GB2153125A - Improvements in or relating to apparatus for monitoring gas flow - Google Patents

Abstract

Gas flow is monitored using a thermistor placed in the flow, and an alarm which indicates when the flow is below a desired value. The alarm may also operate when the flow is above a desired value. The alarm may be visual or audible. The device may monitor separately the flow to head and suit of a protective suit. In practice the thermistor is employed as one limb of a potentiometer connected across a voltage source, and the voltage across the thermistor is monitored, eg by inputting said voltage to terminals 3, 13 of a circuit as shown in Fig. 3. <IMAGE>

Description

SPECIFICATION Improvements in or relating to apparatus for monitoring gas flow This invention relates to apparatus for monitoring gas flow, particularly but not exclusively air flow.

According to this invention apparatus for monitpring gas flow comprises a thermistor for exposure to the gas flow so that the thermistor resistance varies with the rate of gas flow, and electrical circuitry connected to the thermistor and including alarm means operable to indicate when the flow falls below a desired rate.

The alarm means may also be operable to indicate when the flow rises above a desired rate.

The alarm means may comprise an audible alarm. The audible alarm may be arranged to produce one noise when the flow rate is below the desired rate and a different noise when the flow rate is above the desired rate.

The alarm means may include a visible warning device. The visible warning device may be arranged to indicate when the flow rate is at a desired rate or within a desired range.

This invention may be performed in various ways and two specific embodiments with possible modifications will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a thermistor circuit; Figure 2 shows a thermistor mounting; Figure 3 shows part of an alarm circuit; Figure 4 shows another part of the circuit; Figure 5 shows a further part of the circuit; Figure 6 shows a voltage source circuit for the alarm circuit.

Figure 7 shows another arrangement similar to Figure 2; and Figures 8, 9 and 9A show another alarm circuit.

Depending upon the rate of flow of air past a directly heated thermistor, the resistance of the thermistor will change, for a constant air temperature.

In the present case a thermistor 40 is removably sealingly mounted in a transverse bore in a block 41, for example of the material sold under the name perspex, the block having a throughbore 42 extending between a male hose connector 43 and a female hose connector 44. The tip 45 of the thermistor is in the bore 42.

To calibrate the arrangement, the thermistor 40 Fig 1 is placed in series with a variable resistor 46 and a fixed resistor 47. A known air flow is caused to pass through the bore 42 and the resistor 46 adjusted to give a particular output voltage at junction 48 between the thermistor and the variable resistor 46.

In use, the alarm circuit of Figs 3 to 5 is concected to the thermistor. The device is arranged for monitoring two air flows, for example to a hood and a suit, and switches S1, 52, S3, S4 (Fig 5) enable voltages appropriate to hood and/or suit flow to be applied to either or both of inputting amplifiers A2, A3, (via junctions 4,6), or C1, C3, (via junctions 10,14).

The negative voltage at junction 3 (13) with its resistor 46 is sampled by a voltage follower Al which draws almost no current from the source. This negative voltage is fed via resistor 50 or 51 to the inputs of operational amplifiers A2 or A3. These are summed with currents (voltages) taken from switches S1,S3 so that when the flow rate across the thermistor is too low, the comparator output voltage goes hard negative. Assuming that switches S2, S4 are open (off) so that there is no input at 10, 14, the output of amplifier C1 is positive and equal to the output of A2 and there is zero current flow at the summing node of amplifier B3 due to these outputs. A one-second oscillator B2 feeds this node and transistors 52, 53, which are normally cut off hard, will switch at one second intervals thus operating an audible alarm 54.This alarm is designed to draw high (large) instantaneous pulses of current under its own direction and is therefore fed by a ballast capacitor 55 to allow a smaller power supply to be used.

When the alarm 54 sounds, a red light 56 is energised. If the flow is at a desired rate, or within a desired range of rates, a green light 57 is energised, the outputs of A2 and A3 both being positive.

If the flow rate is too high, the output of amplifier A3 goes negative a yellow lamp 58 is energised and pin 1 of analogue switch integrated circuit D goes slightly negative before being caught by pin D3 to connect pin D6 to + 1 5 volts and to switch on transistors 52. 53 to give a continuous audible alarm 54.

If additionally or alternatively, there is input from switches S2, S4 to amplifiers C1, C3 then lights 59, 60, 61 are correspondingly energised as described.

Thus one light is always on as an indication of the rate of air flow. If there is a power supply failure, the contacts of relay E change over and sound a continuous alarm 54 for say 3 hours from a battery 63 which is on a continuous low charge. To prevent this power failure alarm from operating during storage or transport of the device, a microswitch 62 at the rear of a box housing the device is normally open and, to enable the alarm 54, must be closed by fixing the box to a wall.

A suitable voltage supply device is shown in Figure 6 in which a transformer 70 is supplied from mains and includes rectifiers 72, 73 and precision voltage regulators 74, 75.

