WO2000005486A1

WO2000005486A1 - Automatic depth sensing detection for portable soil moisture probes

Info

Publication number: WO2000005486A1
Application number: PCT/AU1999/000566
Authority: WO
Inventors: Ric Gatto; Gabriel Levy
Original assignee: Sentek Pty. Ltd.
Priority date: 1998-07-21
Filing date: 1999-07-12
Publication date: 2000-02-03
Also published as: GB2358254B; AUPP478998A0; GB0101267D0; ZA200100368B; GB2358254A

Abstract

A depth sensing arrangement (10) useable for detecting the depth of insertion of a probe (16) into a probe receptacle (17). The probe having at least one data sensor (20) arranged along its length. The probe has a plurality of detectors (30) located along its length at predetermined spacings. There is at least one trigger element (23) located at a reference position which triggers a reaction of the detector.

Description

AUTOMATIC DEPTH SENSING DETECTION FOR PORTABLE SOIL MOISTURE PROBES

This invention relates to depth sensing for data logging devices and in particular to a method and apparatus for detecting the depth of a sensor while the sensor being logged traverses a predetermined path.

BACKGROUND

This invention is useable in a variety of environments and not restricted to the application described in detail herein, which relates primarily to portable soil moisture probes and their associated data logging.

Currently available portable soil moisture probes require the user to manually position the sensor/ probe at a separately carefully measured desired depth and then press a key/ button to enable the logger/ measurement unit to measure and store the output of the sensor detecting a characteristic of the soil for that desired depth. If the user positions the sensor at an incorrect depth, the logger/ measurement unit does not know it is at an incorrect depth and will therefore unknowingly associate the soil moisture measurement for the incorrect depth against the expected depth.

This erroneous depth detection (if not able to be identified or prevented) will cause data to be invalid. Non-valid data can cause disastrous results (if used for irrigation scheduling) including total loss of crop, environmental damage, waste of resources (fertiliser, water, etc), disease, insect infestation, etc. If erroneous data is used in the mining environment (eg. acid leach heaps) the effects can be extremely dangerous (acid run-off, etc) for people, machinery and the environment.

The ideal approach to data logging and depth sensing is to have the probe measure its own depth as it traverses the access tube and furthermore it would also be ideal that the sensor detects and records without stopping at each desired depth. Ideally, the logger/ measurement unit would detect when the sensor passes a predetermined measurement position and this initiates the output of the sensor measurement (eg ' soil moisture) and association of that measurement against the depth, which could be displayed or stored for later analysis. This method of operation would eliminate or minimise incorrect positioning of the sensor/ probe and hence also prevent the possibility of erroneous data.

These features would also be likely to provide a saving in time and cost compared to the time and cost typically associated with the use of known portable soil moisture probes. Currently available probes typically require up to 16 seconds for each measurement to be taken and this happens at each depth, involving the user manually positioning the sensor at the desired depth, keying site/ depth details into the logger and initiating readings to be taken by the sensor which of course excludes the time taken to move between sites at which measurements are to be taken.

For example a site with two 1.6 metre access tubes would typically require up to 8.5 minutes (16 seconds by 16 depths by two access tubes) to take the readings and if for example 20 sites were to be monitored, the time taken to perform the measurement task (ignoring the time to move from site to site) could be up to 2.8 hours.

The automatic depth sensing apparatus and method the subject of this patent application is considered possible to arrange for the sensor to be lowered and extracted from the access tube in less than 5 seconds with no re-positioning of sensor and/ or setting of the logger for each depth level as the sensor and data logger/ display work together to collect the measurement/ depth associations, thus it should only take 10 seconds per site (5 seconds per tube by two tubes) therefore the approximate probable time to perform the task at 20 sites should be less than 3.3 minutes which compares very favourably with the up to 2.8 hours of prior arrangements and assumptions.

As stated, these timing estimations do not take into consideration movement from site to site, removing/ replacing access tube top caps, initial setting up of the logger, etc which will vary depending on the product used and the locations of the sites but will be similar for each method used.

It is anticipated that the savings in time performing measurements will: a) dramatically reduce labour costs and enable a consultant (a person likely to undertake such work) to take measurements and provide irrigation scheduling advice to substantially more customers (assuming similar overheads) making their business more profitable; and b) enable a grower to take many more readings in the same timeframe so as to provide more data, making irrigation scheduling more timely and precise while reduced risk results from having more information and the reduced likelihood of obtaining incorrect data.

