a PDF overview of the 400A.


400A Product Datasheet
400A Manual Rev L
FM certification

621072F 400A pictorial wiring with off range alarm contact


Installation (notched case)
Installation (square case)
Main Electronics Board
Preamplifier Board
Flame Safety Board
Power input filter module
Case heater board
Interconnection diagram
Flow Diagram


Red Dot Capillary
White Dot Capillary
Yellow Dot Mixed Fuel Capillary
Black Dot Capillary
White Dot Fuel Restrictor
Red Dot Mixed Fuel Restrictor
Blue Dot Air Restrictor
Sample Bulkhead Restrictor 4-4 SS Fitting
Sample Bulkhead Restrictor 2-4 SS Fitting
Sample Bulkhead Restrictor – Brass Fitting
Air Bulkhead Restrictor – Brass Fitting

Burner Assembly – Complete
Burner Contact Assembly
Burner Chamber Assembly
Burner Tip/Jet
Burner Cap w/o Glow Plug
Burner Cap w/ Glow Plug

Case Heater Assembly – Complete

Preamplifier Board
Flame Safety Board
Power input filter module

Case heater board
Cables & harnesses
Case & lid
All parts
Nuts & bolts
Front panel


400A Troubleshooting

Unstable or noisy readings may be symptomatic of a contaminated flow system.  Hydrocarbon contamination can occur in multiple areas: external components, like the fuel or air supplies, external frits and filters, regulators or supply tubing; internal flow components, such as pressure regulators and gauges; and finally, with water or condensation infiltration of the burner itself.

Contamination issues can be mitigated with intensive cleaning of the suspect components, or with their replacement.

The “+1” reading is indicative of an out-of-scale sensor response.

The 400A readout is a “percent of scale” display, which means that the display reads out the percentage of the scale of readings available under the current range setting.

If the analyzer’s current output voltage exceeds 200% of the current scale, the readout will return “+1”.

A quick check of the actual current sensor output is to increment the current range selected upward until the sensor output is no longer out-of-scale.   The set range multiples* (see: 400A Calculator) are instructive in determining the real sensor reading.

*Range multiples are variable if the optional 400A range trim board is installed.

(Describe likely diagnoses and specify possible confirmation testing, mitigation, and relevant RIGAS products or services)

Failure of the burner to ignite can be attributed to several separate (or contributing) factors.

Ignition failure can be due to issues with the supply gases: an insufficiently purged fuel supply, pressure regulators for the air and fuel supplies adjusted incorrectly or simply malfunctioning, or flow restrictions of the fuel and/or air into the burner assembly caused by clogging.

It can also be caused by internal electrical problems, such as faulty contacts and connections in the ignition circuit.

Finally, it may be caused by malfunctioning or failed components, such as the igniter itself, the transformer circuit, or the ignition switch.

(Describe likely diagnoses and specify possible confirmation testing, mitigation, and relevant RIGAS products or services)

This symptom shows up in different ways:
The readout shows an upscale reading with no flame present:

This unexpected reading is usually indicative of electrical leakage within the burner assembly.

The analyzer displays a loss in sensitivity, i.e, the same concentration of test gas reports a lower measured concentration over time:

This is often indicative of regular system degradation over long periods of time; the fuel and air supply systems (filters, restrictors, etc) as well as the sample restrictor and capillary can develop clogging, and the electrical elements of the burner can be partially short circuited, by extended operation without cleaning and rebuilds on the pertinent systems.

Further questions about day-day operation of your 400A can be answered with use of the RIGAS 400A Calculator Spreadsheet (PH).

RIGAS is equipped to diagnose and repair 400A analyzers in our Wooster, OH, office.

Please inquire for terms and instructions to ship your analyzer to RIGAS for service or repair.


[email protected]

2310 East Hutton Rd
Wooster, OH 44691

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The RIGAS 400A Calculator

The 400A Calculator is an interactive MS Excel spreadsheet for use in determining optimal operational parameters, settings, and expected readings, given user-entered details of the use application. The 400A Calculator spreadsheet is available for download below; please view the tutorial video for instructions on using the spreadsheet.

Download the 400A calculator here 400A_THC_response_calculator_r1
— 400A Calculator Video Tutorial (in production circa April 2022)