Operation Manual – RITTER MilliGascounters

Initial Operation

Handling after receipt

• 1x MilliGascounter
• 1x Bottle (200 ml) of packing liquid (hydrochloric acid 1.8 vol% = 0.5 mol/ltr)^*
  option for gas mixtures without CO₂: packing liquid Silox
• 1x Gas connection tube (1.5 m)
• 1x Funnel for filling of packing liquid
• 1x Syringe, each per 1 to 5 MGCs (for fine adjustment of the packing liquid level)
• 1x Level for horizontal alignment of the MilliGascounter
• 1x Cleaning rod for micro capillary, each per 1 to 5 MGCs

^* Because hydrochloric acid is classified as a »dangerous good« there are certain restrictions for transportation depending on the mode of transportation, country of destination and quantity. Please check prior to your order with your distributor or RITTER on the details for your individual case.

Installation

• The MilliGascounter should be installed on a horizontal, solid and vibration-free base.
• If condensation from the measured gas might occur inside of the MilliGascounter please refer to: »Condensation«.
• If hydrochloric acid solution is used as a packing liquid: When the packing liquid evaporates, hydrochloric acid vapour may escape from the gas outlet of the MilliGascounter. It is therefore recommended that the supplied tube is connected to the gas outlet and the escaping gas is diverted away from the MilliGascounter to prevent corrosion of the MilliGascounter’s counter unit. In addition, the tube can be directed into a water bottle to neutralise any hydrochloric acid vapour that may be present. When using several MilliGascounters in one location, a neutralisation bottle with a connection possibility of up to 9 MilliGascounters can be supplied. The neutralisation bottle is filled with simple tap water.

Packing Liquid

• The HCl solution is corrosive due to pH value
• Avoid direct contact with skin and eyes as well as inhalation of vapours
• May be corrosive to metals
• Keep container tightly sealed
• Recommended storage temperature: 15 – 25 °C
• In case of emergency: Immediately contact local or national Poison Information Center (Germany: +49-551-19240) or doctor! Don’t effect vomiting!

Filling with Packing Liquid

• Unscrew the screw plug for filling (5).
• Place the supplied funnel into the filling hole.
• Remove any gas tubing at the gas outlet (4) for venting.
• Pour the provided packing liquid through the funnel into the casing.

For fine adjustment of the packing liquid level (16) please refer to »Setting of the correct Packing Liquid Level«.

Screw the screw plug for filling (5) to the casing with a maximum torque of 1 Nm (hand-tight) otherwise the thread might break.

Setting of the correct Packing Liquid Level

PMMA Version (transparent casing)

• Fill the liquid into the casing until the liquid covers the measurement cell completely.
• Tilt the MilliGascounter twice so that the measuring cell performs two tilting movements. Do this by holding the MilliGascounter in front of you, facing the display of the counter unit. Then tilt the MilliGascounter once to the right and once to the left side. This will remove any air trapped within the two measuring chambers.
• Remove the connection tube from the gas inlet nozzle or depressurise the gas inlet. Please wait for approximately two minutes. The liquid level inside the casing will then be levelled with the liquid level inside the vertical gas inlet channel.
• To increase or decrease the packing liquid level, the use of the supplied syringe is recommended. A tracing screw (2) is located at the upper part of the casing (1) to set the correct level of the packing liquid (3).

The position of this screw must not be changed under any circumstances!

PVDF Version

• Please fill as much packing liquid into the casing until the liquid level is reaches the middle of the sight glass located in the casing wall below the counter unit.
• Tilt the MilliGascounter twice so that the measuring cell performs two tilting movements. Do this by holding the MilliGascounter in front of you, facing the display of the counter unit. Then tilt the MilliGascounter once to the right and once to the left side. This will remove any air trapped within the two measuring chambers.
• Remove the connection tube from the gas inlet nozzle or depressurise the gas inlet. Please wait for approximately two minutes. The liquid level inside the casing will then be levelled with the liquid level inside the vertical gas inlet channel.
• To increase or decrease the packing liquid level, the use of the supplied syringe is recommended.
• The MilliGascounter is filled correctly when the liquid level matches the indicated dot in the middle of the sight glass (see image below).

Tube connection

Measurements

Measurement Principle with Schematic

The gas to be measured flows in via the gas inlet nozzle (3), through the micro capillary tube (9) located at the base of the MilliGascounter and up into the liquid casing which is filled with the packing liquid (12). The gas rises as small gas bubbles through the packing liquid, up and into the measurement cell (13). The measurement cell consists of two measuring chambers, which are filled alternatingly by the rising gas bubbles. When a measuring chamber is filled, the buoyancy of the filled chamber causes the measurement cell to tilt abruptly into a position where the second chamber begins to fill and the first chamber empties.

The measurement of gas volume therefore occurs in discrete steps by counting the tilts of the measurement cell (13) with a resolution of approximately 3 ml (= content of one measuring chamber, please also refer to »Calibration / Measurement Error«). This »residual error« (= max. 3 ml) caused by the resolution should be taken into account when estimating / calculating the total measurement error.

