Thermocouples are extensively used in many applications to provide low cost temperature measurements of gases. However, their accuracy at high temperatures is compromised by radiation to the surrounding environment and conduction of heat through the stem of the probe. The durability of thermocouples is limited by the need to make a protective barrier between the sensor and the gas relatively thin to allow the temperature of the sensor to reach that of the gas. Where pressure and temperature measurements are both required in a gas, conventionally two measurement probes have been used, one being connected to or containing a pressure measurement device and one containing a thermocouple. The use of two probes can cause problems when the probes are large with respect to the scale of the gas flow.

It is an aim of the present invention to provide an alternative method and system for measuring one or more properties of a gas. It is a particular aim of this invention to provide a measuring system that is capable of making measurements of temperature and pressure, particularly in high temperature environments at one location in a gas.

It is another particular aim of the present invention to provide a measuring system that is capable of making measurements of the ratio of the specific heat capacities of a gas or mixture of gases and therefore allowing the user to derive the gas concentration of a mixture of gases.

It is another particular aim of the present invention to provide a system that allows the user to determine the temperature, pressure and gas concentration of a mixture of gases.

According to the present invention, there is provided a system according to claim 1.

The reference to a gas includes gas mixtures.

According to another aspect of the present invention, there is provided a system according to claim 14.

According to a further aspect of the present invention, there is provided a method of determining the temperature of a gas according to claim 10.

According to a further aspect of the present invention, there is provided a method of determining the ratio of specific heat capacities of a gas according to claim 11. According to a further aspect of the present invention, there is provided a method of determining the temperature of a gas mixture according to claim 12.

According to one embodiment of the present invention, there is provided a device for the measurement of gas temperature and pressure, including an orifice, downstream a means of cooling the gas, downstream a means of measuring the mass flow of the gas, all the gas that passes through the orifice passing through the cooling system and the mass flow measurement system, the flow of gas through the probe is alternately present when the gas flow through the upstream orifice is choked and not present when the pressure is being measured, from said measurements calculating the temperature of the gas wherein T. sub. t= (C* (P. sub. t*P. sub. t)/ (M*M)) where T. sub. t is the stagnation temperature of the gas, P. sub. t is the stagnation pressure measured when the no gas flows through the probe, M is the mass flow measured when gas flows through the probe, C is a calibration factor that depends on the ratio of specific heat capacities and the specific heat capacity at constant pressure of the gas and which may be determined by calculation or experiment.

Preferably, the mass flow measurement is determined from pressure measurements upstream and downstream of an orifice plate.

According to another embodiment of the present invention, there is provided a device for measuring the ratio of specific heat capacities of a gas including a means of measuring temperature, a means of measuring pressure, a choked orifice, downstream of the choked orifice a means of measuring mass flow rate, from said measurements calculating the ratio of specific heat capacities of the gas wherein. g=D*T. sub. t* (M*M)/ (P. sub. t*P. sub. t) where T. sub. t is the measured stagnation temperature of the gas, P. sub. t is the measured stagnation pressure, M is the mass flow measured when gas flows through the orifice, g is a function of the ratio of specific heat capacities of the gas and D is a calibration factor that depends on the specific heat capacity at constant pressure of the gas and which may be determined by calculation or experiment.

According to another embodiment of the present invention, there is provided a device for the measurement of the ratio of specific heat capacities including a means of measuring temperature, downstream an orifice, downstream a means of measuring mass flow rate, the flow of gas through the probe is alternately present when the gas flow through the upstream orifice is choked and not present when the pressure is being measured, from said measurements calculating the ratio of specific heat capacities of the gas wherein g=D*T. sub. t* (M*M)/ (P. sub. t*P. sub. t) where T. sub. t is the measured stagnation temperature of the gas, P. sub. t is the measured stagnation pressure, M is the mass flow measured when gas flows through the orifice, g is a function of the ratio of specific heat capacities of the gas and D is a calibration factor that depends on the specific heat capacity at constant pressure of the gas and which may be determined by calculation or experiment.

According to another embodiment of the present invention, there is provided a device for the measurement of gas temperature, gas pressure and the ratio of specific heat capacities including an orifice, downstream a means of cooling the gas, downstream the gas being divided into two parallel paths, in the first path a means of measuring mass flow, in the second path a means of measuring the ratio of specific heat capacities of the gas, three measurement phases in time being used, during the first phase the flow being stopped in the second path and the flow passes through the first path while the orifice is choked and the mass flow of the gas is measured, during the second phase the flow is stopped in the first path and the flow passes through the second path and the ratio of specific heat capacities of the gas is measured, during the third of the time periods the flow is both the two paths is stopped and the pressure is measured downstream of the cooler, these three phases may occur in any order, from said measurements calculating the temperature of the gas wherein T. sub. t= (g*E* (P. sub. t*P. sub. t)/ (M*M)) where T. sub. t is the stagnation temperature of the gas, P. sub. t is the stagnation pressure measured when the no gas flows through the probe, M is the mass flow measured when gas flows through the first path, E is a calibration factor that depends on the specific heat capacity at constant pressure of the gas and which may be determined by calculation or experiment and g is a function of the ratio of specific heat capacities of the gas.

Embodiments of the present invention are described in detail hereunder, by way of example only, with reference to the accompanying drawings, in which: Figures 1 and 2 show a system according to a first embodiment of the present invention; Figures 3 and 4 show an embodiment according to a second embodiment of the present invention; Figure 5 shows an embodiment according to a third embodiment of the present invention; and Figure 6 is a graph showing the performance of the system of Figure 2.

