GB2347750A

GB2347750A - Method of measuring temperature differentials

Info

Publication number: GB2347750A
Application number: GB9911876A
Authority: GB
Inventors: Maurice A Yates
Original assignee: ADVANCED ENERGY MONITOR SYST
Current assignee: ADVANCED ENERGY MONITOR SYST
Priority date: 1999-03-08
Filing date: 1999-05-24
Publication date: 2000-09-13
Anticipated expiration: 2019-05-24
Also published as: GB9911876D0; GB9905167D0; GB2347750B; GB2347747A; GB2347747B

Abstract

A method of measuring the difference in temperature between two points includes the use of two temperature to current transducers (T1 and T2), one located at each of said points. The two transducers (T1 and T2) are connected in series and the junction between the two transducers (T1 and T2) is connected to a device (A1) which produces an output proportional to the difference in the currents in the two transducers (T1 and T2). This output is determined and then a correction factor is applied. The correction factor is obtained by calibration of the two transducers (T1 and T2).

Description

2347750 METHOD OF MEASURING TEMPERATURE DIFFERENTIALS

Field of the Invention

This invention relates to a method of measuring temperature differentials.

There are many occasions when it is desired to measure temperature differentials and, although the present invention has been developed in relation to the measuring of differences in temperature between different points in a fluid flow line, the invention is of more general application.

Figure 1 of the drawings of British Patent Specification No. 2 139 388 shows a circuit for use in measuring temperature differentials which includes two AD590 terminal integrated circuit temperature transducers which produce an output current proportional to absolute temperature.

All AD590 transducers are, however, not perfectly matched and it is accordingly an object of the present invention to provide an improved method of measuring differentials which includes the use of two temperature to current transducers and in which provision is made to compensate for any mismatching of the transducers.

2 It is a further object of the present invention to provide an improved method for determining the efficiency of a pump utilising the information obtained by measuring the temperature difference between the fluid entering the pump and the fluid leaving the pump.

Summary of the Invention

According to a first aspect of the present invention there is provided a method of measuring the difference in temperature between two points, said method including the use of two temperature to current transducers, one located at each of said points, said transducers being connected in series with the junction between the two transducers connected to a device which produces an output proportional to the difference in the currents in the two transducers, determining said output and applying thereto a correction factor obtained by calibration of the two transducers.

Calibration is preferably effected by placing the two transducers in a controlled constant temperature bath so that they are both at the same temperature and determining the output then obtained. The output is preferably recorded as a function of temperature over the range of temperatures which, in use, are likely to be encountered.

An equation is given on British Patent Specification No. 2 139 388 (to which reference should be made) for the determination of pump efficiency based on a determination of the temperature rise

I 3 across the pump, i.e. the difference in temperature between the fluid at the exit side of the pump and that at the inlet side of the pump.

This equation (which is hereinafter referred to as the pump efficiency equation as defined) is E = 1 1 + Cp.AT/gH), where E = Pump Efficiency, Cp the Specific Heat of the Fluid, AT the Temperature Differential, and H the Total Generated Head.

According to a second aspect of the present invention there is provided a method of determining pump efficiency by determining the temperature differential across the pump by the method defined above and using the relationship between the pump efficiency and the differential temperature as set out in the pump efficiency equation as defined to determine the pump efficiency.

Brief Description of the Drawings

Figure 1 shows a circuit for measuring the temperature difference between two points in a fluid flow line, one upstream of the point at which heat is added and the other downstream thereof, and Figure 2 shows a voltage measuring circuit.

Description of the Preferred Embodiment

4 As shown in Figure 1, two AD590 temperature to current transducers T1 and T2 are connected in series between voltages V+ and V- relative to a quality ground (represented by a dagger symbol). The transducers T1 and T2 are housed in two separate probes, each in good thermal contact with the fluid flowing through a flow line upstream and downstream of, for example, a pump.

The probes within which the transducers T1 and T2 are contained will also include pressure transducers if measurements of the pressure of the fluid are required in addition to temperature measurements.

Each AD590 transducer T1, T2 regulates a current that is linearly proportional to absolute temperate in degrees Kelvin with a constant of proportionality that is nominally 1 WOK.

The choice of a current-based transducer system is very beneficial as it overcomes cable losses experienced by voltagebased systems. This thus allows any length of transducer cable to be considered, provided that sufficient potential difference, i.e. voltage, is available to drive the currents required.

Another benefit arising from the use of a current-based system is that it is far less susceptible to electrical interference, which can be a problem with long leads around motors and generators or other electrically noisy environments.

I The voltages V+ and V- are stabilised using a circuit containing four resistances r and a connection to a reference voltage Vref but other methods of stabilisation may be employed. Stabilisation is necessary to remove secondary errors in the characteristics of the transducers Tl and TZ which exhibit a small variation of current with voltage.