An example of a prior depth sensing arrangement is disclosed in US Patent No 4146796 to Campbell which describes a method of sensing the depth of a sensor using a chain of resistance components arranged along the internal length of an access tube connected to detection circuitry associated with the logger unit. The resistance of the chain of resistors varies (depending on depth) by means of connected switching components (reed switches, etc). The logger unit measures the resistance and thereby deduces the depth the sensor is positioned at.

There are several problems associated with the use of this type of depth measurement which relies on contact switched resistances to determine depth a) resistance changes due to temperature of components, tolerance of components, detector resistance, contact resistance (plug/ socket), cable resistance (length of cable, temp, etc) all of which may cause erroneous or invalid depth detection; b) changing probe lengths (number of depths sensed) may require re-calibration of the measurement/ logger unit. This is a major problem as it is then impractical to produce individually set up probes for all the variety of depths (eg 0.5m, lm, 1.6m, etc) and which are suitable for connection to more than one typically dedicated measurement/ logging unit. It is envisaged that customers will have numerous monitoring sites with varying depths (due to soil type, crop type, etc) and possibly different loggers so they may be required to purchase probes of varying lengths to be used with one common measurement/ logger unit which is considered unrealistic and ultimately costly and inconvenient for the user; c) limited probe length due to the number of steps to be sensed. The number of depths sensed is limited by A/D converter resolution (ie number of bits). Mining applications require depths from 10cm to over 100 meters to be monitored thus the accuracy of the depth may suffer or smaller increments at specific depths becomes impractical; d) contact bounce of switches may cause erroneous depth detection; e) failure of any one resistor along the chain of switched resistors would likely render the depth detection circuitry inoperable; f) failure of any solder joint (dry joint, etc) on switched resistors or detector switches would render the depth detection circuitry inoperable; g) variations in resistance of any one resistor along the chain of switched resistors would cause erroneous depth detection or render the depth detection circuitry inoperable; h) variations in supply voltage to a chain of switched resistors would vary the voltage measured by the detector circuitry hence cause invalid depth detection; i) any noise or voltage spikes/ surges on the supply voltage to the resistor chain could result in erroneous depth detection; and j) the sensor/ probe would need to be stopped at each depth to be measured in order for the measurement/ logger unit to measure resistance (to determine depth) then measure and record the output of the sensor. This is not an efficient method when ideally it is best to traverse the sensor/ probe access tube in one movement without stopping. To avoid the above problems it is proposed that a digital like depth sensing mechanism be utilised for depth detection as opposed to the analog method (using switched resistances) described in above patent.

There are many benefits in utilising digital depth sensing which include: a) length of probe (hence depth of detection) is unlimited; b) detecting unit does not require modification or re-calibration if various length probes are used. Probes can be interchanged without user intervention or requiring the logger unit to be re-calibrated for each probe length; c) probes can be manufactured at any length without consideration for the maximum resistance of components or need for calibration for different resistances depending on the number of sensing elements; d) digital depth detection involves digital circuitry which is inherently more reliable than analog circuitry (as in an analog resistance measurement); e) failure of a sensing element is more easily identified and can be automatically accounted for; f) speed of detection is not limited to the time delay requirements of analog circuitry (eg A/D conversion delays); g) with resistance type depth detection the resistance of detectors may vary from detector to detector, vary with varying temperature and vary over time which could cause false measurements and would require regular re-calibration; h) contact bounce is not an issue, since electromechanical devices are not used or if some are used, digital circuitry allows the measurement/ logger unit to detect that the sensor/ probe is at a certain position irrespective of contact bounce by using edge-triggered measurement. Analog detection would require measurement delay while the depth indicating switch finishes bouncing in order for analog circuitry to measure the resistance (hence depth); i) digital circuitry is relatively immune to variations of supply voltages; j) digital circuitry is relatively immune to noise and voltage spikes/ surges on supply voltage; k) failure of detecting circuitry (due to faulty detectors, unreliable plug/ socket contacts, dry solder joints, etc) are easily identified using digital circuitry by ' detecting invalid code sequences while also ensuring that the invalid measurement is not used and subsequent measurements are correctly associated with an appropriate depth. Failures in detecting circuitry (using resistance based measurement/ detection as described in Patent No. US 4146796) may be unknowingly processed by the logging unit as a valid depth resulting in invalid depth/ data measurement being recorded; and 1) using a digital detection method the sensor/ probe does not need to be stopped at each depth to be measured. The sensor/ probe can be inserted into the access tube in one movement. The digital circuitry immediately detects that the sensor/ probe is positioned at a valid depth (using edge triggered digital circuitry) and measures/ stores the sensor output.

