Frequently Asked Questions

About RM Frobisher

How Can I contact RM Frobisher

Our contact details are here.

Can I pay with a Credit of Debit Card

Yes. RM Frobisher (1986) Ltd. accept payment by Credit or Debit Cards on our website. (We use 'Stripe' to process your payment.)

What currencies can I pay in?

RM Frobisher (1986) Ltd use ‘’ to process debit and credit card payments. We also accept ‘BACS’ and ‘CHAPS’ bank transfer payments in UK pounds, USD United States Dollars and Euros. Please contact us for prices in other currencies.

Where can I see your products?

If you would like to see our products you could visit us at our offices in Barnsley, South Yorkshire. (Please make an appointment or check that we are available before visiting.) We have an area close by where you can use a unit. RM Frobisher visit shows and have stands at archaeology conferences where we have samples you can handle see Meet Us for more information about where we will be this year.

Do you sell spare parts?

Yes. We sell all our products and components individually. Check our website or contact us for a price list.

Can a buy a TAR-3 control unit to use with my existing frame and probes?

Yes. The TAR-3 uses standard 4mm gold plated jack plugs and can be used with any manufactures frame and probes. We would be happy to advise or produce adaptor leads to suit your current frame and probes.

You do not have an agent in my country – can I be your agent?

We recognise the value of having agents and/or resellers. Please contact us if you would like to become an agent or reseller.

Do you supply and ship your products to my country?

Yes. Our products meet the essential requirements of many countries and we are willing to ship (at your cost) to any destination. If we have a reseller or agent in your country, we may direct you to them as they would be better placed to offer support.

Can I resell your products for a profit?

Yes, we would welcome resellers. Please contact us so that we can come to an arrangement regarding supporting our products and supporting you in your efforts.

Can I buy a unit for a friend and claim a rebate?

Yes, but the unit must be shipped to you! If you are buying on behalf of a friend please let us have their details so we can register the unit to them. This will enable us to let them know about updates and allow us to carry out any repairs under warranty.

Do you give a discount?

No, but we do offer a ten percent rebate to customers (companies or individuals) who purchase more than one unit at the current list price. The rebate reflects the savings we make by selling to one person or company. The rebate is available on purchases of multiple units and on the purchase of more units at a later date. (This does not apply to accessory kits)

RM Frobisher’s products contain the latest 32bit ARM Cortex based processors

Our products contain one of the latest 32bit ARM Cortex based processors, capable of running at speeds of up to 48MHz. The processor features built in Digital Signal Processing functions (DSP) allowing it to perform complex Fast Fourier Transform (FFT), Infinite Impulse Response (IIR), and Finite Impulse Response (FIR) calculations in a single instruction. This enables unwanted 50/60Hz mains “hum” to be removed using modern Digital Signal Processing techniques, giving it unrivalled power and performance.

Can you interface the TAR-3 with my equipment.?

We at RM Frobisher consider ourselves to be experts at interfacing equipment together. Please contact us with your requirements. This is an area that we wish to develop and are actively seeking partners.

Can you make a special or one off piece of equipment for ….?

Yes. We can produce one-off, customised, equipment. Producing customised equipment helps us at RM Frobisher increase our knowledge and understanding. A word of caution: One offs or the first off can be expensive as the cost of development is not spread over tens or hundreds of units. Please contact us with your problem or ideas, we may be able to help or suggest ways of solving your problem.

DFG-1 Magnetic Gradiometer

Is the DFG-1 Magnetic Gradiometer difficult to use?

No! There are only a few settings and it takes less than 60 seconds to zero. The sensor does not need to be aligned. Once you have entered your settings they are remembered by the DFG-1 unit and reloaded ready for use when you switch the DFG-1 on.

What is the "warm up time" for the DFG-1

The DFG-1 and Sensys FGM series sensors do not have a warm up time. Modern electronic techniques and components have eliminated the need to allow the equipment to warm up. We do recommend that you avoid subjecting the equipment to sudden thermal shocks just before use. (Such as moving the unit from a heated room to a cold outdoor environment or from an air conditioned environment to a hot one.) If there is a difference between the storage temperature and the operating environment you should allow the temperature of the unit to stabilise before using it. Fifteen minutes is sufficient.

