Be.T.A. - Model BetaCAP30X100 - Capillaries Based Dual Range Gas Dividers
BetaCAP30X100 is a gas divider in two stages with 100 + 30 equal capillaries, designed to operate 31 dilution ratios linearly scaled between 0 and 100%) and 30 more between 3.000: 1 and 100: 1. While the first range of dilutions gets the main application of the analyzers for emissions, the second is aimed to tests of analyzers air quality (immission). BetaCAP30X100 is built in a 19 'rack to hold two modules: BetaCAP1A100 is a pre-divider by 100 fixed ratio with 100 capillaries, automatic regulation of the input pressures , automatic regulation (bypass) of the output pressure and with a bypass solenoid that allows the inclusion / exclusion of the pre-divider. BetaCAP30 is the diluter with 30 capillaries, to achieve 30 dilution steps. This module is described in detail in the corresponding page, where you can download the relevant technical data sheet.
The pre-diluter is controlled by a separate electronics, but his functions are coordinated by the main interface (display / keypad) so that the operator does not have to worry about the interfacing of the two modules
The mechanical stability of the capillaries, combined with fine filtration at the inlets, do ensure the maintenance over time of the low-uncertainty in dilution factors. The inlet pressures of both sides are electronically regulated (PID) and the operator is just requested to provide enough pressure to inlet fittings for the gas to be diluted and the diluting gas To reduce the flow of diluted gas is sufficient to reduce the set point of the pressure at the inlet of the final diluter.
Any difference in viscosity of the two inlet gases is easily compensated unbalancing the pressures applied to the capillaries according to the formula P1 / P0 = η1 / η0 : when the user writes the value η1 / η0 in the test gas menu, the viscosities compensation is automatically handled by the diluter logics. The viscosities ratio is easily calculated by the spreadsheet you may find in this web site. The user may set the wanted output gas flow by setting the P1 pressure : P0 will be managed accordinghly by the diluter and the flow is linearly growing with the applied pressures.
- The use of 'equal' capillaries makes it simple and very accurate thr selection process (pre-selection and final selec-tion) and thus allows for low uncertainties on dilution ratios.
- PID electronic control of pres-sures, along with the quality of the sensors ensures high repeatability (± 1 hPa of 2000) of the pressure applied to the capillaries.
- The ability to assign set point calculated has allowed the implementation of additional functions (compensations and operating modes) that add capabilities to the gas divider without asking the user for a particular engagement.
- The user is informed 'in the clear' through display 4 x 40 characters on everything that happens in the diluter that could interfere with the correct results.
- Three analog inputs are avai-lable to acquire the concen-tration values measured by the analyzers subject to the test. This feature is necessary to run the software InfoCAP30 in a really automatic way, produ-cing automatically the test reporting.
Pneumatic diagram and pressures control function
From the diagram it is evident an apparent inconsistency: while the flow of gas leaving the pre-diluter is constant, the required flow (gas to be diluted) to the final diluter is variable (due to changes of the dilution factor) with a dynamic range of 0 to 30. The the bypass valve (CVby) has precisely the function of adapting the two stages venting, when necessary, the excess gas pre-diluted. Activating the solenoid valve SV07 the pre-diluter, is bypassed and the span gas is applied directly to final diluter. In this case, the bypass regulator is automatically set to off.
Continuous dilution range with capillaries ? Yes, we do.
Even for BetaCAP30X100 it's available the 'linear capillaries' operation mode : it adds a menu that allows the user operating the direct entry of the required concentration value without the limitation of discrete steps. The mail electronics manages the task of converting the request of the user into the required actions (activating the solenoid valves to set the correct dilution step and setting the setpoint of pressures ratio to move the dilution at the wanted target)
Even in case of different viscosities is provided automatically offsetting the effects. The range of possible dilutions thus is extended to the interval 0.025% ... 98,33%, in addition to zero and 100%.
Even for BetaCAP30X100 it's available the option 'multiple test gases selection': only in 'remote operation mode' offers the possibility to select one of six test gas cylinders to be diluted and of one out of two diluting gas cylinders. The option is provided ti estend the tests (managed by PC) to multi-component analysis benches. With this option 10 isolated analog inputs are kept available for receiving the measurement signals from analyzers in the test: the 10 signals are used two at a time for each of the additional 5 test gas cylinders . An additional analog input is always available (normally used for the sixth cylinder, the one connected to the front fittings).