BRIEF DESCRIPTION OF THE INVENTION

In its broadest form a depth measuring arrangement for a data logging device comprises a probe rod along which is located a plurality of detector elements at predetermined spacings, a trigger element located at a datum point past which said probe rod is moved such that as said detector elements pass said trigger element the passing is detected without reliance on mechanical contact between said trigger element and a said detector element, characterised in that there are at least two signal paths passing by all said detectors, to which at least one path said detector is connected, but not necessarily adjacent said detector and wherein the existence of a signal on one or more of said signal lines is indicative of a predetermined depth when a preceding or succeeding signal combination is known.

In a further aspect of the invention the signal paths carry digital coding of the passing of a said trigger element past a detector element.

In yet a further aspect of the invention said first detector element along the length of said probe to encounter said trigger element is connected to separate circuitry providing a specific code on said signal paths useful for identifying the beginning of a series of signals which are representative of probe depth.

Embodiments of the invention will now be described in some further detail with reference to and as illustrated in the accompanying figures. These embodiments are illustrative, and not restrictive of the scope of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1 depicts a probe and data logging arrangement; and

Figure 2 depicts a circuit diagram of the detectors within the probe rod.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

A depth sensing probe apparatus, as depicted in its preferable configuration in Figure 1, comprises a single moisture sensor probe assembly 10 which is connected to a data logger/ display unit 12 by a spiral cable 14.

The probe rod 16 is typically and in practical terms, likely to be of fixed length, but could be of any required length.

Typically, an access tube 17 is pre-located in the ground however the invention is not limited to use in terra-firma nor required to have a vertical orientation. The access tube is typically plastic and provided with a top cap assembly (top cap body 27 and top cap lid 28) to prevent the ingress of water, soil and insects. The top cap body 27 is permanently fixed to the access tube 17. The top cap lid 28 is fitted to the top cap body 27 when not being used and removed to allow insertion of the sensor 20 into the access tube 17. The access tube is also sealed at the bottom with a bottom stopper 29 to prevent water entering the bottom of the access tube although it is understood that eliminating water ingress is a desirable feature and not an essential one.

With the top cap lid 28 removed, the probe top cap 18 of the probe assembly can be attached to the top cap body 27 of the access tube 17 once the single sensor 20 is inserted into the tube. The datum/ locator plate 22 is thereby located in a recess in ^• the top cap body 27 which is restricted from entering the access tube 17 by its diameter, which is larger than the inner diameter of the top cap body 27, and also restrained from travelling upwards by a resilient means, preferably a spring 24 located between the datum/locater plate 22 and the probe top cap 18.

The spring allows restricted movement of the plate 22 while the rod 16 is being retracted from the in situ tube 17 so as to prevent the plate 22 from moving from the datum recess in the top cap body 27.

The top cap body 27 has tabs (not shown) arranged to fit within a slot 26 in the plate 22 to prevent the plate 22 from rotating when positioned in the top cap body 27. The square probe rod 16 moves through a square hole in the centre of plate 22 so that when the plate 22 is located in the top cap body 27 the probe can not rotate while being inserted so as to avoid errors due to translocation/ rotational effects. The plate has, in this embodiment, only one sensor triggering means, which in this embodiment is a magnet 23, which is in a fixed position relative to the ground or other body into which the in situ tube has been placed. Other, preferably non- contact triggering means could be used such as electromagnetic radiating bodies etc.

The location of the plate 22 serves as the reference point for depth detection.

Clearly the reference point may be higher or lower than the actual soil commencement level, but as long as the sensor 20 is at the predetermined depth when the corresponding first depth detector 30 passes the plate 22 and subsequent detectors 30 are appropriately spaced, the depth measurement process will be accurate within practical limitations and sensor characteristics.