What is the battery Life of the DFG-1?

The DFG-1 Magnetic Gradiometer's battery lasts for twelve hours after charging.

What is the lead time for the DFG-1?

We keep the DFG-1 in and associated equipment in stock! There are three types of sensor. Sensors are usually available in five working days. Please contact us for details of sensor lead times.

How long does it take to align the sensor align?

The sensor does not need aligning! The design of the sensor eliminates the need to align it.

What software can I use with the DFG-1?

The output is compatible with

DW Consulting's TerraSurveyor and Snuffler

Data can be imported directly from the SD card. If required the data can be exported from Snuffler in different formats enabling it to be used with other software.

TAR-3 Resistance Meter

Is the TAR-3 difficult to use?

No! There are only two setting the user may have to change:

Is the TAR-3 a Resistivity Meter or a Resistance Meter?

The TAR-3 is Resistance Meter. It measures the resistance between the P1 and P2 probes and using the value of the injected current and records the measured (apparent) resistance. If the user has recorded the probe separation and offsets the TAR-3 results can be converted to resistivity.

What Injection current does the TAR-3 use

The "Factory Default" setting is 4mA. If the P1 and P2 probe resistance is very high, or the resistivity of ground being measured is high, the TAR-3 unit will reduce the current in stages to 2mA, 1mA or 0.5mA before reporting an error. Conversely, if the resistance being measured is very low the injected current is increased, up to a maximum of 8mA. This is one of the features of the TAR-3 that make it simple to use.

Ergonomic - The TAR-3 has been designed from the "ground up" to be ergonomic

The TAR-3 has been designed with the user experience in mind. The TAR-3's light weight minimises user fatigue, enabling operators to use the unit for extended periods. The shape of the TAR's handle allows users to change their grip and the rounded shape has no "Hard" corners to injure the operator's hands. The TAR-3 has a 32mm diameter grip to reduce the pressure on the operator's hands. The TAR-3's slim probes (8mm) reduce the force needed to insert them into the ground minimising operator fatigue.

What size is the TAR-3 frame.

For users who are not of average height, the frame of the TAR-3 is available in custom lengths. Please contact us with your requirements.

Lightweight - The TAR-3 is the lightest Resistance Meter in its class

The TAR-3 is uses a lightweight Aluminium Alloy for its frame and high power lightweight Lithium Iron batteries, making it the lightest Resistance Meter in its class.

When I measure the TAR-3's P1P2 impedance I get a very low value.

You can't measure the input impedance of the P1P2 voltage measuring circuit directly, as the full impedance is only developed when the circuit is powered up and actively measuring. The P1P2 probes are shorted together periodically (many times per second) to reduce the polarisation of the probes.

I think I've blown up my TAR-3 by touching a 240/110V cable/supply with a probe

Voltages in excess of 60V will damage the TAR-3! If your TAR-3 comes into contact with voltages greater than 60V report yourself to the relevant health and safety body! If your TAR-3 is damaged by voltages greater than 60V consider yourself lucky - you could have been damaged or killed!

What happens if I short the probes together?

Nothing bad will happen! You can short any combination of probes together without causing any damage. The voltage probes P1 and P2 can measure voltages higher than the maximum produced by C1 and C2. Shorting C1 to C2 results in the voltage dropping so that the current flowing is the set current.

Can I buy a frame and probes to use with my own control unit?

Yes. The TAR-3 unit uses standard 4mm gold plated jack plugs and sockets and can be used with any controller. Please let us know what unit you have and what connector and adaptors you require. We would be happy to advise or produce adaptor leads to suit your existing equipment.

At what frequency does the TAR-3 operate?

