Innovative solutions to pressure sensing in biogas production

Innovative solutions to pressure sensing in biogas production

Microbiological analysis is an important component of the biogas manufacturing process. In this instance, combined pressure and temperature transmitters from STS are employed.

The Institute for Agricultural Engineering and Animal Husbandry at the Bavarian State Research Center for Agriculture has been examining, amongst other things, the influence of activating or toxic substances on the process of biogas production. In contrast to the continuous flow process of a biogas plant, investigations of potential such as this are conducted in intermittent batch procedures. For these investigations, a mini-batch system has been specially developed, based upon combined pressure and temperature transmitters from STS.

Measuring microbial activity

To guarantee a reliable temperature control, which is essential to such investigations due to its vital role in microbiological activity, the mini-batch system is submerged in a water bath. Within this bath, some 33 measuring points are situated so that ten variants, as well as a control sample, can be tested for both parallel and statistical evaluation. The measurement of microbial activity takes place indirectly by a continuous determination of biogas production aided by the ATM/N pressure transmitter from STS.

To additionally calculate methane productivity, the gas composition is regularly analyzed using a gas chromatograph. After adding 100 ml of fermenter content to 300 ml Schott-Duran bottles, the ATM/N pressure transmitters are capable of exactly recording the pressure increase brought about by biogas production. From this, an exact statistical evaluation and assessment of the addition of substances is possible in the process of biogas production, as is a comparison between those individual variants.

Combined sensors are highly versatile

A substantial advantage of combined sensors for pressure and temperature is the recording of both process parameters from only one pressure port. The temperature probe here is submerged in the medium and provides a measurement range of – 25 … + 50 °C. All connections are welded and conform to the IP68 protection rating. This has the advantage that these sensors, apart from their industrial usage, can also be applied in the foodstuffs and pharmaceuticals industries. Other typical applications for the transmitters are in plant and machine engineering, in testing and calibration technology, process engineering and environmental technology, as well as shipbuilding. These sensors are also deployed in the industrial environment of biogas plants for determining the filling level inside fermenters.

The following characteristics set these pressure sensors apart: Measurement ranges from 0 … 50 mbar to 0 … 25 bar, high dynamic response and precision (< 0,1 % FS), mechanical and electrical adaptation to end user applications due to the manufacturer’s modular system. Upon request, intrinsically safe designs can also be supplied. It is through these technical properties that the pressure sensors are suited to various fields of application in measurement technology, as well as in equipping test beds and calibration facilities.

Original publication: INDUSTRIELLE AUTOMATION 2/2014 

Density measurement in gas flow meters

Density measurement in gas flow meters

Gas consumption is calculated using gas meters measuring the flow volume. Since the density of gas, and thus its volume also, is both pressure and temperature dependent, the measured quantity can deviate due to the prevailing  pressure or temperature. The gas volume, depending upon pressure and temperature, can be described by the formula p · V/T = Constant (p: pressure, V: volume, T: temperature).

Whilst the pressure with which gas flows through the pipes can be relatively easily controlled and monitored, this is not the case with the temperature. The resulting differences in density have an influence on the measured flow rate. What remains negligible here to the normal consumer due to relatively light usage becomes an important cost factor to those major consumers.

With the Measurement Instruments Directive (MID), an EU-wide guideline for measuring instruments was issued to establish a uniform approval procedure for all EU states and some other nations. Further objectives of the directive include a one-time and unified test for the approval of measuring instruments, as well as a uniform and transnational regulation for initial calibration. With these designated, transnational regulations an even better product quality is striven for and a level playing field ensured. Ten types of measuring instruments in the sphere of legal metrology are covered by the MID, with the requirements for gas meters and volume converters laid out in Annex MI-002.

Pressure and temperature must be taken into account when calculating exact gas quantities. And this requires appropriate sensors in the gas meters. Instead of the volume, the gas mass must be indicated, since this is the more precise measure in light of fluctuating density. To reliably determine this, it is necessary to measure both pressure and temperature and thus determine the density.

High precision through computational compensation

There are two types of pressure and temperature sensors to be connected to gas meters. In the first variant, the pressure transmitter is screwed onto the gas-delivery pipe and connected to the gas meter by means of a cable. In variant two, however, the sensor is installed directly into the device (the specific example below describes variant two).

The pressure ranges used for gas metering generally fall between 0.8 and 3.5 bar (absolute) and 2.5 to 10 bar (absolute). The requirements in terms of precision are enormous: Demanded is 0.2% of the measured value at temperatures from -20 °C to 60 °C. This figure, however, cannot be achieved with conventional pressure sensors. To maintain this level of accuracy, computational compensation must be applied. For this reason, STS supplies its pressure and temperature transmitters not only functionality-tested, but also parameterized (coefficients for polynomial compensation).

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