The tilting procedure of the measurement cell generates a pulse through the permanent magnet (11) on top of the cell and one of the two magnetic sensors (reed contacts) (10), which is registered by the counter unit (1).

For external data logging (PC) the switching pulses of the second reed contact can be obtained via the signal output socket (2). Please also refer to: »Signal Output«.

The measured gas exits through the gas outlet nozzle (4).

Legend

1. Counter unit with LCD display
2. Signal output socket (reed contact)
3. Gas inlet
4. Gas outlet
5. Screw plug for filling
6. Gas inlet channel
7. Casing
8. Base plate
9. Micro capillary tube
10. Two reed contacts
11. Permanent magnet
12. Packing liquid
13. Measurement cell (tilting body) with twin-chambers
14. Tracing screw for liquid level (with MilliGascounter MGC-1 PMMA)
15. Inspection screw gas inlet channel

Calibration / Measurement Error

Static Correction of Manufacturing Tolerances

Due to manufacturing tolerances, the exact (real) volume of a measurement cell is generally ≠ 3.0 ml. Therefore, it is determined by an individual calibration of each MilliGascounter unit. The exact cell volume …

• is determined at the standard flow rate of 500 ml/h.
• is stated in the calibration certificate.
• is programmed into the counter unit.

Dynamic Correction of the Measurement Error

Effect of Dead Space Volume

Whereby:

• V_D = Deficit volume
• V_DS = Dead space volume
• P_a = Current ambient pressure (mbar)
• P_DS = Remaining pressure in dead space volume = 5 mbar

Condensation

Influence of Particles (Dirt & Dust) in the Gas Flow

Effect of Temperature Changes

Due to the extreme resolution of MilliGascounters in the millilitre range, »volume flows« can also be registered as a consequence of changes in temperature. A temperature rise (or decrease) at the gas source or in the supply system causes an expansion (or contraction) of the gas present in the system proportional to its volume. While an expansion of the gas generates a »virtual« gas flow (with an consequent indication at the counter unit), a contraction causes an under-pressure in the supply system. This underpressure allows packing liquid to flow through the micro capillary tube into the gas feeder tubing. Packing liquid in the gas feeder tubing causes …

• an increased admission pressure.
• a time delay until the first indication of values on the counter unit (until the micro capillary tubing free of packing liquid).
• fluctuating measurement results.

No measurement should be started until the temperature of the entire system has been adjusted^*. An expansion of the gas during adjustment of the temperature and the subsequent build-up of an overpressure can simultaneously be used as an operational check of the MilliGascounter (description of reset button, please refer to »Reset Button«).

The room temperature should be approximately constant during the entire measurement. (Please mind a temperature decrease during the night and over the weekend.) Otherwise the temperature has to be monitored so that an integrating correction of the measurement values can be performed (please also refer to »Temperature and Pressure Corrections«). Another alternative is the installation of the MilliGascounter, gas feeder tubing and gas source in a temperature-controlled enclosure.

^* With fermentation tests: After equalization with the fermentation temperature.

Effect of System Gas Pressure

A rise (or decrease) in pressure at the gas source or respectively in the gas supply system causes an expansion (or contraction) of the gas present, proportional to its volume. The same applies to air pressure as described under »Effect of Temperature Changes«.

Effect of Water Vapour Partial Pressure^*

If the measurement result has to be corrected for the volumetric fraction of water vapour, the values in the following table (which are adjusted for temperature) can be used in the equation in accordance with »Temperature and Pressure Corrections«:

^* These values are only valid for gases which contain water vapour and only if the volume of the water vapour must be mathematically eliminated from the measurement result. If the water vapour is a »natural« element of the gas and its volume should therefore be taken into account, then no (partial-)pressure correction should be carried out. In that case pV = 0 must be used in the equation in accordance with »Temperature and Pressure Corrections«.

Temperature and Pressure Corrections

The MilliGascounter is a volumetric gas meter and therefore measures gas volume in the current operating state at the time of measurement. The gas volume is dependent on gas temperature, air pressure and water vapour partial pressure (please refer to »Effect of Water Vapour Partial Pressure«). These measurable variables are therefore needed to recalculate to norm conditions. The gas temperature has to be measured at the gas outlet.