The TAR-3 does not have an operating frequency. The internal electronics of the TAR-3 are digital and use the Digital Signal Processing (DSP) features of the embedded processor. To prevent electro chemical polarisation of the probes the polarity or the probes is switched between readings but 50Hz and 60Hz signals are digitally filtered from the result using advanced digital signal software techniques.

What is the maximum voltage between the current in injection probes?
The maximum current injection voltage is limited by safety regulations. The voltages of the TAR-3 are classed as Safety Extra Low Voltage (SELV) which limits them to a maximum of 42.4Vac or 60Vdc. WARNING Voltages above 42.4Vac or 60Vdc are no longer legal if they can be touched by a finger.
What is the input impedance of the TAR-3 voltage measuring circuit?

The input impedance of the Voltage measuring probes P1P2 is very high, in the order of 100,000,000,000 Ohm, 100,000 Mega Ohm, or 0.1 Tera Ohm. (That's 10 to the power 11 Ohm). The TAR-3 uses electrometer grade components and design techniques for its input circuits which have been tested with very high impedance voltage sources to confirm the input impedance.

Why use 4mm jack plugs and sockets and not IP rated sockets?

Gold! 4mm jack plugs and sockets have more gold contact surface area than waterproof connectors. The standard (and military spec) connectors have a limited open/close, connect/disconnect, life span. Gold plated 4mm jack plugs have a longer life as they have a much greater surface area and the plugs orientation changes with each insertion which extends their life further.

How accurate is the TAR-3?
The TAR-3 uses a 32bit processor and 18.5 bit Analogue to Digital Converters (ADC) with a range of 262,144 (as opposed to 2047 for 12bit systems) . Using precision electronics in the analogue circuits it is possible for resistance measurements to have a standard deviation of 0.12% of the reading (better than one in eight hundred). The current between C1 and C2 and the voltage across P1 and P2 are determined using the full 18bit (plus sign bit) range of the ADC and are processed using modern digital signal processing techniques (DSP) using the processor's built in DSP functions and hardware to produce a reading that is unaffected by 50Hz (and 60Hz) mains "Hum".

To put the accuracy in to perspective. 0.12% of a ton of water (1000kg) is 1.2kg which is about two pints.

What software can I use with the TAR-3?
The output is compatible with

DW Consulting's TerraSurveyor and Snuffler

Data can be imported directly from the SD card. If required the data can be exported from Snuffler in different formats enabling it to be used with other software.


What is 'Line Mode'?

Battery Information

Aren’t Li-Ion-Ion batteries expensive?

Yes, they do cost more than NiMH or NiCd but they have less “charge memory” and are more efficient than NiMH when charging 80 to 90 percent compared to approximately 60% for NiMH batteries. They charge much faster for a given input current which means less time waiting!

How should I store my control unit?

Our products have no special storage requirements. Like all electronics they should be stored in a dry location at temperatures between -20 Celsius and +30 Celsius: Long term storage above 30 Celsius is not recommended

We recommend that that units are stored with the back panel removed and the storage case slightly open. This is to enable the foam and case to dry – if the unit has been used in damp/humid conditions any moisture in the sealed case can condense on the pcb, which over months and years may cause corrosion.

What does “Topping Up” mean?
Li-Ion batteries are charged in two stages “Constant Current” and then “Constant Voltage”. During the second stage the current flowing into the batteries drops as the batteries’ voltage rises to the that of the charger. As the voltage rises the charging current drops to zero. The final milli-Volt rise is takes a long time in comparison to the rest of the charging schedule. “Topping Up” or saturation charging is squeezing the last one percent (or less) of charge into the Li-Ion cells.

Users can switch the charger off and omit the “Topping Up” if they want to as it’s unlikely that they will need the last one percent (or less) of battery charge 99% of the time.

