RM85 Technical Support - page 1

Page 1
Probe Mode - How do I tell if a problem is due to the RM85 or the (Twin) probe array ?
Probe Mode - Why does the display show "HCR/Open cct." with the probes correctly inserted ?
Probe Mode - Why am I having difficulty obtaining stable readings?
Probe Mode - Why do I see a steady drift in the reading ?
Probe Mode - Why do I see banding and/or level shifts in my data ?
Wheel Mode - Why does the displayed reading flicker ?
Wheel Mode - Why does using the Wheel Encoder result in traverse under/overshoot ?
GPS

Page 2
How do I download data if the RM85 is not listed in my software ?
What do I do if I am having difficulty downloading data ?
Why is the data scrambled when observed in software ?
What do I do if there is water in the RM85 or EPIB ?
Why does the NiMH battery pack not hold its charge ?


Probe Mode - How do I tell if a problem is due to the RM85 or the (Twin) probe array ?
In the vast majority of cases reading problems (HCR, slowly drifting background reading, speckley looking data when viewed in software) are associated with incorrect array connections or breaks in the external cabling - it is very rare for there to be a problem with the RM85. See the next two questions as well. You can do two tests to see if the RM85 is working correctly.

In the first test, which is not fully definitive but easier to do, set Log Mode = Single, Probe Configuration = Twin. Disconnect all cabling from the RM85 and then identify the "Mobile Probes" and the "Remote Probes" connectors. The connectors have six and two pins inside respectively and for this test you should short together all eight pins; it is quite safe to do this since the outputs are current limited. To do this use some aluminium foil to act as a bridge between all eight pins - short together the pins in each connector (especially the two pins either side of the notch in the "Mobile Probes" connector) and also connect these two connectors together at the same time. If you see a very low value displayed then it is likely the RM85 is OK and the problem lies with the probe array. If it shows HCR then there is problem with the RM85.

In the second, more definitive test, we are going to create a Wenner array using the probes, jump leads and remote probe adapter lead - this will eliminate the 50m cable drum (which is the most common problem area) and the frame wiring. Set Log Mode = Single, Probe Configuration = Twin. Again identify the "Mobile Probes" and the "Remote Probes" connectors which have six and two pins inside respectively. Also identify the notch on the six way connector, to the right hand side. We are going to create a setup similar to figure 2-16 in the RM85 manual. First take two probes from the frame along with the two L shaped remote probes and place in the ground in a line about 25cm apart - have the pair of remote probes at one end, frame probes at the other. Next connect the remote probe adapter lead to the RM85 "Remote Probes" connector and plug the red 4mm plug into the outer remote probe, and black 4mm plug into the inner remote probe. Next take a red and green jump leads and plug the red one into the outer frame probe and green one into the inner frame probe. Switch on the RM85 and hold the other ends of the jump leads to the RM85 "Mobile Probes" connector with the red jump lead touching the pin just above the notch and the green jump lead touching the pin just below the notch. You should now see a reading in the range 5-25 ohms, depending on the resistivity of the ground. If it shows HCR then there is problem with the RM85.

In this second test you may still see HCR if there is a fault with either the jump leads or the remote probe adapter lead. Try swopping to other jump leads if available and replace the remote probe adapter lead with jump lead connections that connect the outer remote probe to the top pin of the "Remote Probes" connector and the inner remote probe to the bottom pin of the "Remote Probes" connector.

Probe Mode - Why does the display show "HCR/Open cct." with probes correctly inserted?
First check that you do have the correct connections by referring to the RM85 instruction manual. For example if you are using a single 0.5m Twin and a PA5/PA20 probe array then note that the short black jump lead goes to the red socket and the green jump lead goes to the black socket. Do not just match plug and socket colour - you need to follow the wiring diagrams figures 2-12 and either 2-11 or 2-13 in the manual.

If "HCR/Open cct." is still shown then it is very likely that there is a break in the cabling of the probe array (in the "current" cable). You will require a multi-meter that measures continuity or resistance to trace where the break is in the probe array, flexing the cables in turn until an open circuit is found. Where possible replace the cable; cable type is not critical and any mains type cable can be used.

There are several places where the cable can break. Firstly, the cable that runs up the side of the probe array frame may be broken at the top of the frame where it exits and is held in place with a pair of cable ties.

Secondly the 50m cable (the most common problem area) can have a break anywhere along its length : (a) the break may be near the resistance meter end especially if it is tied in a knot round the frame for strain relief (in this case cutting off a short length and rewiring the black Bulgin connector may isolate the problem - see appendix E for details on how to rewire a connector), (b) the point where the 50m cable exits the centre of the cable drum, (c) the internal connection of the cable to the external connectors mounted on the drum side.

Thirdly, the cables to the remote probes may be broken or corrosion on the jump lead connectors can cause open circuits.

Probe Mode - Why am I having difficulty obtaining stable readings?
First check that you do have the correct connections by referring to the RM85 instruction manual. For example if you are using a single 0.5m Twin and a PA5/PA20 probe array then note that the short black jump lead goes to the red socket and the green jump lead goes to the black socket. Do not just match plug and socket colour - you need to follow the wiring diagrams - see Figure 2-12 and either 2-11 or 2-13 in the manual.

