Post by thread:[EE:] Measuring high resistence (of wood)

Hi All! I am looking into making an Ohm meter which will measure the high resistance of wood. This is to make a wood moisure meter. The resistence of wood goes down as the moisure goes down. This meter would have to measure resistances in the range of 1 MOhm to about 50 MOhms. If it is possible to read higher/lower values that would also be great. I have done some googling and other searching, but can't find any cicuits for measuring high resistance. What I have found is a paper which describes the conductivity of wood at different moisture levels, and the different techniuques of measuring the resistence (As in invasive, non invasive, but no info on actual circuits) I will use the invasive method wich is done by having two metal "needles" about an inch apart rammed into the wood. This seems to be the most commin method of doing it if you look at already available equipment. Is it possible to do it using a simple potential divider as shown below? ------o 5V | > < Wood of unknown resistance > | |-----> to PIC A/D | > < Known resistance > | |-----o gnd Now, the known resistence would have to be high I guess. 100 MOhms or so? Or would it be better to use a smaller known resistance, and use an OpAmp to scale the signal suitably for the PIC? Can this be done this simply at all?? I know I could go out and buy one of these, but this is not a commercial project, just something I wanted to see if I could pull off (and something I need because I don't know if my firewood is dry enough to burn this winter...). Any thoughts, links, circuits, rants and flames appreciated. Thank you, Øyvind Tjervaag. -- http://www.piclist.com hint: To leave the PICList spam_OUTpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

Oyvind Tjervaag wrote: >Hi All! > >I am looking into making an Ohm meter which will measure the high resistance >of wood. This is to make a wood moisure meter. The resistence of wood goes >down as the moisure goes down. This meter would have to measure resistances >in the range of 1 MOhm to about 50 MOhms. If it is possible to read >higher/lower values that would also be great. > Uh, don't you mean resistance lowers when moisture increases? David... -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam@spam@mitvma.mit.edu

> I am looking into making an Ohm meter which will measure the high resistance of wood. This is to make a wood moisure meter. The resistence of wood goes down as the moisure goes down. This meter would have to measure resistances in the range of 1 MOhm to about 50 MOhms. If it is possible to read higher/lower values that would also be great. /> Quick thought. - Consider using a constant current source and measuring the voltage drop across the sample with a VERY high impedance buffer. The 3 amplifier instrumentation amplifier discussed here recently would be a good starting point. - Many multimemters have a 200 or 400 megohm range. Looking at how they do it may be useful. - It's usual to use higher than somewhat voltages for such high resistances unless there is a reason not to. I suspect that DC may change the wood's resistance due to moisture, and high voltage more so. - A bridge circuit would allow much more usable values in the actual measurement input. At 50 MOhm and 10v you have 0.2 uA flowing. Many many opportunities for errors at such low currents. An AC bridge may be needed. - 4 wire "kelvin sensing" measurement may be needed at such high resistances. - Depending on other properties of the material, other methods such as thos which rely on the dielectric properties (eg capacitive measurement) may be more useful. - Banging in pins may require standardisation of pin size and depth and material for consistent results. For starters, why not try an existing high ohm range ohm meter and see where/if it is inadequate. Also: http://www.keithley.com/servlet/Data?id=4645 http://www.keithley.com/servlet/Data?id=6584 http://builder.itpapers.com/abstract.aspx?&scid=106&x=40&docid=637 Russell McMahon -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestKILLspammitvma.mit.edu

>I know I could go out and buy one of these, but this is not a commercial >project, just something I wanted to see if I could pull off (and something I >need because I don't know if my firewood is dry enough to burn this >winter...). >Any thoughts, links, circuits, rants and flames appreciated. >Thank you, >Xyvind Tjervaag. If you get enough flames, maybe you won't need the firewood? I have been involved in the design of a device designed to measure the conductivity of wood, in this case to find decay pockets in wooden utility poles (see Shigometer). This design applied a 5V AC square wave (no net DC component) to the wood under test, and measured the current flow across a series resistor. An opamp was used to amplify the rather tiny voltage before converting it. Bob Ammerman RAm Systems -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam.....mitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspam_OUTTakeThisOuTmitvma.mit.edu

Oyvind Tjervaag wrote: > I am looking into making an Ohm meter which will measure the high > resistance of wood. This is to make a wood moisure meter. The > resistence of wood goes down as the moisure goes down. No, it's the other way around. {Quote hidden}> Is it > possible to do it using a simple potential divider as shown below? > > > ------o 5V >> >> > < Wood of unknown resistance >> >> >> -----> to PIC A/D >> >> > < Known resistance >> >> >> -----o gnd Not as shown. The PIC A/D has too low an input impedence. I would use a higher voltage going into the wood with around a 2Mohm current sense resistor to ground. The voltage from the current sense resistor would be buffered and amplified by an opamp with CMOS inputs, then go into a PIC A/D. Since this is a high impedence circuit, special attention needs to be given to leakages, guard traces, etc. In other words, the circuit is as you diagrammed above, except 5V is preferably higher, the known resistance is about 2Mohm, and a high impedence buffer with gain is inserted before the PIC A/D input. It might also be a good idea to divide the actual voltage and feed it into another A/D input so that its absolute value can be factored out. Varying that supply from the PIC will also allow for larger dynamic range of measurement. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestspam_OUTmitvma.mit.edu

> Is it possible to do it using a simple potential divider > as shown below? I'd say yes. But it has limitations. If you use a known resistance low enough to not affect PIC A2D accuracy excessivley then the input voltage should be >> 5v. eg if Rknown in 10k and Rmax tested = 50 Mohm and max A2D input = 5 volts then Vin = 50E6/10E3 x 5v = 25,000 volts (!) (Actually 25,005 :-) above ground). A voltage clamp across Rknown would be "wise" :-) At lower voltages the max voltage will be lower. Increasing Rknown reduces Vin BUT currents reduce markedly and the A2D starts getting inaccurate so a buffer would be needed. At Rknown = 1M Vin = 250 volts. Clamp still needed! Voltage is typical of "Meggers" used for insulation testing. (They use high voltage to induce breakdown of suspect connections). Power in wood = 250^2/50M = 1.25 mW. Runknown = 10M gives 25 volts (actually 30) BUT current = 25/50M = 0.5 uA A buffer is needed AND while 0.5 uA is measurable, it is not simple to measure it with stability or with simple circuitry. Some sort of compromise is going to be needed. A bridge is probably going to help heaps. RM -- http://www.piclist.com hint: To leave the PICList @spam@piclist-unsubscribe-requestKILLspammitvma.mit.edu