Principle of operation
The measuring principle is based on the different thermal conductivity of gases.
The CALOMAT 6 works with a micromechanically produced Si chip whose measuring membrane is equipped with thin-film resistors.
The resistors are kept at a constant temperature. This requires an current intensity depending on the thermal conductivity of the sample gas. This "raw value" is processed further electronically to calculate the gas concentration.
The sensor is located in a thermostatically-controlled stainless steel enclosure in order to prevent the influence of changes in ambient temperature.
To prevent the influence of changes in flow, the sensor is positioned in a bore located to the side of the main flow.
Note
The sample gases must be fed into the analyzers free of dust. Condensation (dew point sample gas < ambient temperature) is to be avoided in the measurement chambers. Therefore, the use of gas modified for the measuring tasks is necessary in most application cases.
CALOMAT, principle of operation
Essential characteristics
- Four freely parameterizable measuring ranges, also with suppressed zero point, all measuring ranges linear
- Smallest measuring spans up to 1 % H2 (with disabled zero point: 95 to 100 % H2) possible
- Measuring range identification
- Galvanically isolated measured-value output 0/2/4 to 20 mA (also inverted)
- Autoranging or manual measurement range switchover possible; remote switching is also possible
- Storage of measured values possible during adjustments
- Wide range of selectable time constants (static/dynamic noise suppression); i.e. the response time of the analyzer can be matched to the respective measuring task
- Short response time
- Low long-term drift
- Measuring point switchover for up to 6 measuring points (programmable)
- Measuring range identification
- Measuring point identification
- External pressure sensor can be connected – for the correction of sample gas fluctuations
- Automatic range calibration can be parameterized
- Operation based on the NAMUR recommendation
- Two control levels with their own authorization codes for the prevention of accidental and unauthorized operator interventions
- Simple handling using a numerical membrane keyboard and operator prompting
- Customer-specific analyzer options such as:
- Customer acceptance
- TAG labels
- Drift recording
- Clean for O2 service
Measuring spans
The smallest and largest possible spans depend on both the measured component (type of gas) and the respective application.
The smallest possible spans listed below refer to N2 as the residual gas. With other gases which have a larger/smaller thermal conductivity than N2, the smallest possible span is also larger/smaller.
|
Component |
Smallest possible span |
|---|---|
|
H2 |
0 ... 1 % (95 ... 100 %) |
|
He |
0 ... 2 % |
|
Ar |
0 ... 10 % |
|
CO2 |
0 ... 20 % |
|
CH4 |
0 ... 15 % |
|
H2 in blast furnace gas |
0 ... 10 % |
|
H2 in converter gas |
0 ... 20 % |
|
H2 with wood gasification |
0 ... 30 % |
Influence of interfering gases
Knowledge of the sample gas composition is necessary to determine the influence of residual gases with several interfering components.
The following table lists the zero offsets expressed in % H2 resulting from 10 % residual gas (interfering gas) in each case.
|
Component |
Zero offset |
|---|---|
|
Ar |
-1.28 % |
|
CH4 |
+1.59 % |
|
C2H6 (non-linear response) |
+0.04 % |
|
C3H8 |
-0.80 % |
|
CO |
-0.11 % |
|
CO2 |
-1.07 % |
|
He |
+6.51 % |
|
H2O (non-linear response) |
+1.58 % |
|
NH3 (non-linear response) |
+1.3 % |
|
O2 |
-0.18 % |
|
SF6 |
-2.47 % |
|
SO2 |
-1.34 % |
|
Air (dry) |
+0.50 % |
For residual gas concentrations differing from 10 %, the corresponding multiple of the associated value in the table provides an acceptable approximation. This is valid for for residual gas concentrations up to 25 % (dependent on type of gas).
The thermal conductivity of most gas mixtures has a non-linear response. Even ambiguous results, such as e.g. with NH3/N2 mixtures, can occur within a specific concentration range.
In addition to a zero offset, it should also be noted that the gradient of the characteristic is influenced by the residual gas. However, this effect is negligible for most gases.
In case of correction of the influence of interfering gases with additional analyzers (ULTRAMAT 6/ULTRAMAT 23), the resulting measuring error can – depending on the application – amount up to 5 % of the smallest measuring range of the respective application.
Example of correction of cross-interference
Specification for the interface cable
|
Surge impedance |
100 ... 300 , with a measuring frequency of > 100 kHz |
|
Cable capacitance |
Typ. < 60 pF/m |
|
Core cross-section |
> 0.22 mm2, corresponds to AWG 23 |
|
Cable type |
Twisted pair, 1 x 2 conductors of cable section |
|
Signal attenuation |
Max. 9 dB over the whole length |
|
Shielding |
Copper braided shield or braided shield and foil shield |
|
Connection |
Pin 3 and pin 8 |
Bus terminating resistors
Pins 3-7 and 8-9 of the first and last connectors of a bus cable must be bridged (see image).
Note
It is advisable to install a repeater on the device side in the case of a cable length of more than 500 m or with high interferences.
Up to four components can be corrected via the ELAN bus, correction of cross-interference can be carried out for one or two components via the analog input.
Bus cable with plug connections, example