Thus the apparatus is arranged to provide depth indicating (detector triggered) signals concomitant with the location of the sensor 20 within the in situ access tube 17 at the desired depth. The automatic nature of the depth detection procedure ensures that data logging provided by the data logger/ display unit 12 is as complete as it can be and the information provided by the data logger can thus be relied upon. Such as, for example, providing historical data based on reliable depth detection presented in a manner which better assists irrigation scheduling decisions.

However, it will be appreciated that the sensor does not need to be a soil moisture sensor but could be another type of sensor with which data at a corresponding depth can be usefully interpreted.

The following preferable components are depicted in Figure 1:

Data logger/ Display unit 12

. having a keypad for the selection of modes and entry of settings

. acts as a data logger for future down-loading of that data to a computer

. having a display (LCD) to view data and unit settings in the field

Spiral cable 14

. for minimising the tangling of cables

Square probe rod 16

• is square to prevent translocation effects (but other non rotatable shapes would suffice) . made of light non-magnetic (ie magnetically neutral) material such as aluminium if magnetic non-contact detection is used . has depth detectors located at predetermined intervals inside the rod . has depth units (mm and inches) displayed externally along its length visible by the operator and arranged to be placed so that the sensor depth with respect to ground level is indicated to the operator Probe top cap 18

. holds spring down on datum/ locator plate . supports probe rod at top Spring 24 . holds down datum/ locator plate

Datum/ locator plate 22

. acts as a datum for depth reference

. locates sensor at same axial orientation

. prevents probe rod from rotating (prevents translocation)

. has a magnet to actuate depth and zero detectors

Depth detector magnet 23

. actuates depth/ zero detectors inside probe rod

Sensor 20

. senses soil water content (preferable for this embodiment)

. is encapsulated or sealed for environmental/ mechanical protection

. sensing rings made from stainless steel so as to reduce adverse environmental effects if exposed Water-in-tube detectors 21

. detects presence of water at bottom of access tube . acts by detecting resistance between detectors (in particular very low resistance will be encountered when the water in the tube is touching both the detectors).

Referring to Figure 2 only 1 detector (eg Hall Effect device, reed switch, optoelectronic device, etc) is placed at each predetermined depth along the probe rod to provide depth detection. The use of only one detector per depth avoids invalid transient codes which may be generated if multiple detectors are used (per depth) due to: detectors not being simultaneously triggered due to variations/ movement of magnets in relation to detectors and/ or varying trigger levels and trigger delays/ timing of detectors.

References to logic states as "low" means "logic state 0" (which is approximately 0 volts for TTL logic devices) whereas reference to "high" means "logic state 1" (which is approximately 5 volts for TTL logic devices). Each of the A,B,C lines have a pull-up resistor (not shown) typically to 5V (for TTL devices) so that when no detectors are at predetermined depths (where magnets are placed) then all A,B,C lines are high (A,B,C = 1,1,1). Pull up resistors are not used in the circuit the same as resistors of prior art, they merely set an otherwise random state (floating voltage) to a known state (in this case logic 1 ie 5 volts) when either the depth detector magnet is positioned between detectors, when power is not applied to the probe (under the control of the data logger/ display unit) or when the probe/ sensor is disconnected from the data logger/ display unit.

The detectors have an open collector output so that when they are positioned at the "zero" point or at desired measuring depths (10cm, 20cm, 30cm...) their output goes low and (via diodes) takes either one, two or all the A,B,C lines low. It will be appreciated by those skilled in the art that other detector devices (eg. non-Hall Effect, without open collector output, etc) can be utilised with appropriate circuitry to provide the necessary code sequence.

When the sensor is positioned between valid depths no detectors are triggered so the voltage representative of a code on the A,B,C lines remains 1, 1, 1.

The "zero" level detector is connected to all A,B,C lines (via diodes) so that when the sensor is positioned at the "zero" point then all A,B,C lines are pulled low (A,B,C = 0,0,0). The code of A,B,C = 0,0,0 is unique to when the sensor is positioned at the "zero" point.

The A, B and C lines are connected to circuitry (preferably the parallel port inputs of a microprocessor) which decode the lines and determine the depth of the sensor by knowing where the sensor is in relation to the "zero" point by keeping track of the changing/ cycling code on the A, B and C lines. As will be explained the code change follows a particular sequence when the probe travels in one direction and in another particular sequence when the probe travels in the opposite direction, which is merely a preferred arrangement as other digital arrangements and code sequences are possible using different arrangements of lines and detectors.

