Drum-type Gasmeters TG-Series (Wet-Type)
Operation Instructions – Packing Liquid (Chapter 1)
1.1. General
By all means, the meter must be filled with the very same packing liquid which the meter was calibrated with! Otherwise major measurement errors will occur!
The Gas Meter (which is shipped dry) must be approximately half-filled with a suitable so-called "Packing Liquid" before first use. The measuring drum which rotates in this packing liquid forms the actual measuring unit in conjunction with the liquid.
The packing liquid has two functions: Firstly, it seals off the active measuring chamber (= measuring chamber inside the measuring drum which is being filled with gas) and secondly, the level of the packing liquid inside the measuring chamber defines the volume of the measuring chamber. The latter function is the basis for calibration of the gas meter's measuring accuracy which is performed at the factory. Because of this, the measuring accuracy is directly dependent on the packing liquid level and so an incorrectly set level at time of installation is bound to cause incorrect measurements (see 2.3).
1.2. The mutual influence of packing liquid and gas
Irrespective of the chosen packing liquid, the packing liquid and the flowing gas inevitably affect each other with respect to evaporation and dissolving:
- Absorption of evaporated particles of the packing liquid by the gas,
- Dissolving of the gas in the packing liquid up to the saturation limit.
Generally valid figures and limit values for the mutual influence of gas and packing liquid cannot be stated, because they depend to a very great extent on the particular gas and its state. For example, when water is used as a packing liquid, a dry, warm gas absorbs significantly more evaporated water particles than a moist, cold gas.
The solubility of gases in the packing liquid also varies greatly: for example, the solubility of nitrogen is 6 %, air 7 to 8 %, oxygen 12 %, carbon dioxide 90 % (volume % at 20 °C) in "Autin-B" White Oil (mentioned below). The gas can, of course, only dissolve in the packing liquid up to the saturation limit. A measuring error caused by solubility can be avoided if the gas can dissolve in the packing liquid up to the saturation limit during test operation before the experiments are subsequently carried out.
1.3. Selection of packing liquid
The criterion for choosing a packing fluid should be that any mutual influence between the packing fluid and the flowing gas should be as small as possible, or that the effects can be ignored. In most cases, water can be used as the packing fluid. No special requirements for the water are necessary - that means that normal clean tap water can be used.
When water is not suitable to be used as the packing fluid, oils or synthetic fluids can be used. Generally speaking, a thin-bodied fluid (ideal: viscosity of water) with a low vapour pressure (ideal: <0.1 mbar/hPa) should be selected. A thin-bodied fluid causes a small friction resistance of the rotating measuring drum only, and hereby a small pressure difference between gas inlet and outlet of the meter. This, in return, results in a better (more flat) calibration curve. A low vapour pressure reduces the (unavoidable) evaporation of the packing liquid. Hereby a better long term stability of the packing liquid level is obtained and thus more stable measuring results.
RITTER recommends, and can supply, the following alternatives:
- "Ondina 909" or "Autin-B". These oils are paraffinic White Oils. They are colourless, clear and odourless.
- "Silox", a silicone oil belonging to the group of polydimethyl siloxane. It is colourless and clear with a weak odour.
- "CalRiX", which is a completely synthetic fluid on a fluorine base. It is almost totally inert, even to the most aggressive gases. It can also be used without difficulty under the most demanding and critical application situations. Further advantages of "CalRiX" are: low evaporation rate; a viscosity similar to that of water; 1.8 times the density of water and very low surface tension, which result in a more even rotation of the measuring drum; dry gases remain dry.
Subject to alterations