testo 350

Two Boxes, One System

The testo 350 comes in two parts, and the split matters in practice.

The analyser box holds the gas sensors, electronics, pumps and gas preparation. The control unit operates it, sets up the test, defines sites and locations, runs calibrations and displays results.

The useful result is that the analyser box can carry out tests and save results on its own, once programmed. So it can be left running a long term measurement at the stack while the operator is elsewhere. The two connect by the Testo data bus, with USB and Bluetooth also available.

Choose Six From Nine

The analyser box ships with an oxygen sensor and can take up to five more, for six in total.

The options are carbon monoxide (0 to 10,000 ppm), a low range hydrogen compensated carbon monoxide (0 to 500 ppm), carbon dioxide by infrared (0 to 50 vol %), nitric oxide (0 to 4,000 ppm), low range nitric oxide (0 to 300 ppm), nitrogen dioxide (0 to 500 ppm), sulphur dioxide (0 to 5,000 ppm), hydrogen sulphide (0 to 300 ppm) and hydrocarbons (CxHy). The gas sensors are pre calibrated and can be swapped or added on site without test gas, so a configuration can change as the work changes.

That flexibility is why this instrument suits contractors and service teams. One analyser covers industrial boilers, engine testing, turbine tuning and emissions monitoring, as long as it’s configured for the job in front of it.

Why NO and NO2 Are Measured Separately

This is the technical point that decides whether an engine emissions figure is worth anything.

NOx is the combination of nitric oxide and nitrogen dioxide. Many analysers measure NO alone and estimate NOx from it using an assumed ratio. That works where the ratio is stable. In engine exhaust it isn’t. The share of NO2 can be high and changes with load and after treatment, so a NOx figure based on NO alone can be wrong, by an amount that varies.

Measuring both separately gives a real NOx value rather than a calculated approximation. On engine set up, and on any work where the number goes into a compliance record, that matters.

Low NOx Turbines and 0.1 ppm

Emission measurements during inspection and adjustment of low NOx gas turbines have to be very accurate, because the concentrations are very low.

The NOlow sensor measures 0 to 300 ppm with 0.1 ppm resolution. Combined with the NO2 sensor, also at 0.1 ppm resolution, it gives the detail that work needs. A standard NO sensor with 1 ppm resolution isn’t enough when the reading itself is in single figures.

The Hose Problem

This is worth understanding, because it explains why measurements taken on different days sometimes disagree for no obvious reason.

Nitrogen dioxide and sulphur dioxide are absorptive gases. They stick to the inside of a sampling hose, so part of the gas drawn from the stack never reaches the sensor. How much depends on the hose material, its condition, its temperature and what it’s carried before.

The testo 350’s industrial engine probe uses a special hose for NO2 and SO2 measurement, and the integrated gas preparation conditions the sample before it reaches the sensors. For a monitoring programme rather than a one off test, consistent sampling is what makes a trend meaningful.

When the Reading Goes Off the Scale

Industrial combustion can produce concentrations that are extreme and unexpected. An analyser that saturates gives you nothing, and risks the sensor.

The testo 350’s measurement range extension dilutes the sample so measurement can continue. Individual dilution, with selectable factors of x2, x5, x10, x20 or x40, handles a single high channel. Dilution of all sensors at a factor of 5 handles the whole sample, extending CO measurement up to 50,000 ppm, for example. Fresh air or nitrogen can be used as the diluting gas.

The practical effect is that a test doesn’t have to be abandoned and repeated with different equipment, and a sensor isn’t exposed to a concentration that would shorten its life.

Measuring Before and After

The Testo data bus allows flue or exhaust gas to be measured at the same time before and after an exhaust after treatment system. Up to 16 analyser boxes can run on the bus, over up to 800 metres of cable, controlled from a PC or a control unit.

That’s how you show a scrubber, catalyst or SCR system is working. It can’t be done properly with one instrument taking two readings at different times, because the plant changes between them. Two analyser boxes sampling at the same moment give a removal efficiency, rather than two unrelated numbers.

Gas Preparation

An integrated thermoelectric (Peltier) chiller conditions the sample before it reaches the sensors. It’s recommended where flue gas humidity is high, and for long term measurements.

Water in a sample line affects readings and damages sensors. On a short test in dry gas, that can be tolerated. On a four hour measurement in a wet stack, it can’t, which is why the gas preparation option is usually specified with the special main gas pump for long term work.

Two Versions

Built for the Environment

The analyser box housing is sealed, with built in impact protection, for use in harsh conditions. An opening on the underside gives easy access to the pumps and filters, the parts needing attention most often. Dirt filters can be changed without tools.

Modular flue gas probes come in 335 mm and 700 mm lengths, with thermocouples rated to 500°C or 1,000°C, and versions with a pre filter for dusty gas. A dedicated industrial engine probe covers engine exhaust work. The analyser box handles up to 20 g/m³ of dust in the flue gas.

Power is mains, 100 to 240 V AC, or the built in lithium ion battery, which runs for about 4.5 hours without the gas cooler and IR module. An 11 to 40 V DC input is available as an option for vehicle and site use.