A first embodiment allows the user to calculate the temperature of a gas. It consists of three main parts, an orifice, downstream of the orifice a method of cooling the gas and downstream of that a method of measuring the mass flow of the gas. The flow is alternately stopped and started through the probe. When the flow is stopped the pressure downstream of the cooling system is the same as at the head of the probe and so a pressure measurement device mounted downstream of the cooler can be used to measure the probe's inlet stagnation pressure. When there is a flow through the probe the first orifice is choked and the mass flow is measured downstream of the cooler. This means that both the mass flow and pressure measurements can be made at low temperatures. The stagnation temperature at the orifice is then given by T. sub. t= (C*P. sub. t*P. sub. t/ (M*M)) where T. sub. t is the stagnation temperature of the gas, P. sub. t is the stagnation pressure measured when no gas flows through the probe, M is the mass flow measured when gas flows through the probe, C is a function of the ratio of specific heat capacities and the specific heat capacity at constant pressure of the gas and which may be determined by calculation or experiment.

With reference to schematic Figure 1, the gas enters the probe at 1 and passes through an orifice 2, is then cooled 3 and the mass flow is measured 4. The valve 5 is used for stopping and starting the flow.

Figure 2 shows an example of the implementation of the first device. The gas passes through a nozzle 6, and is then cooled by a water jacket 7. The mass flow is measured using an orifice plate 9, a means of measuring the upstream pressure 8a and a means of measuring the upstream temperature 8b and a means of measuring the downstream pressure 10. There is a valve 11 for starting and stopping the gas flow.

Figure 6 shows a plot of the temperature measured by the system of Figure 2 against the temperature measured by a thermocouple mounted next the probe head when the head of the probe is inserted into a large volume of hot gas. The temperature of the gas was set to 43 temperatures between 300K and 900K. Six tests were completed and are plotted on top of each other.

The second embodiment allows the user to calculate the ratio of specific heat capacities of a gas. This may allow the user to calculate the gas concentration in a mixture of gases. It consists of two main parts, an orifice with upstream temperature measurement and a downstream method of measuring the mass flow of the gas. The flow is alternately stopped and started through the probe. When the flow is stopped the pressure is the same as at the head of the probe and so a pressure measurement device mounted in the mass flow measurement device can be used to measure the probe's inlet stagnation pressure. When there is a flow through the probe the first orifice is choked and the inlet temperature of the first orifice and the mass flow through the second orifice is measured. From the said measurements the ratio of specific heat capacities of the gas can be determined g=D*T. sub. t* (M*M)/ (P. sub. t*P. sub. t) where T. sub. t is the measured stagnation temperature of the gas, P. sub. t is the measured stagnation pressure, M is the mass flow measured when gas flows through the orifice, g is a function of the ratio of specific heat capacities of the gas and D is a calibration factor that depends on the specific heat capacity at constant pressure of the gas and which may be determined by calculation or experiment.

With reference to schematic Figure 3, the gas enters the probe at 12 and its temperature is measured upstream of the inlet orifice 13. The mass flow of the gas is measured 14. The valve 15 is used for stopping and starting the flow.

Figure 4 shows an example of the implementation of the second device. The gas enters the device 16, then its temperature is measured 17 and it passes through an orifice 18. The mass flow is measured using an orifice plate 20, a means of measuring the upstream pressure 19a and a means of measuring the upstream temperature 19b and a means of measuring the downstream pressure 21. The valve 22 is for stopping and starting the flow.

The third embodiment allows the user to calculate the temperature and the ratio of specific heat capacities of the gas and may also allow the concentration of a mixture of gases to be determined. It is made up from a combination of the first and second embodiments. The device consists of four main parts. The first two consist of an orifice and a downstream method of cooling the gas. Downstream of the cooler the gas path is split into two parallel streams, the first, passing the gas through a device to measure mass flow and, the second, passing the gas through a device that allows the user to calculate the gas concentration and ratio of specific heat capacities of the gas mixture.

The device has three operating phases. In the first phase a valve is shut so that the flow only passes through a device for measuring the mass flow through the upstream choked orifice. In the second phase a valve is shut so that the flow only passes through a device that allows the user to determine the ratio of specific heat capacities of the cooled gas and gas concentration. During this phase the upstream orifice is unchoked. In the third phase both valves are shut and no gas flows through the probe. The stagnation pressure throughout the probe is then constant and the probe's inlet stagnation pressure can be measured.

If a mixture of two gases is being measured the stagnation temperature at the orifice may then be calculated using the measured stagnation pressure, mass flow rate, the gas concentration and knowledge of the ratio of specific heat capacities and the specific heat capacities at constant pressure of the two gases.

Figure 5 is a schematic view of the third embodiment. The gas enters the probe at 23 and passes through an orifice 24, is then cooled 25. The flow is then split and passes through a mass flow measurement device 26 and a valve 27 on one path and a device to measure the ratio of specific heat capacities and gas concentration 28 and a valve 28 on the second path.

Inlet devices 2,6, 13,18 and 24 may be an orifice, a nozzle or a venturi.

In each of these three embodiments, the valves 11,15, 22 and 27 may be connected to a device for developing a pressure ratio across the passage i. e. between the inlet and the valve, of a sufficient size to cause the gas to flow through the inlet orifice, nozzle or venturi at Mach Speed 1 when the valve is opened, thereby allowing the temperature of the gas at the inlet to be derived from the mass flow and the pressure at the gas inlet.