6. PROBE CALIBRATION
There are three methods for the calibration of the TC probe
1. For COMPARISON with data coming from isokinetic analysis in the gas to be sampled.
2. For SIMULATION with pre-weighed quantities put in the duct if the datas from the isokinetic analysis done in apposite laboratories are not
available.
3. Calibration to detect only the broken bag or broken cartridge.
6.1 CALIBRATION FOR COMPARISON (SEE PRACTICAL EXAMPLE CALIBRATION FOR COMPARISON 6.6)
In order to proceed with the calibration of the probe, you must know the data concerning the quantity of dust present in the gases in mg/m3.
These data are normally released by special laboratories during the periodical gas analysis These analysis must be done according to the
law restrictions.
6.2 CALIBRATION FOR SIMULATION (SEE PRACTICAL EXAMPLE CALIBRATION FOR SIMULATION 6.7)
The calibration of the probe is recommended after a certain period of operation of the plant. The above mentioned period is necessary to
allow the filtering material to reach the typical characteristics of the functioning (filter partially dirty).
6.3 CALIBRATION OF THE 4÷20 mA SIGNAL
The function of the TC probe is to convert the triboelectric ‘T-E’ physical phenomenon, produced by a certain quantity of dust present in the
gases, in an electric signal ‘STC’ 4÷20 mA.
In order to calibrate the STC signal, it is essential that the sampling duct contains a measurable quantity of dust.
In default of dust, it is impossible to proceed with the calibration.
The STC signal must be calibrated in order to cover the whole range of emissions.
In order to proceed with the calibration, the probe must be connected to an instrument that detects the 4÷20mA output signal.
1. Without dust emissions, the value must be 4mA (value pre-determined by the calibration from the manufacturer’s laboratory).
2. With the maximum value of emissions estimated, the signal must be around 14÷18 mA.
In practice, if according to law regulations for certain types of material the maximum level of emissions allowed is equal to 20 mg/m3, the
STC signal must be adapted to this value in the presence of maximum emissions.
You can get this calibration by acting on the rotary switch present inside the head of the probe (SW21).
By leaving a margin of 20÷30 % against the maximum value allowed, you can consider that: 0÷20 mg/m3 = 4÷17 mA.
Over 20 mA the STC signal saturates by making insignificant the higher values of dustiness.
This fact does not constitute a problem, since the goal of this application is to control by means of an alarm threshold that the emission
limits envisaged by the law are not overtaken.
The STC signal generated is handled by electronic equipments (ESA sequencers, PLC, PC) that convert it into a digital value.
The digital value can easily be adapted to the value of the quantity of dust present in the duct.
6.4 SENSITIVITY
The TC probe, by triboelectric effect generates a STC electric signal which is function of 4 parameters:
1 Length of the probe
2 Speed of the gases in the sampling duct
3 Type of material present in the smokes
4 Density of dust
The degree of sensitivity of the probe means the possibility to convert the value of the T-E signal (source) in a certain STC 4÷20mA electric
signal that can be used by the various equipments connected to it.
The ratio KTC between the two entities is called SENSITIVITY: STC = KTC x T-E.
6.5 SENSITIVITY REGULATION
KTC is a variable coefficient depending by the switch SW21 place inside of the TC probe (See Pg. 2).
SW21 allows to change the sensibility amplifying or attenuating the response of STC.
Normally SW21 comes set in position 4 to a low level after the laboratory tests.
Pg. 4