In a preferred arrangement the probe is a 160cm long square aluminium tube with OD of 20mm X 20mm and an ID of 16mm X 16mm.

The probe has detectors (Hall Effect, reed switch, opto, etc) fixed internally along its length at 10cm intervals (for 10cm probe) or 4" intervals (for 4" probe). The spacing is selected to provide typical data collection requirements for irrigation scheduling needs however spacings may be altered to suit other requirements. A magnet (the preferred sensor trigger device) is fixed to the datum/ locator plate 22 which is positioned inside the top cap body 27 which itself is attached to the top of the tube. The magnet 23 in the stationary datum/ locator plate triggers each detector as the probe is lowered into the access tube 17.

A preferred process for taking and storing readings at regular intervals along the length (depth) of a soil profile is as follows:

1. remove top cap lid 28 from top cap body 27 which is fixed to top of access tube 17;

2. place probe in access tube 17, position datum plate 22 and align tabs in recess inside top cap body 27 and fix probe top cap 18 to top cap body 27;

3. user turns on display unit and waits for the initial screen;

4. user selects mode of operation (by pressing "SCAN");

5. the user then is prompted to select profile ID which is a unique identifier of the access tube;

6. once the profile ID is selected the display unit instructs the user to "position the probe at zero depth point";

7. the display unit detects that the probe is at zero point then instructs the user to "insert probe";

8. the user inserts the probe down the tube in one steady movement;

9. the unit automatically senses the depth the sensor is at, measures and records the data;

10. the unit detects the probe is being removed then automatically switches to default screen (such as summed graph); and

11. the user views the screen where they can see historical data (eg the last 10 data sets) for that profile ID, so that a trend can be determined and thereby be able to identify where between the full point and re-fill point of soil moisture exists and whether there is a need for irrigation, its timing and duration.

After turning on the display unit the user only needs to press predetermined buttons at step 4 (selecting mode) and step 5 (select profile) and then insert and extract the probe to obtain information as to the soil moisture take-up trends and need for irrigation. The user can also down-load collected data into a stand alone computer at any future time for further analysis and printing if required.

Further to point 10 above it is envisaged that the unit could also detect depths and measure moisture content during removal of the probe rod to verify measurements made during insertion so as to provide verification or increased accuracy with the additional data collected. It would also be possible to further increase accuracy/ reliability by recording measurements when detecting the rising and falling edge of the signal from the detectors during both the insertion and removal of the probe rod which would thus result in 4 measurements at each sensor location.

By operating the "Scan" mode button on the display unit the scan mode for reading the next profile is set.

The probe as described previously has a "zero" detector to determine when it is positioned at "zero" depth prior to the commencement of data logging which is used as a starting reference for the automatic digital depth detection process. The detectors output codes will be as follows as the probe rod is lowered past the datum/ locator plate: DETECTORS Output DEPTH

A B c cm inches

0 0 0 0 0

1 1 1 in between depths

1 1 0 10 4"

1 1 1 in between depths

1 0 1 20 8"

1 1 1 in between depths

1 0 0 30 l'O"

1 1 1 in between depths

0 1 1 40 1'4"

1 1 1 in between depths

0 1 0 50 1'8"

1 1 1 in between depths

0 0 1 60 2'0"

1 1 1 in between depths

1 1 0 70 2'4"

1 1 1 in between depths

1 0 1 80 2'8"

1 1 1 in between depths

1 0 0 90 3'0"

1 1 1 in between depths

0 1 1 100 3'4"

1 1 1 in between depths

0 1 0 110 3'8"

1 1 1 in between depths

0 0 1 120 4'0"

1 1 1 in between depths

1 1 0 130 4'4"

1 1 1 in between depths

1 0 1 140 4'8"

1 1 1 in between depths

1 0 0 150 5'0"

1 1 1 in between depths

0 1 1 160 5'4"

1 1 1 in between depths Where:

A = line A (code bit A)

B = line B (code bit B)

C = line C (code bit C)

Depth = depth positioned at (10cm intervals for 10cm probe)(4" intervals for 4" probe)

If the detected code is A,B,C = 0,0,0 the sensor is positioned at the "zero" point.