BetaCAP30X100 is the clever combination of two diluters :
- the well known 30 steps gas divider built in a 19” rack mod. BetaCAP30RK
- The more recent fixed divider 1:100 compact module mod. BetaCAP1A100.
Evaluating the combination of those devices we did realize that a new product did born with a new functions set.
From the applications point of view, the diluter is suitable both for emissions monitoring and for ambient air quality.
From the operational point of view, this device may be operated in two ways :
- traditional way where the user selects the dilution ratio (0:30….30:30 and 1:3000...30:3000) and may read the concentration
- of the diluted gas ;
- a new way, never seen before in a capillary type gas divider, where the user selects the diluted gas concentration in the range 0,02% to 98,3% of the span gas, without being conditioned by the discrete steps typical of capillaries.
This gas divider offers the flexibility that we may find in MFCs type gas diluters, but joined to the accuracy, stability and availability typical of capillaries.
- 30 + 30 dilution ratios available in two ranges (with pre-divider on or bypassed)
- PID pressures regulation in pre-divider, bypass and final divider (repeatability ± 1 mBar)
- Internal circuits made by materials (glass, PVDF, PEEK, PTFE, AISI316L, Viton) corrosion resistant
- Compact construction with manifolds to minimize fittings and tubes number (reduction of leakages)
- Automatic compensation of certified errors for both dilution stages
- Compensation of different gas viscosities (viscosity is the sole gas property that may affect dilution with capillaries)
- Analog signals isolation and acquisition for 3 measuring signals from the gas analyzer under test
- Available options : Software for automatic testing, Multiple test gas switching unit.
The main diluter is well described in the data sheet BetaCAP30, than this document will focus on pre-divider and interface between the two modules.
The pre-divider inlets (gas to be diluted and diluting gas) are protected by two fine filters to prevent dusty fittings from releasing dust in the circuits.
All the gas contacted parts are corrosion proof using fluorinated plastic compounds, glass, stainless steel 314L. It’s also available an “all steel” version to reduce the delays due to plastic saturation and permeability (with HF, NH3, O2 very low ranges).
All the internal parts of the pre-divider module BetaCAP1A100 are built to accept input pressures of max 5 Bar, and the final divider BetaCAP30 to accept pressures of max 2,5 Bar. The pressure applied to the pre-divider is higher than the pressure applied to the 30 steps divider.
A detail that is immediately evident is the apparent incompatibility between the flow variable (with dynamic 0 to 30) at the entrance of the final diluter, compared to flow produced by the prediluter, which instead is basically fixed.
The back pressure regulating valve VRby leaves vent (bypass) a pre-diluted gas flow which compensates for variations in the flow of gas from the entrance of the final diluter : In this way, it maintains a constant flow of pre-diluted gas and consequently the pressures at the ends of pre-diluter.
The differential pressures (input-output) of the two ways (diluent gas and gas to be diluted) in both modules are accurately regulated (± 1 mBar) to control the dilution process : in fact, a change of the applied pressure corresponds to a proportional variation of the induced flow in the capillary and than a dilution change.
As we shall see, this ratio P (TG1) / P (TG0) is of fundamental importance in both the pre-diluter diluter and the final diluter.
This is the function that faithfully represents the behavior of a diluter with equal capillaries:
- KDIL is the dilution ratio
- N is the number of capillaries involved by the gas to be diluted
- TOT is the total number of capillaries
- η is the viscosity of the gas or the gas mixture
The index TG0 refers to the diluent gas, and TG1 refers to the gas to be diluted.
For a diluter as BetaCAP30, N is an integer between 0 and 30, while TOT = 30
For a diluter as BetaCAP1A100 (our pre-diluter), N = 1 and TOT = 100. In fact in the pre-diluter there are 100 capillaries, one of which is crossed by the span gas and 99 by the diluting gas.
The total dilution ratio (the series between the pre-diluter and the diluter final) corresponds to the product between the dilution ratios of the two elements.
In both modules, the pressures regulation function accepts two set points : P(TG1) and the ratio P(TG0) / P(TG1) . An additional set point is available to set the bypass control (interfacing the two modules).
Viscosity compensation : is a practice we use since longtime. In the earlier versions of BetaCAP30 it was manual (the user sets the pressures ratio setpoint at the same value of the viscosities ratio) now is automatic.
- when the pre-diluter is used, viscosity compensation is applied to the pre-diluter only (the final diluter will handle two gases very similar because one is pure diluting gas and the other is diluted 100 times with diluting gas).