Li-Ion batteries are expensive, need a dedicated charging circuit and have to be declared when shipping or travelling with them but their advantages far outweigh their disadvantages. Li-Ion batteries’ power to weight ratio makes them the best choice for modern equipment. The batteries in the TAR-3 last for up to twenty hours and only weigh 100g (less than four ounces) that’s about the same weight as ten and a half one pound coins or thirteen Euros. As the TAR-3 has to be picked up 400 times per twenty metre grid the saving in weight does matter. The built-in dedicated battery charger in RM Frobisher’s products uses any 5V USB power supply or socket
The Battery voltage drops overnight – is this normal?

es, the voltage can drop slightly after charging. This is often referred to as “relaxing” or “settling”. The voltage drop can be around 0.1V. You will quickly develop a “Nose” for how long the batteries last for you. Don’t be too worried about battery life. They last a long time and can be charged up enough in 30 minutes to last a few hours. The battery voltage / charge relation ship is not linear. At the nominal voltage 3.7V large changes in charge only give small changes in the battery’s measured voltage. As a rule of thumb charge the unit if the battery voltage is 3.7V or less.

The batteries can also ‘self discharge’ or go ‘flat’ over a few weeks – this is normal if the unit is left for a few weeks it will need a full charge before using. After a day’s use it will only need a few hours charging. Always allow time for an overnight charge if you haven’t used the unit for a few weeks.

The unit keeps switching its self off?

When powering up the TAR-3 check the battery status. If the voltage is too low, a warning is displayed and the unit should be recharged (TAR-3 units will power down after 20 seconds). If during use the battery voltage drops below 3.7V “BAT” is displayed at the top right-hand corner of the LCD display. The unit can still function but consider ending the grid and saving the results

How often do the batteries need replacing?

It depends on usage and how long you use the unit per day but all batteries fitted by us are guaranteed for two years. In our experience the batteries in a unit used two or three days per week for six to eight hours and charged before each use can last up to five years (or longer).

Can I take my TAR-3 on an aircraft?

Yes. See

The TAR-3 contains two 9.5Wh Li-Ion batteries that are fixed in position in a tough plastic case. Net weight 50g each. A TAR-3 may be carried in hand luggage or in “check-in” (hold) luggage.
The DFG-1 contains One 20Wh Li-Ion twin cell battery that is fixed in position in a tough plastic case. Net weight 100g. A DFG-1 may be carried in hand luggage or in “check-in” (hold) luggage.

WARNING Do not crush, heat, incinerate, short circuit, dismantle or immerse in any liquid that may vent or rupture the batteries.

Can I over charge my unit?

No. The built in charger stops charging when the batteries are fully charged. Leaving the charger on charges the batteries and then enters maintenance mode. During the following weeks or months the charging circuit monitors the voltage of the batteries and will charge them as they self discharge. It is not recommended, or environmentally friendly, to charge the unit for long periods (weeks) as the external USB supply will consume energy and the TAR-3 unit consumes around 150mW when in standby mode during charging.

What type of battery do your products use?

The our products use two two LI-ION 3.7V 2600mAh battery cells with a charge voltage of 4.2V; fully discharged at 2.75V and a nominal capacity of 2600mAh each.

N.B. The operational temperature for Li-Ion batteries is -20C to +60C but the temperature range for charging is zero Celsius to +45 Celsius. Do not charge in freezing conditions!

Can I replace or change the Li-Ion batteries myself?

Yes – but we don’t recommend it. Opening the unit will invalidate your warranty! If your unit is still under warranty contact us as the batteries are covered by our warranty and we will replace them. If your unit is out of warranty then we recommend that you return it to us. We will check your unit and update the software if required. During battery replacement we will also replace the waterproof seal. Replacing the batteries is not for the fainthearted as the batteries are sealed in place with a specialist silicon sealant (NOT bathroom sealant) and the terminals are soldered to the circuit board using resin free solder to prevent corrosion. Using the wrong solder or sealant can reduce the life of your unit and may reduce it’s performance. The circuit boards are lead free and require solder that most users do not have. A low wattage, temperature controlled soldering iron is also required. It is unlikely that most users have access to the correct equipment. If you must replace the batteries yourself, contact us, we will help get you up and running again and we have a list of parts and instructions to help you.

Update (June, 2015)
Having visited sites and seen the equipment available to users (soldering irons, etc.) I recommend that users do not change the batteries themselves! (TA)

How often should I recharged my meter?

