GB2283328A

GB2283328A - Fluid speed monitor

Info

Publication number: GB2283328A
Application number: GB9421460A
Authority: GB
Inventors: Stephen David Marshall
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-10-26
Filing date: 1994-10-25
Publication date: 1995-05-03
Anticipated expiration: 2014-10-25
Also published as: GB9421460D0; GB2283328B

Abstract

A fluid speed monitor e.g. for an air conditioning installation comprises two thermal devices 10, 12 which each provide an electrical output dependent upon the temperature of at least part of the device. The devices described are temperature-dependant resistors; the device 10 is at ambient temperature while the other one 12 is maintained at a temperature which is higher than the ambient temperature by a predetermined amount, e.g. 10 DEG C, by supplying current to a heater (e.g. a Zener diode Z1) thermally coupled to the device 12. Means are provided to give an indication (V4) of the heat being generated by the heater Z1 and thus of the fluid speed. Offest and linearity correction arrangements are briefly described, and constructional details of the devices 10, 12. <IMAGE>

Description

Fluid sPeed monitor The present invention relates to a fluid speed monitor, especially but not exclusively an air speed indicator.

It is necessary in many disciplines to have an accurate measure of the speed at which air is flowing past a given point. One such application is air conditioning where it is useful to ensure a constant throughput of air.

Previously proposed air speed sensors involve the use of an orifice plate or pressure sensors. These sensors are relatively expensive to produce.

It is an aim of the present invention to provide a fluid speed monitor which is simple to produce.

Accordingly the present invention is directed to a fluid speed monitor which comprises two thermal devices which each provide an electrical output which is dependent upon the temperature of at least part of the device, one of which devices is at ambient temperature and the other one of which devices is maintained at a temperature which is higher than the ambient temperature by a predetermined amount, as indicated by the electrical outputs of the thermal devices when the monitor is in use, by a variable heater, and means to provide an indication of the heat being generated by the heater.

Such a fluid speed monitor has the advantage that it involves no moving parts and is temperature compensated.

Advantageously the thermal devices each comprise a thermistor and a resistor in series with one another.

Preferably the electrical output from the heated device is amplified. That electrical output is therefore most suitably an electrical voltage. In a preferred embodiment voltage off-set means are provided to off-set a voltage output from the heated thermal device. The voltage off-set may be variable.

Advantageously the heat being generated by the heater is indicated by the current supplied to heat the heater.

Alternatively, the heat being generated by the heater is indicated by the voltage generated across a resistor through which the heater current flows.

Preferably the heater comprises a Zener diode.

Preferably the fluid of which the speed is being measured is air.

To reduce the likelihood of an oscillation in the output as the circitry hunts for the correct heat flow to one of the thermistors in order to maintain a given temperature difference between the two thermistors, at least that one of the thermal devices which is maintained at a temperature which is higher than the ambient temperature may be provided with temperature-oscillation damping means.

Preferably, the temperature-oscillation damping means comprises a metal capsule incorporating the thermal device and the variable heater.

In order to ensure that the heat-exchange characteristics between the the fluid the speed of which is being monitored and each thermal device is substantially the'same, the other thermal device, which is a t ambient temperature, may also be within a metal capsule having substantially the same dimensions and construction as the capsule in which the heated thermal device is contained.

These thermal characteristics may be made even more precisely the same if a device having substantialy the same dimensions and construction as the variable heater is contained within the capsule which also contains the unheated thermal device, in substantially the same manner.

The invention extends to a method of monitoring and/or measuring fluid speed.

Examples of fluid speed indicators made in accordance with the present invention are shown in the accompanying drawings in which: Figure 1 is a schematic circuit diagram of a first embodiment of the present invention; Figure 2 is a circuit diagram of a second embodiment of the present invention; Figure 3 shows further parts of the circuitry shown in Figure 2; Figure 4 shows an explanatory graph; and Figure 5 is a cross-sectional view through a part of a sensor shown in Figure 2.

The air speed indicator shown in Figure 1 comprises two thermal devices. One will be referred to as a cold sensor 10 which is at ambient temperature and the other will be referred to as a hot sensor 12 which is heated.

The two sensors 10 and 12 are connected in parallel across a potential difference provided by input lines 14 and 15.