- when the pre-diluter is bypassed, viscosity compensation is applied to the final diluter.
When the active gases concentrations in the span gas are lower than few percent, the difference of viscosities may be neglected ( ηTG1/ηTG0 ~ 1)
Continuous dilution range : is a new practice that uses the above formula to move the dilution ratio by manipulating the ratio P(TG0) / P(TG1) . It’s evident that increasing this ratio value, the flow of the zero gas do increase and the dilution ratio too, and vice versa decreasing the ratio. In the side chart, we may see the 30 lines, one for each N (number of capillaries crossed by the span gas) in the space x= dilution ratio and y= ratio P(TG0) / P(TG1) . Using this practice, the entire area of dilution factors may be covered just playing with the pressures ratio. Knowing the span gas concentration (and his viscosity) and the wanted concentration, the gas divider may find the best “N” and the correct pressures ratio value to get the target.
The operator is faced with a simple interface: alphanumeric display (4 x 40 characters), and 5 function keys, with whom he may select the menu and set a few parameters needed to operate.
The menus hierarchy originates from 4 independent lines :
- Initial configuration : concerns the construction details (installed options, basical adjustments, PID parameters and correction parameters that, on the base of the metrological certificate, correct the measured errors.
- Calibrations: contains the sub-menu for the calibration of the analog signals acquisition and for the two groups of three pressure sensors. The pressures calibration is “static” (without gas flow, the pressure is uniformly distributed inside the unit) and don’t requires traceable references (the sole purpose is of aligning the sensitivities of all the measurements after having corrected the offset (using environmental pressure).
- Parameters accessed by the user : like the concentration of span gas, his viscosity, the set points of the pressure con-trols (both for pre-diluter and final diluter)
- Operation modes : with three possibilities : “dilution factor selection” mode , “diluted concentration selection” mode, “remote” mode
The first three menu groups are password protected
We see more in detail just the operation mode menus :
- Mode “dilution ratio selection” : is classical for a capillaries type gas divider. The user must set the integer value of N(0<N
- Mode “diluted concentration selection” : it’s an unconventional way, that get advantage of the above theory to drive the gas divider obtaining the target result. In the indicated order, it manage automatically the following choices :
- including or bypassing the pre-diluter module
- Defining the number N of capillaries to be crossed by span gas
- Calculating and applying the set points of pressures ratio for pre-divider and final divider
The function that is applied, do also uses the viscositiy values, to find the best N value to minimize the difference between P(TG1) and P(TG0) and corrects according to the certified errors
- “Remote” mode : through the serial port RS485 with AK protocol, both pre-diluter and final diluter receive commands from a PC : the command types do include all the functions that may be called locally . The optional software Info-CAP30 is designed to handle programmed sequences : receiving the measuring values acquired by the diluter it may show the trends of real and theoretical concentrations and write reports complying with the norm EN14181.
- Dilution ratios (discrete mode):
- 31 discrete values regularly spaced between 0:30 and 30:30
- 30 discrete values regularly spaced between 1:3000 and 1:100
- Dilution ratios (continuous mode): all the diluted values between 0,022% and 98,33% of the span gas
- Dilution accuracy (natural): better than 0.5% of reading + 0,0003% of the input concentration (after calibration): better than 0,2% of reading + 0,0001% of the input concentration
- Dilution repeatability: < ± 0.2 % of reading
- Input pressure: up to 5 Bar at inlet fittings
- Flow of diluted gas: depends on the regulated pressure and capillaries size
- small capillaries size from 0,2 to 2 L/min.
- big capillaries size from 0.6 to 6 L/min.
- Gas connections
- inlet: Span gas and diluting gas
- outlet: diluted gas and bypass
- Connection types .: compression fittings 4 x 6 mm PVDF (AISI316L upon request)
- Metrological certificate: optional, from an European accredited laboratory
- Gas contacted materials: AISI 316, Glass, PVDF, PPS, PEEK, Kalrez, Viton, epoxy resin.
- Main measurements: 3 rel. pressures for pre-divider and 3 rel. pressures for final divider
- Other measurements: barometric pressure and internal temperature
- Acquired measurements: 3 concentration signals from gas analyzers (isolated from diluter)
- Serial communication port: RS485 (with USB converter) open protocol type AK
- Electric power: 100 ... 240 Vac - 0,8 A max.
- Size and weight: 19” std. h 3UT dept. 250 mm - weight 10 kg
** The uncertainty of the certifying laboratory is not considered.