One use of the device is in connection with air-hoods or air-suits worn for man-entry into cells or spaces containing radioactive material.

Compressed air is supplied to the hood or suit through a panel which has a pressure-control valve, a pressure gauge and two rotameters for indicating the rate of air flow and each provided with a flow control valve. The present device can give an audible and visible warning of inadequate or excessive supply of air.

The block 41 is secured in the panel and the air flow to, for example, a hood passes through zone 42, the thermistor being connected to input Al. A second block 41 can also be used, the associated thermistor being connected to input C2. Thus a two-channel arrangement can be used. The desired band of flow rates may differ to suit and hood, and the respective thermistor is calibrated accordingly (Fig 1).

The alarms are self-cancelling.

A typical operating sequence might be: 1. With no air hose connected, switch on the required detector. The red light should be on and an intermittent alarm given.

2. Connect the suit or hood hose and set the suit/hood switch.

3. Open the air valve and observe that the red light goes out, the alarm ceases and the green light is on.

4. Continue opening the valve until the green light goes off and the yellow goes on and a continuous alarm is heard.

5. Adjust the flow back to the green light condition and finally adjust by observing the rotameter float.

6. Cover the switches.

In a modified arrangement shown in Figures 7-9, all mechanical switches are omitted to give improved in-service robustness, and the arrangement has a minimum air flow alarm using an in-built rotameter thereby compensating for various water/heat content of the air and consequently allowing a wider general application of the device. The thermistor 40 may be cast in resin 80 (Figure 7) and the assembly include thin rubber washers 81 If the desired minimum flow is set on a rotameter for either suit or hood, a variable resistor (VR 1 or VR 2 in Figure 9) may be adjusted to the alarm point for low flow. If the flow is increased, the alarm 54 will cease and then change as before with the upper level of either suit or hood providing another alarm.

Suppose the instrument is switched on without thermistor probes connected then flipflops F1, F2 and F3 F4 are set so that analogue electronic switches B feed a signal voltage between the upper and lower level alarm points, derived from resistors 72-75 via D2, D3. No alarm will be produced and the green lights 57, 60 will both be on. When thermistor probe Thl is connected, F1, F2 is flipped so that the switch between pins 11 and 10 of integrated circuit B makes and the output of amplifier D4 is fed to the common inputs amplifiers of A2, A3. An alarm will probably sound and the flow is adjusted to the minimum for suit or hood as required. By adjusting VR1 to the green light off/red just on position the system is set up for this detector. The potentiometer is then locked and the flow increased to check the upper level and then adjusted back to the working level with the green light on.In an exactly similar fashion F3, F4 can be flipped and another suit or hood accommodated.

Suppose a detector (thermistor) is detached, then an alarm will sound until either the detector is replaced or the mains supply is switched off for 10 seconds and then on again.

If a detector is connected and the power supply fails, a continuous alarm will sound as in the alarm system described above.

A socket 70 and a plug 71 is connected to a thermistor as shown. When the thermistor is connected by inserting the plug into the socket the DIN connections are made as shown. A similar arrangement may apply in respect of another thermistor.

The alarm levels are set by adjusting a potentiometer; there are several advantages to the system: 1. The design contains no mechanical switches and will have solid-state reliability.

2. An operator can set the potentiometer and be required to log the dial setting for low level alarm to provide a permanent record that the correct flows are always set up for the user before cell-entry.

3. There can be no confusion about hood or suit alarm levels as the setting up is done with the hood or suit in position.

4. Long term drift of thermistors will be checked by the operator-logged dial settings.

5. A wider range of air inlet temperatures and humidity will be possible making the instrument more widely applicable.

Claims (9)

1. Apparatus fur monitoring gas flow comprising a thermistor for exposure to the gas flow so that the thermistor resistance varies with the rate of gas flow, and electrical circuitry connected to the thermistor and including alarm means operable to indicate when the flow falls helow a particular rate;

2. Apparatus as claimed in claim 1, in which the alarm means is operable to indicate when the flow rate rises above a predetermined rate;

3. Apparatus as claimed in claim 2, in which the predetermined rate is the particular rate;

4. Apparatus as claimed in any preceding claim, in which the alarm means comprises an audible alarm.

5. Apparatus as claimed in claim 4, in which the alarm means produces one noise when the flow rate is below the particular rate and a different noise when the flow rate is above the predetermined rate.

6. Apparatus as claimed in any preceding claim, in which the alarm means is adapted to indicate when the flow rate is at a desired rate or within a desired range.

7. Apparatus as claimed in claim 6, in which the alarm means comprises a visual warning device to indicate when the flow rate is at a desired rate or within a desired range.

8. Apparatus as claimed in any of claims 1 to 5, in which the alarm means comprises a visual warning device.

9. Apparatus for monitoring gas flow substantially as hereinbefore described with reference to and as shown in Figs. 1 to 6, or Figs.

7 to 9 and 9A, of the accompanying drawings.