The temperature sensors 10 and 12 are potential dividers formed by thermistors Thl and Th2 respectively in series with fixed resistors R1 and R2. The values of the resistors R1 and R2 are chosen so that respective voltages V1 and V2, which are each voltages taken off between the thermistor and the resistor, are nearly linear functions of temperatures T1 and T2, where T1 and T2 are the temperatures of the thermistors over the ranges T1=00C to 600C and T2=100C to 700C. The permitted range of the ambient temperature of the indicator as a whole is the range of T1. By using different values for the resistors R1 and R2, other permitted ranges are possible. It may be desirable to have variable resistors for R1 and R2, to enable the ranges to be selected.

The voltage V2 is amplified by an amplifier Al and offset by a voltage adder 16. The adder 16 has the other of its inputs connected to a variable resistor VR1 which is in parallel to the sensors 10 and 12 and provides a voltage V3 at the said other input to the adder 16. The output voltage V2' from the adder 16 is applied to one of the differentiated inputs of a differential amplifier A2, and is equal to the voltage V1, which is applied to the other of the differentiated inputs of the differential amplifier, when a temperature difference AT (given by T2-T1) is 10 C for any temperature T1 in the permitted range. If the effective voltage V2' falls below V1, being the value corresponding to this 100C temperature differential, the amplifier A2 increases the current I supplied to a Zener diode Z1 which is connected across the output of the amplifier A2 and the low voltage level line and which heats the thermistor Th2. In this way, the temperature difference AT is maintained substantially at 100C regardless of the temperature T1. In practice, the amplification and offsetting could alternatively be achieved by resistors on the inputs of a single amplifier.

When in operation air is passed over the two sensors 10 and 12. The heater current I is a function of the airspeed over the hot sensor 12 but not a function of the ambient temperature. It can be measured as a voltage V4 across a resistor R3 which is connected in series with and between the amplifier A2 and the Zener diode Z1, or alternatively it can be measured as the supply current to the whole circuit.

It will be appreciated that the cold sensor 10 is maintained at the ambient temperature by the airflow. The hot sensor 12 is warmed by the heater, being the Zener diode Z1, to a temperature which is higher than the ambient temperature by a fixed amount hT. The amplifier A2 effectively monitors the temperature difference and provides the necessary heater current I.

As the airspeed increases, there is a greater cooling effect of the air on the hot sensor 12 and the measured heater current I required also increases. As the heater current I is substantially independent of the value of the temperature T2, the sensor is temperature compensated over the permitted range. Any residual temperature dependence can be trimmed out by adjusting the gain of the amplifier Al.

The inherent accuracy of the sensor is largely independent of the chosen value of temperature difference AT. However, if the temperature difference AT is too small the sensor is vulnerable to local fluctuations in air temperature while if it is too large the power consumption is excessive. The value of the temperature difference AT can be chosen to suit the application.

Although the example works for a permitted range of OOC to 600C, the fixed temperature difference AT principle can be used for any temperature range restricted only by the differing non-linearity of the temperature sensors and their ability to withstand the operating temperatures. A 600C range is the largest that can be operated using two identical thermistors with a simple resistor network maintaining reasonable temperature compensation. By using other sensors, which could be two differing thermistors, or by processing the temperature readings, larger ranges are possible.

The heater can be any kind of resistive device. A Zener diode gives a better scale of output than a resistor.

Advantages of the air speed indicator illustrated in Figure 1 are that - there is approximately a 10 seconds response time suitable for many control purposes; - the device is temperature compensated; - there are no moving parts; - it is cheap to manufacture; - it is cheaper to install than an orifice plate / pressure sensor; - the fixed temperature difference AT principle can be applied to any gas at any temperature where solids can exist; and - it can be powered by 4 to 20mA loop for industrial purposes; - it provides a linear output that is substantially directly proportional to the air speed for the temperature range of T1.

The parts of Figure 2 which correspond to those of Figure 1 have been given the same reference characters.