The units may be recharged at any time. Li-Ion batteries do not have a “memory” like other rechargeable batteries and the built in charger will charge and then maintain the charge in the batteries without damaging them.

Four and a half hours to recharge the batteries is that too fast?

Batteries that have been left for several months or a year or more and have completely self discharged will take ten hours to charge. After a day’s normal use the TAR-3 will typically require four and a half hours charging (or less).

The DFG-1 uses much more power than the TAR-3 and takes eight to ten hours to recharge.

How do I charge my unit’s battery?

The battery can be recharged with any USB power supply or port. Both products have a built in Li-Ion battery charger and only requires a five volt 500mA supply. Any USB power source can be used.

  • A mains USB supply,
  • a USB hub,
  • a 12V car to USB 5V adapter, or
  • a standard USB port.

WARNING Do not charge below zero degrees Celsius (when it is freezing)

How long does the battery last?

How long is a piece of string? The battery life depends on many factors. Tests have shown that the TAR-3 can take one measurement per second for over eighteen hours, over sixty-five thousand readings or one hundred and sixty 20m by 20m grids. That’s a lot of readings!

The DFG-1 battery lasts for twelve hours after a full charge. You can attach a 'Power Bank' and charge the battery while using the DFG-1!

The exact battery life will depend on;

  • the ambient temperature,
  • the age of the batteries, and
  • the rate of measurement.
LI-Ion battery charging

Lithium Ion Batteries should not be charged when the temperature is below zero degrees Celsius (when it is freezing).

Can I use a 'USB Power Pack' to charge my TAR-3 or DFG-1?

Yes, USB Power Packs can be used to charge the DFG-1 and  TAR-3  - just like mobile phones and tablet devices!

Note You can charge and use the DFG-1 at the same time, but you cannot charge and use the TAR-3 at the same time.

SD Card and Results

Are the results are compatible with my devices or operating system?

The SD Card and results are compatible with Microsoft Windows (XP to 10 and 2012) including mobile versions, RT and Phone. Results can also be transferred to Android, Linux, and *BSD based PCs, tablets and phones. Apple PCs, phones and devices are also seamlessly supported! The results are stored in a plain text file containing only ASCII characters

What cables do I need to transfer my results?

You do not need a cable to transfer your results are transferred between the meter and your PC without having to install any software, drivers, and without cables. No "Com" ports are required!

I can't open some files (*.SET, *.DAT and *.SWP)
My replacement SD Memory Card doesn't work - do I have to buy a one from RM Frobisher?

No. We do supply replacement parts, but any card will work. New cards need to be formatted with a FAT32 file system and initialised before they can be used. The card will work with the TAR-3 and can be used by most PCs and Tablet devices after formatting and initialisation.

Do I need to use high speed SD Memory Cards?

No. Units are supplied with a Micro SD Standard capacity card with a Normal Speed (NS) rating. Should the SD card be damaged it can be replaced with readily available replacements.

Help! My memory card is stuck!

Don't panic! The memory card socket is a "Push/Push" socket. To remove the card push it in until you here a "click" the card will then be pushed out of the socket by an internal spring. The card socket has an "anti-flyout ramp" which prevents the card flying out and getting "lost" in the case. Once pushed the card can be removed.

Can I lose data or settings when I switch the unit off?

If you switch the unit off in the middle of a grid the results for that grid are lost. Saved and completed grids are unaffected. If you "End" the grid the results are saved and the unit powers off.

No need for cables to transfer results?

Results are transferred between the meter and your PC without having to install any software, drivers, and without cables. No "Com" ports are required!

Results saved in a simple "txt" file

Results are available in plain text comma separated values (CVS) format and can be viewed or edited without specialist software. The results can be edited using Windows Notepad, Word, Excel, Vim, or OpenOffice.

What type of memory cards does the TAR-3 use?