The junction between the two transducers Tl and T2 is connected to op-amp Al, wired as a current to voltage converter. The op-amp Al may alternatively be integrated into a complex semiconductor circuit, such as an analogue to digital converter. The resulting output from the op-amp Al is an output voltage Vout that is proportional to the difference in currents in the transducers Tl and T2 and hence to AT, i.e. the difference in temperature in the fluid between the two points at which the transducers Tl and T2 are located. Vout can then be measured by any voltmeter including, for example, an analogue to digital converter which is linked to a computer so that further computations can be effected if required.

A resistance R is connected across the op-amp Al. The value of this resistance R determines the sensitivity of the system. For example, with the nominal 1WOK, a lMQ resistor will give a sensitivity of 1 mV of Vout per mK of AT. If the two probes are at the same temperature, there will be no current difference and Vout will be zero.

A major benefit of the particular circuit shown in Figure 1 is that the op-amp Al forces its -ve input to quality ground voltage.

6 The two cables joining the measurement unit containing the two probes can, therefore, be shielded at ground, as indicated at S1 and S2 making the provision of shielding easy and safe. In addition, there will be no voltage between the sensor connections to the opamp Al and the cable sheath and, since there is no voltage, there will be no leakage current. This safeguards the accuracy of the measurement of the difference current. There will also be no problem due to leakage from V+ and V- to the shield, because these voltages are stabilised.

All AD590 transducers are not, however, perfectly matched. Their sensitivity varies slightly from 1 WOA, from one component to the next. This is overcome by calibration. The method of calibration is as follows:

Place the two transducers T1 and T2 in a controlled "constant temperature" bath so that they are both at the same temperature and so that any output Vout must then correspond to some mismatch of the transducers T1 and T2.

This error is recorded as a function of temperature over the range of temperatures which, in use, are likely to be encountered and subsequently used to correct the readings which are obtained. Any suitable method may be used to measure the temperature of the "constant temperature" bath.

This procedure can also be used to monitor the long-term stability of a pair of transducers.

I 7 The temperature measurement is preferably effected using the circuitry shown in Figure 2. This includes a shunt resistor SR1, SR2 in series with each transducer Tl, TZ with a differential amplifier A2 or A3 to measure the voltage across each shunt resistor SR1, SR2. This voltage is proportional to temperature. The provision of the differential amplifier A3 is optional, but it helps to maintain symmetry and hence reduce errors.

The shunt resistors SR1 and SR2 need to be low value resistances so as not to compromise the shielding advantages referred to above. For 1 00fl the voltage is only about 30 mV.

As well as possible asymmetries in the transducers Tl and T2, there are other potential sources of errors in the electronics. Long-term stability and temperature dependence of active components are the main practical problems, while noise introduces a practical limit to the sensitivity of measurement.

The main source of amplifier error is due to the temperature dependence of offset in amplifier Al. This should, therefore, be a high-gain chopper amplifier whose offset is negligible. Amplifiers Al, A2 and A3 should all have negligible input currents.

There are two types of noise, i.e. interference and inherent thermal noise in the components. The quality of cable screening is important. Filters can be added to the transducer cable connections or in the cables themselves to reduce susceptibility to radiated 8 interference, and the cables need to be connected in such way as to avoid earth loop pick-up from the device being monitored. The main source of electronic noise is in the AD590 transducers themselves.

Signal averaging can reduce the effects of both types of noise but there is a limit to what can be achieved when the parameters are themselves changing. For steady parameters and a measurement time of about one second, the practical limit of sensitivity is of the order of 0.5 mK.

Having determined the temperature differential between the fluid downstream and upstream of the pump, i.e. the increase in temperature in the fluid as a result of passage thereof through the pump, this increase in temperature, AT, can be used to determine the pump efficiency using the equation set out in British Patent Specification No. 2 139 388, i.e. E = 1 / {1 + Cp.AT/gH), where E = Pump Efficiency, Cp = the Specific Heat of the Fluid, AT = the Temperature Differential, and H = the total Generated Head.

I 9

Claims

Claims:-

1. A method of measuring the difference in temperature between two points, said method including the use of two temperature to current transducers, one located at each of said points, said transducers being connected in series with the junction between the two transducers connected to a device which produces an output proportional to the difference in the currents in the two transducers, determining said output and applying thereto a correction factor obtained by calibration of the two transducers.

2. A method as claimed in Claim 1, in which calibration is effected by placing the two transducers in a controlled temperature bath so that they are both at the same temperature and determining the output then obtained.

3. A method as claimed in Claim 2, in which the output obtained when the two transducers are in the controlled constant temperature bath is recorded as a function of temperature over a range of temperatures.

4. A method of measuring the difference in temperature between two points carded out substantially as hereinbefore described with reference to the accompanying drawings.

5. A method of determining pump efficiency by determining the temperature differential across the pump by the method claimed in any one of the preceding claims and using the relationship between the pump efficiency and the differential temperature as set out in the pump efficiency equation as defined to determine the pump efficiency.

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