The differences between the circuit shown in Figure 2 and the Figure 1 circuit are as follows (1) the amplifier Al between the adder 16, which is in the form of a resistance network, and the junction between the thermistor Th2 and its associated resistor R2 has been omitted so that junction is connected directly to that input; instead, a voltage resistor VRd is connected between the differential amplifier A2 and the junction between the thermistor Thi and its associated resistor R1. Thus, instead of amplifying the output from the heated thermistor Th2, an equivalent result is obtained by reducing the output from the unheated thermistor Thl, this is less expensive, the voltage reducer comprising, for example a resistor network acting as a potential divider; (2) a voltage regulator VRg has been included in the input line 14 to maintain a potential difference between the input lines 14 and 15 at substantially 14.7v, substantially regardless of the voltage applied to those lines within the range 16v to 30v; (3) voltage lineariser circuits are connected across the resistor R3 which is in series with the Zener diode Z1; in view of the connection of these linearising circuits in this manner, a visual indication of the speed of fluid which is being monitored is provided by means of an ammeter Am connected in series with and forwardly of the voltage regulator VRg on the input line 14; (4) a diode D1 may be connected between the ammeter Am and voltage regulator VRg to ensure that no reverse polarity of voltage is connected across the circuitry.

Further parts of the circuitry shown in Figure 2 are shown in Figure 3. These further parts comprise linearisers to compensate for what would otherwise be a non-linear variation of the current through the ammeter Am as a function of the fluid speed being monitored, that is to say the speed of the fluid passing over both probes.

Thus Figure 3a comprises a differential amplifier A3 connected across the resistor R3 of Figure 1, with an adder 40, which again is in the form of a resistance network, connected in series between the resistor R3 and the positive input of the amplifier A3. This adder 40 has its other input connected to an offsetting variable resistor VR2 connected across the input lines 14 and 15. The output from the amplifier A3 is connected to the input line 15 via a resistor R4 and a Zener diode Z2 connected in series.

The value selected for the different components are such as to operate the Zener diode in its non-linear range away from its Zener threshold.

The part of the circuitry shown in Figure 3b acts as a lineariser for high fluid speeds, and comprises a differential amplifier A4 having it differentiated inputs connected across the resistor R3 shown in Figure 1 and its output connected to the input line 15 via a resistor R5 and a Zener diode Z3 connected in series with one another.

Again, the values of the components of this circuitry are selected to operate the Zener diode Z3 in its non-linear range away from its Zener threshold.

In particular, values of the different components of the linearisers are so selected that, if a current characteristic for the basic circuit has a function of fluid speed as represented by the curve C1 in Figure 3, the low-speed lineariser draws a current as a function of fluid speed represented by the curve C2 and the higher speed lineariser draws a current as a function of fluid speed represented by the curve C3. The additions of these curves is represented by a substantially straight slanting line C4, and it is the current represented by this line which is measured by the ammeter Am to provide a substantially linear parameter as a function of fluid speed.

Figure 5 shows in greater detail the thermal link between the Zener diode Z1 and the thermistor Th2. In particular, the Zener diode Z1 represented physically by the device labelled 22 in Figure 4, is incorporated within a brass or bronze generally circularly-cross-sectioned cylindrical capsule 20 within which is also encapsulated the thermistor Th2 here represented by the device 30. The diode 22 and the thermistor 30 are in close proximity, and are physically connected via an adhesive loaded with silver particles to bond the thermistor 30 to the diode 22. Leads 24 and 26 from and to the diode are connected to the resistor R3 and the input line 15 respectively with reference to the circuit shown in Figure 2. Similarly, leads 32 and 34 to and from the thermistor 30 are connected to the input line 14 and the resistor R2 respectively with reference to the circuitry shown in Figure 2.

Reference numerals have been used to designate the various parts of what is illustrated in Figure 5, instead of the alphanumeric references of Figure 2, because what is shown in Figure 5 also represents an encapsulation of the other thermistor Thl. Thus, the device labelled 30 in Figure 5 is also representative of the thermistor Thl of Figure 2. The Zener diode 22 in this case is also present, bonded to the thermistor 30 in precisely the same way as the Zener diode Z1 is bonded to the thermistor Th2, but in this case the leads 24 and 26 are present but not connected to any of the circuitry shown in Figure 2. This ensures that the thermal characteristics of the probe comprising the thermistor Th2 are substantially identical to those of the probe comprising the thermistor Thl.