If the detected code is A,B,C = 1,1,1 the sensor is positioned between sampling depths.

Moving the sensor away from the "zero" reference point and before it is positioned at the first depth level (eg 10cm) the A,B,C code goes from 0,0,0 to 1,1,1 which enables the logger unit to identify that all A,B,C lines are operational.

Note that the length of probe (as indicated in above table) could be much greater and is limited only by the ability of the operator to handle a very long rod, since the repetition (ie circular nature) of the A,B,C sequence can be used ad infinitum. Note that the A,B,C sequence from the "zero" reference point is used for digital positioning and is not a digital representation of the position. Digital representations would limit the length of the probe to the number of bits used for digital representation. For example 4 bits (eg A, B, C, D) would provide a hex representation of depth (maximum of 16 codes) which would limit the probe length to 16 depths (ie 1.6 meters). If longer probes are required then additional bits would be required (eg 5 bits for 32 depths = 3.2 meters, 6 bits for 64 depths = 6.4 meters, etc).

It would be obvious to those skilled in the art that the described code sequence is not the only one that can be utilised for the purpose of depth detection and to enable early detection of the direction in which the probe is being moved. Once the unit detects it has left the zero point (by the detection of a 1,1,1 code after^' 0,0,0 code), monitoring of the status of A, B and C provides the ability to determine when a detector is encountered. As long as the unit starts from a zero point, it can determine at what depth it is positioned by monitoring the changes in A,B,C code sequences. The unit also has the ability to detect that it is being removed since the sequence of the A, B and C code is the reverse of the code sequence when the probe rod is being inserted.

In a preferred arrangement the code sequence detected enables the measurement unit to determine when the probe rod is positioned at zero depth or for that matter any initialisation position since the probe itself may not be actually at zero depth relative to the ground or any other body into which the probe is inserted. For example: a) after leaving "zero" point the A,B,C bits go to 1,1,1 which indicates that the sensor is between depths. Note if A,B,C is 1,1,1 then sensor is between valid depth points along the length of probe; b) when the measurement unit detects 1,1,0 on the A,B,C lines it knows it is at 10cm depth since that is the agreed spacing between detectors and that is the first sensor detection code; c) when it leaves the 10cm depth point the A,B,C lines go to 1,1,1 indicating it is between valid depth points; d) when the measurement unit detects 1,0,1 on the A,B,C lines it knows it is at 20cm depth since that is the agreed depth of the second detector and is the first sensor detection code detected beyond the first (disregarding the code 1,1,1 which occurs while the sensors are between depths); e) when it leaves the 20cm depth point the A,B,C lines go to 1,1,1 indicating it is between valid depth points; f) when the measurement unit detects 1,0,0 on the A,B,C lines it knows it is at 30cm depth; g) when it leaves the 30cm depth point the A,B,C lines go to 1,1,1 indicating it is between valid depth points; h) when the measurement unit detects 0,1,1 on the A,B,C lines it knows it is at ^'

40cm depth; i) when it leaves the 40cm depth point the A,B,C lines go to 1,1,1 indicating it is between valid depth points; j) when the measurement unit detects 0,1,0 on the A,B,C lines it knows it is at

50cm depth; k) when it leaves the 50cm depth point the A,B,C lines go to 1,1,1 indicating it is between valid depth points; 1) when the measurement unit detects 0,0,1 on the A,B,C lines it knows it is at

60cm depth; m) when it leaves the 60cm depth point the A,B,C lines go to 1,1,1 indicating it is between valid depth points; n) when the measurement unit detects 1,1,0 on the A,B,C lines it knows it is at

70cm depth. This is the start of the repetition of the A,B,C code; o) when it leaves the 70cm depth point the A,B,C lines go to 1,1,1 indicating it is between valid depth points; and p) etc, etc;

The above process continues until all depths determined by the location of a sensor in the probe rod have been read, whereby the logger/ measurement unit, which could be programmed to expect a maximum number of A,B,C codes (or repetition thereof), can be arranged to alert the user of the completion of the insertion of the probe rod into the access tube and which is independently verifiable by noting the depth measurement written on the side of the probe rod 16.

However, if for some reason the probe is removed before it has finished scanning all depths the measurement unit monitoring the sequence of A,B,C codes can detect if the probe is being removed.