If you still experience large up and down, rapid, fluctuations in the reading check, and consider changing, the Operating Frequency and Auto-Log Delay setting - see section 4-12(4) for further guidance. Getting stable readings (with a Twin array in an urban locations can especially be a problem sometimes. Avoid using fast auto-log settings below 300ms in an urban situation. Try moving the remote probes to a very different location. Also see section 5-6 (8) for Twin array Instrument settings where use of higher currents 5mA and 10mA and lower gain (Advanced only) can help improve the signal to noise ratio and overcome interference problems. In the worst cases you may have to change to a different probe array such as Wenner, Double-Dipole, mini-Schlumberger or Square array to minimise interference - see section 4-12 (3).

On urban sites interference is usually due to underground mains power earth currents, telephone earth returns, or currents conducted along underground pipes. On rural sites interference can occur when operating in the vicinity of radar, radio or television transmitters or booster stations. Interference will increase as you get nearer the source, though it is difficult to give precise indications of the level of interference. Some radar stations including those of air traffic control, ship harbour and military establishments can transmit at very large signal strengths (especially military), sometimes causing interference at up to 15-100 Km distances.

Probe Mode - Why do I see a steady drift in the reading?
First check that you do have the correct connections by referring to the RM85 instruction manual. For example if you are using a single 0.5m Twin and a PA5/PA20 probe array then note that the short black jump lead goes to the red socket and the green jump lead goes to the black socket. Do not just match plug and socket colour - you need to follow the wiring diagrams - see Figure 2-12 and either 2-11 or 2-13 in the manual.

If you still experience a steady drift in the readings, yet the resulting plots still seem to produce some meaningful results then it is likely that there is a break in the cabling of the Twin probe array (in the "potential" cable). Another symptom of a broken cable is you may also find in dry weather that the meter gives very high readings when the probes gently touch the ground but if they are firmly pushed into the ground the meter then produces normal readings.

You will require a multi-meter that measures continuity or resistance to trace where the break is in the probe array, flexing the cables in turn until an open circuit is found - the 50m cable is the most common problem area. Where possible replace the cable; 50m cable type is not critical and any mains type cable can be used. See previous question for indications of where the break may be.

Probe Mode - Why do I see banding and/or level shifts in my data?
If you observe banding in the data and/or level shifts (yet otherwise the resulting plots still seem to produce some meaningful results) then it is likely that there is a break in the cabling of the Twin probe array (in the "potential" cable).
You will require a multi-meter that measures continuity or resistance to trace where the break is in the probe array, flexing the cables in turn until an open circuit is found - the 50m cable is the most common problem area. Where possible replace the cable; 50m cable type is not critical and any mains type cable can be used. See previous question for indications of where the break may be.

Wheel Mode - Why does the displayed reading flicker?
Some flickering of the reading will be observed in normal circumstances in the field. Often this will be insignificant to the changes observed in resistance as the MSP25 or MSP40 moves over a target.

To overcome excessive flicker ensure you are using a current of 10mA and a gain of x1, rather than 1mA and x10 which will improve the signal to noise ratio. Use of current Boost = 'On' (resulting in 5mA at x2 gain) is recommended since this will reduce power consumption and more importantly give extra contact resistance compliance.  If the source of interference is directional (which can be the case in urban situations) then, since the alpha and beta arrays are directional in nature, you may find one or the other array rejects the interference and you can just do a survey with just one array. Check this out by setting Probe Configuration to Alpha then Beta and observe any difference in stability. Changing the traverse angle can also help.

On urban sites interference is usually due to underground mains power earth currents, telephone earth returns, or currents conducted along underground pipes. On rural sites interference can occur when operating in the vicinity of radar, radio or television transmitters or booster stations. Interference will increase as you get nearer the source, though it is difficult to give precise indications of the level of interference. Some radar stations including those of air traffic control, ship harbour and military establishments can transmit at very large signal strengths (especially military), sometimes causing interference at up to 15-100 Km distances.

Wheel Mode - Why does using the Wheel Encoder result in traverse under/overshoot?
Normally you will calibrate the encoder over a representative area of ground. However, if the nature of the ground changes (i.e.. goes from soft soil to hard ground) then the depth that the spikes on wheel #1 drop into the ground changes and hence it's effective circumference. If this happens then you can nudge the calibration by going into the Setup Menu and select Mode, Adjust - see 4-9(3) for details. You may need to do this progressively over a site if its nature changes noticeably.

If the encoder system appears to be not working properly, then ensure the system is all clean inside the housing for wheel #1. Also clean the faces of the optical sensor, figure 2-64 (MSP25), figure 2-31 (MSP40). For later versions of the MSP25 ensure the felt washer is lying flat. If you are unable to get the system working then consider using the Timer system 4-9 (4) and ((7). This is a viable approach for short traverse lengths of 20 or 30m and will allow data to be collected that may not otherwise be done so.

Wheel Mode - GPS
GPS is normally used with Wheel Mode. If you see 'GErr' in the top right hand corner it means there is either no valid GPS signal, the baud rates are mismatched or you are using the wrong COM port setting in your GPS unit. If you continue to survey in these circumstance then at the end of each traverse you will get a warning message reporting bad data points (equal to the number of readings in a traverse).

If you are surveying with Map Mode = 'Gridded' in an area with surrounding tree cover then the GPS signal may be blocked, resulting in bad data points reported at the end of each traverse. The RM85 will insert dummy GPS readings allowing you to continue surveying. In this situation you can only make use of the gridded resistance data, discarding any GPS referencing information. You should merge/paste in this data set with any GPS referenced data collected successfully.

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