Further comments now follow in regard to the fluid speed monitor thus described with reference to the drawings: Metal capsules Metal (specifically brass) capsules as shown in Figure 5 work well for the following reasons: they give a good thermal link between the hot thermistor and the air cooling it which makes the device work well at high airspeeds; they provide thermal mass which reduces the extent to which the circuit oscillates; they provide protection from erosion by any abrasive particles in the fluid stream; they are easy to make accurately; the capsules may be cylindrical or square shaped.

Thermally conductive adhesive An adhesive loaded with silver particles is used to bond the thermistor to the diode within each probe. This provides a good thermal link between them which reduces the tendency of the circuit to oscillate. Without the conductive adhesive, either a larger capsule is needed or the circuit gain would need to be reduced. Either of these reduces performance by indirectly making the circuit more vulnerable to temperature effects.

Loot powered The current taken by the whole circuit can be made small enough for the whole circuit to be contained within a 4mA to 20mA current-loop powered device, in which the power current and indicator current are the same. This significantly reduces the installation cost in many applications.

Linearisins The current taken by the heater circuit is an approximately logarithmic function of the airspeed over the two probes over the whole temperature range. For use in a 4mA to 20mA circuit, it is desirable to make this into a linear function. This is achieved by using two additional circuits which monitor the heater current and take additional current such that total current taken by all the circuits is part of the straight line shown in Figure 4.

The lineariser circuits are shown schematically in Figure 3. The linearised device is accurate to approximately 6% when designed for a 600C temperature range. Smaller ranges with higher accuracy are possible, for example 2% over a 400C range.

Advantages of the illustrated monitor The illustrated monitor is advantageous to the extent that it is rugged and maintenance free; fairly accurate over a wide temperature range; possible to make versions which work at very low fluid speeds; effective over the airspeed and temperature ranges required by the HVAC industry; 4mA to 20mA loop powered and hence relatively cheap to install.

Claims

1. A fluid speed monitor comprising two thermal devices which each provide an electrical output which is dependent upon the temperature of at least part of the device, one of which devices is at ambient temperature and the other one of which is maintained at a temperature which is higher than the ambient temperature by a predetermined amount, as indicated by the electrical outputs of the thermal devices when the monitor is in use, by a variable heater, and means to provide an indication of the heat being generated by the heater.

2. A fluid speed monitor according to claim 1, in which the thermal devices each comprise a thermistor and a resistor in series with one another.

3. A fluid speed monitor according to claim 1 or claim 2, in which the electrical output from the heated device is amplified.

4. A fluid speed monitor according to any preceding claim, in which the electrical output from the heated device is an electrical voltage.

5. A fluid speed monitor according to claim 4, in which voltage off-set means are provided to off-set a voltage output from the heated thermal device.

6. A fluid speed monitor according to claim 5, in which the voltage off-set is variable.

7. A fluid speed monitor according to any preceding claim, in which the heat generated by the heater is indicated by the current supplied to heat the heater.

8. ' A fluid speed monitor according to any one of claims 1 to 6, in which the heat generated by the heater is indicated by the voltage generated across a resistor through which the heater current flows.

9. A fluid speed monitor according to any preceding claim, in which the heater comprises a Zener diode.

10. A fluid speed monitor according to any preceding claim, in which the fluid of which the speed is being measured is air.

11. A fluid speed monitor according to any preceding claim, in which at least that one of the thermal devices which is maintained at a temperature which is higher than the ambient temperature, is provided with temperatureoscillation damping means.

12. A fluid speed monitor according to claim 11, in which the temperature-oscillation damping means comprises a metal capsule incorporating the thermal device and the variable heater.

13. A fluid speed monitor according to claim 12, in which the other thermal device, which is at ambient temperature when the monitor is in use, is also within a metal capsule having substantially the same dimensions and construction as the capsule in which the heated thermal device is contained.

14. A fluid speed monitor according to claim 13, in which a device having substantially the same dimensions and construction as the variable heater is contained within the capsule which also contains the unheated thermal device, substantially in the same manner.

15. A fluid speed monitor substantially as described herein with reference to and as illustrated in Figure 1, or Figures 2 to 5 of the accompanying drawings.

16. A method of monitoring fluid speed using a fluid speed monitor as claimed in any preceding claim.