For example: a) after leaving "zero" point (A,B,C = 0,0,0) the A,B,C bits go to 1,1,1 which indicates that the sensor is between depths. Note if A,B,C is 1,1,1 then sensor is between valid depth points along the length of probe; b) when the measurement unit detects 1,1,0 on the A,B,C lines it knows it is at 10cm depth; c) when it leaves the 10cm depth point the A,B,C lines go to 1,1,1 indicating it is between valid depth points.; d) when the measurement unit detects 1,0,1 on the A,B,C lines it knows it is at 20cm depth; e) when it leaves the 20cm depth point the A,B,C lines go to 1,1,1 indicating it is between valid depth points; and f) if the measurement unit detects 1,1,0 (instead of expected 1,0,0) on the A,B,C lines it knows it has returned to the 10cm depth.

If all depths have not been detected the unit can instruct the user to repeat the lowering process from the start.

If an incorrect A,B,C sequence occurs the measurement unit monitoring the sequence of A,B,C will recognise there has been a failure of one or more of the depth detectors and if programmed appropriately can determine which detector is faulty and advise the user. So as to allow measurement to continue, the process may be continued but the data for the missing depth sensor will not be available, and although that is not ideal, the correctly collected information may be useful anyway. Furthermore, if either A, B or C stay high or low while it is being inserted down the tube, the measurement unit will detect an incorrect sequence and report a failure immediately.

It will be appreciated by those skilled in the art, that the invention is not restricted in its use to the particular application described and neither is the present invention restricted in its preferred embodiment with regards to the particular elements and/ or features described herein. It will be appreciated that various modifications can be made without departing from the principles of the invention, therefore, the invention should be understood to include all such modifications within its scope.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A depth measuring arrangement for a data logging device comprises a probe member having a data sensor located thereon and a plurality of detector elements located at predetermined spacings along the length of said probe member having at least one of a plurality of signal paths connected to a said detector element; and a probe member receptacle having a trigger element located at a fixed point past which one or more of said detector elements of said probe member move as said probe member is inserted into or retracted from said probe member receptacle wherein said trigger element causes a detector element to cause a signal on at least one of a plurality of said signal paths when a detector element passes by or is located between one and another of said detector elements, such that a signal initiated by said detector on one or more of said signal paths when passing a detector element is indicative of a predetermined depth of said data sensor on said probe member within said probe receptacle when a preceding or succeeding combination of signals on said plurality of signal paths is known.

2. A depth measuring arrangement for a data logging device according to claim 1 wherein said signal paths carry a digital coding which when changing is representative of the movement of a said detector element past said trigger element.

3. A depth measuring arrangement for a data logging device according to claim 1 wherein said first detector element along the length of said probe member to pass said trigger element is connected to said signal paths in such a manner as to provide a unique code on said signal paths for identifying the beginning of a series of signals on said signal paths subsequent of which are representative of the distance of said first detector member from said trigger element within said probe receptacle.

4. A depth measuring arrangement for a data logging device according to claim 1 wherein said signal path terminates in a signal detector adapted to detect a binary value for use in a digital computer means which compares said binary value with ^' preceding and succeeding binary values to determine the direction of travel of said probe member and depth of said probe member within said probe member receptacle.

5. A depth measuring arrangement for a data logging device according to claim 1 wherein said detector element is a Hall Effect device.

6. A depth measuring arrangement for a data logging device according to claim 1 wherein said trigger element is a magnet.

7. A depth measuring arrangement for a data logging device according to claim 6 wherein there are more than one trigger element.

8. A depth measuring arrangement for a data logging device according to claim 1 wherein said probe member receptacle comprises a tube into which said probe member may be inserted and a said trigger element is located in a removable top for said probe member receptacle.

9. A depth measuring arrangement for a data logging device according to claim 1 wherein there are three signal paths and a said detector is connected to at least one of said three signal paths.

10. A depth measuring arrangement for a data logging device according to claim 9 wherein a said connection between said detector and a said signal path is via a signal diode arranged to conduct only when a said respective detector element passes a said trigger element.

11. A depth measuring arrangement for a data logging device according to claim 1 wherein a said combination of signals is predetermined when there are no detectors passing a said trigger element.

12. A depth measuring arrangement for a data logging device substantially as hereinbefore described with reference to and as illustrated in the figures on the accompanying drawings.