Our products use standard Multi Media or Secure Digital memory cards. The same type used in your camera or phone. You can swap cards between grids allowing results to be analysed while continuing to take readings. Swapping cards allows results to be "archived" without the need for a PC. You can copy results to your phone, camera or tablet. We recommend cards of one, two, four or eight gigabyte you do not need to use high speed cards.

What units are used and what are the results displayed in?

Common Features

Do I have to enter my settings every time I power it on?

No. Setting are saved automatically and reloaded on power up. The settings can be returned to their factory default value by the user.

Text flashes on the LCD display too fast for me to read.

This is normal. Status messages are written to the screen during normal operation and updates or clears them when the unit starts a new step. During startup the unit goes through its startup tests and displays the current status message. When the test is completed a new message is displayed. For example, during start up the MMC Memory Card's File Allocation Table is checked and initialised. During this process the LCD will display "MMC FAT Init" (Please wait). If the control unit "hangs" at this point it means that the Multi Media (Secure Digital) Memory card is missing, faulty or incorrectly inserted in the card reader. When the card is inserted the message is cleared and the unit will continue.

What do the buttons on your products do?

There are two sets of five buttons on the front of the unit and one button ON button just below the LCD display and an OFF/Reset button under the access cover.

Reset Button

Switches the unit OFF and resets the unit. (Located underneath the access cover next to the USB charging connector)

ON Button
On the top of the meter (below the LCD Display), powers the unit up.

The two sets of five buttons allow the user to operate the unit with either their left and/or right hand.
Our products are "Menu Driven" in that the option groups are scrolled through and the desired options selected. The groups options can then be scrolled through and the required setting selected. The selected setting can then be changed - Increased or Decreased before it is Selected or Entered. Back/Previous/Up Button Increase Button Enter/OK/Select Button Decrease Button.

Why do your products do (or not do) it this way

RM Frobisher aim to produce reliable equipment at affordable prices. To do this we manufacture in volume and try to have a stable product design. This helps us maintain quality and our competitive prices. Our software is thoroughly tested and has the functions required to carry out surveys accurately and reliably. We recognise that our products donot do everything (they are not designed to) but if you have a request for a feature that would be useful to many users please let us know. The unit may already have the capability or it may be possible to incorporate it in updates.

This FAQ seems to be concerned with problems. Do you have satisfied customers?

Yes! We do. Unfortunately (or fortunately depending on your point of view) when products work as expected, just like the last time it was used, users do not ask questions or contact us. We would love to share users' experiences. Please send us a link to your work or website.

What does LOAD DEFAULTS do?
The LOAD DEFAULTS option replaces all of the user settings with the factory default values. This is how the equipment is supplied. Users cannot change the default values (they are fixed when the unit is manufactured).
Do your products meet the current 'EAC Guidelines 2'?
Yes! RM Frobisher (1986) Ltd' meets or exceeds the current guidelines. You can download a copy here
The text on the LCD changes before I can read it

This is normal. When running the unit writes information to the screen almost continuously. Most of the messages are overwritten before they can be seen - every milli second (mS) or so. Some information may take longer to overwrite and be displayed for tens of mS. Writing the results to the SD/MMC card is relatively slow and formatting a SD or MMC card can take up to thirty seconds. The information written to the screen is the current status message

Why haven’t you used a colour display?

The two line LCD Display used on the TAR-3 was selected in preference to a colour LCD display because it has a low power consumption, which extends battery life, it's usable in bright sunlight and readable over a wide range of viewing angles.

Why are there Capital Letters in words in your documentation?

Words in all capitals like "ENTER", "BACK" and "SAVE" use the same capitalisation as used on the LCD display. Words in capitals are selectable options . Words that have an initial capital letter or contain one or more capitals are used in a restricted manner and can be found in the glossary.

Where is my cursor?

The Cursor is only visible when the TAR expects an input from the user. The Cursor is not visible when the TAR-3 is taking a measurement or writing data to the Memory Card.

What is the cursor?

The LCD Cursor is the black flashing rectangular box on the LCD display.

RM Frobisher (1986) Ltd © 2020