According to the general gas laws the following equation is used for temperature and pressure corrections:

Whereby:

Special characteristics of Fermentation Tests

• In incubators without compulsory ventilation, uneven temperature distribution can cause under-pressure in the reaction vessels.
• To determine the total gas production as accurately as possible, it is advisable to to remove the dissolved CO₂ from the fermentation tank through acidification to pH 1-2 after the fermentation test has ended. However, this can lead to foam formation and leave residue in the tubing.
• The MilliGascounter was calibrated at room temperature (21 ºC). If the user defined standard temperature is 21 °C (instead of the international standard of 0 °C / 273.15 K), any temperature correction is not necessary when the gas is cooled down to 21 °C. At a fermentation temperature of 37 ºC, this can be achieved by using tubing with a length of 1.5 m.
• Experiments to determine the methanogenic potential of organic substances in the laboratory of Prof. Dr. Paul Scherer^* (University of Applied Sciences Hamburg, Paul.Scherer@rzbd.haw-hamburg.de) have shown that the dry matter content of the seed sludge has not only an influence on the velocity of the gas production, but also on the total amount of produced gas. In all cases parallel to the gas production of a test substance a reference without added organics was subtracted. Based on these findings it is recommended to use at least 3% dry matter content of a seed sludge. It is important to homogenize the sludge with a blender before use. It is also of importance that thickened sewage sludge often contains small amounts of polymers to support the coagulation. Added polymers often contain substantial amounts of biodegradable alkanes to facilitate the addition. These additives increase the background production of biogas during a test period. If the background production of biogas is too high this could complicate the calculation of the gas production of the test substance.
• Should the gas production of the seed sludge be too low, the pressure in the fermentation bottles might drop below the atmospheric pressure. According to the principle of connected tubes this can lead to a flow of oily packing liquid into the test vessel. In such cases it is recommended to increase the background gas production by the addition of cellulose powder (e.g. Avicel). Furthermore, the test approach should be started at room temperature so that the temperature in the incubators (mostly 35 – 37 °C) increases evenly and generates a small overpressure.

^* Scherer, P.A. (2001) Influence of high solid content on anaerobic degradation tests measured online by a MilliGascounter® station for biogas. In: Proceedings of the 9th World Congress on »Anaerobic Digestion 2001« (L. van Velsen, W. Verstraete, Eds.), Antwerpen

Counter unit

Display

Reset Button

Signal Output

Reed Contact

Output Socket

Maintenance

Inspection of the Packing Liquid Level

The rate of evaporation of the packing liquid in the MilliGascounter occurs very slowly but dependent upon the gas flow rate as well as the operating temperature. Also the diameter of the gas outlet nozzle contributes to this process. The evaporation can be further diminished by closing the outlet with a stopper and piercing it with a syringe needle. To ensure stable measurement accuracy, the packing liquid level must be inspected regularly. Regarding the correct level please refer to »Setting of the correct Packing Liquid Level«. When using hydrochloric acid solution as a packing liquid, please refer to »Installation«.

Exchange of packing fluid

An exchange of the packing liquid …

• is necessary if particles or substances from the gas which have been introduced into the liquid cause bubbling or foaming.
• is recommended if a large quantity of particles is floating in the liquid.

Cleaning the Micro Capillary Tube

1. Empty the MilliGascounter either by pouring out the packing liquid through the gas outlet nozzle or by removing it out through this nozzle with a pipette.
2. Remove the 4 closing screws underneath the casing base plate.
3. Remove the 4 screws of the measurement cell support fixture (bearing block) which is located at the base plate.
4. The micro capillary should only be cleaned with the cleaning rod containing a fine wire supplied with the MilliGascounter. A wire with a smaller diameter would not have the desired cleaning effect, a larger diameter could damage the micro capillary and consequently lead to an alteration in the calibration and measurement errors.
5. Mount the fixture of the measurement cell to its original position.
6. Mount the casing base plate to the casing. Ensure that the sealing ring is in the correct position. Tighten the 4 base plate screws crosswise. The tightening torque of the screws must not exceed 3 Nm (»handtight«) to avoid damaging the plastic threads.
7. Fill the MilliGascounter with liquid according to »Filling with Packing Liquid«.
8. In addition, the tube can be directed into a water bottle to neutralise any hydrochloric acid vapour that may be present. If more than one MilliGascounter is used in one location, a neutralisation bottle can be supplied to connect up to 9 MilliGascounters. The neutralisation bottle is filled with simple tap water.

Counter Unit Battery Exchange

Disassembly / Exchange of the Measurement Cell

It is recommended that the entire unit be returned to the manufacturer if the measurement cell needs to be replaced. If this is not possible or uneconomical, the measurement cell (including the cell bearing block) can be replaced as follows:

• Disassemble the unit according to »Cleaning the Micro Capillary Tube« (steps 1 – 3).
• After receiving the replacement cell (including the cell bearing block), follow the instructions to reassemble the cell according to »Cleaning the Micro Capillary Tube« (steps 4 – 7).

After the assembly is completed, it is recommended to perform the following function tests:

• Hold the MilliGascounter upside down and swing the whole unit slightly. The measurement cell should be able to swing freely.
• To ensure the gas tightness of the MilliGascounter, close the gas outlet nozzle by inserting a sealed tube. Apply a gas pressure of approximately 10 to 20 mbar to the gas inlet and monitor the pressure indication (manometer). The pressure should remain constant.

Long-term Storage