Resistance of wood

[EEC]

I was using a DMM to find resistance of a popsicle stick. Using all the ranges, I could not find any measurement. What will it take to measure this?

 

[S'mise]

a Megometer

 

[mivey]

Resistivity

Resistivity

Oven dry wood: 100 trillion -10 quadrillion ohm-meters
Wet wood: 1,000-10,000 ohm-meters

 

[steve66]

I have to ask: Why do you want to find out how much resistance a popsicle stick has?

The applied voltage may have a big difference. It may read very high with a DMM working from a 9 volt battery, while touching 480V with one might give quite a shock.

 

[K8MHZ]

Oven dry wood: 100 trillion -10 quadrillion ohm-meters
Wet wood: 1,000-10,000 ohm-meters

It also depends on the wood.

Live pine (as in trees) conducts very well due to the sap and the moisture.

Pine wood for construction is nearly a perfect insulator if dry.

Any wood that is soaking wet, especially with salt water or water with fertilizer n it, is decent conductor. (Actually it's not the wood that is the conductor, it's what the wood contains)

So, you should be able to 'see' a 'perfect' insulator with a DVOM on a dry popsicle stick. Looking at Mivey's specs, above, we could then expect to see 1 to 10 ohms for every mm in length of a wet stick. We should then expect to see 100 to 1000 ohms with the probes 100 mm (4 inches) apart which is easily visible on a regular old DVOM.

To see the true value of a wooden stick you will need a megohmeter as has been mentioned.

 

[K8MHZ]

I have to ask: Why do you want to find out how much resistance a popsicle stick has?

The applied voltage may have a big difference. It may read very high with a DMM working from a 9 volt battery, while touching 480V with one might give quite a shock.

Not with a dry popsicle stick. Kid's don't try this at home, but touching 480 with a dry popsicle stick won't even hurt.

I have even poked around in HV power supplies (1kv and up) with dowel rods and not got zapped. Some of the amps hams poke around in approach 4kv to run the tubes. Before there was plastic, wood was used as an insulator. So was cloth and paper.

 

[steve66]

Not with a dry popsicle stick. Kid's don't try this at home, but touching 480 with a dry popsicle stick won't even hurt.

I have even poked around in HV power supplies (1kv and up) with dowel rods and not got zapped. Some of the amps hams poke around in approach 4kv to run the tubes. Before there was plastic, wood was used as an insulator. So was cloth and paper.

Yea, I would agree - a dry one wouldn't conduct much at 480. But from the original post, for all we know, it still has the popsicle on it :grin:

 

[wireguru]

Oven dry wood: 100 trillion -10 quadrillion ohm-meters
Wet wood: 1,000-10,000 ohm-meters

so to measure the resistance of wood, one would need an ohm meter meter?

 

[jghrist]

so to measure the resistance of wood, one would need an ohm meter meter?

ohm-meter is a measure of resistivity, not resistance. Your typical 0.002x0.013x0.1 meter wet popsicle stick with 5,000 ohm-meter resistivity would have an end-to-end resistance of about 19 Mohm.

 

[K8MHZ]

ohm-meter is a measure of resistivity, not resistance. Your typical 0.002x0.013x0.1 meter wet popsicle stick with 5,000 ohm-meter resistivity would have an end-to-end resistance of about 19 Mohm.

So, an ohm-meter is not 'ohms times length'?

If you have a 5000 ohm-meter stick 1/10 of a meter long, would you not have 500 ohms?

What am I missing here??

 

[K8MHZ]

Real world test results

Real world test results

Since I didn't have a popsicle stick, I used an incense holder soaked in tap water.

Using a DVOM, it acted like a capacitor. I could see around 1750 ohms for an instant and then infinity. I could also see the effects that lying in water vs. dry had on the discharge rate.

With a 500 volt megger, I saw 3000 ohms at a length of 7 inches, 1500 at a length of 3.5 inches.

I figure this to be about 16,000 ohm-meters.

Now, at 480 volts, 1500 ohms would allow for 320 mA of current. Figure in another 1500 for the body and we would see 170 mA, just under the 'lethal limit' at 7 inches and quite deadly at 3.5 inches.

To me, the above scenario seems very possible and I would rather trust my measured values than one based upon some theory that was way over my head and resulted in a value a few orders of magnitude off what I actually measured.

My uneducated prediction was 500 ohms based upon a 5k ohm-meter sample. I stand by my prediction as my other uneducated opinion is that I created a wet stick of a value of 16,000 ohm-meter using well water and an unknown species of wood covered with ash. The ash will skew the results as it parts of it becomes ionic when dissolved in water.

 

[shepelec]

When you find out let me know so I can fix this meter socket.:grin:

 

[K8MHZ]

Drier sample

Drier sample

After drying for about a half hour, the sample now shows 50 megs at 3.5 inches. 5 minutes ago it was 20 megs.

So, a damp sample would show 19 megs at 3.5 inches, or about 2 billion ohm-meters.

 

[K8MHZ]

When you find out let me know so I can fix this meter socket.:grin:

I don't see anything wrong with it!

:grin:

Great pics! Such a level of craftsmanship is hard to come by now days.

But not all together gone, I fear......

As I type this I am listening (on a scanner) to the fire department working a fire started in the main panel of a home about 10 miles from here.

 

[K8MHZ]

Like a capacitor

Like a capacitor

Realize that I did not let the water permeate the sample. I can see how I created a capacitor using wet wood as a conductor and dry wood as the insulator. If the sample was soaked through it would have not only presented less resistance, but would have also presented less reactance as well.

My still uneducated prediction is that the resistance of a totally soaked piece of wood would be measurable with a DVOM with a value of 1000 ohms or less at 7 inches, with no reactance to speak of.

I am not going to soak a perfectly good incense holder over night to find out though. Someone is going to have prove me wrong or just take my uneducated word for it.

 

[rcwilson]

Resistivity of a material is given in ohm-meters. The resistance of an object, like a wire, is proportional to its length and inversely proportional to the cross section area. More area for current to flow = less resistance. Longer path = more resitance. A 500 kcmil wire has less resistance than a 250 kcmil of the same length.

To make it simple, assume a block of material W x W in width and L long.

Resitance = resitivity in ohm-meter x (length L meters)/(area = W x W)

Look at the units: Ohms = ohms-meter x meter/ (meter x meter) = ohms.

Common misconception about ohm-meters. Many engineers specify ground resitivty incorrectly as "x ohm/meters."

Wood is a good insulator. The performance of a distribution line depends on the insulating value of the wood crossarms.

 

[dbuckley]

One can purchase wood dryness meters (link to the one I have from Amazon.com) that are no more than ohmmeters with a differently calibrated scale. But dry wood has a very high resistance indeed.

Bit of googling found this Department of Agriculture (19 page PDF) article on the subject, which has tables of wood resistance...

 

[Rick Christopherson]

Just FYI, but in the world of woodworking, there are pin-type moisture meters to determine the moisture content of the wood as part of the drying process. These types of meters are nothing more than calibrated ohmmeters. The conductivity of the wood is a function of the moisture content and the species of wood.

 

[mivey]

Wood is a good insulator. The performance of a distribution line depends on the insulating value of the wood crossarms.

But not for normal voltage insulation. It has to do with getting the BIL of the pole top at 300 kV or better to reduce lightning problems. With metal crossarms, we have to use higher-rated insulators.

 

[SG-1]

Once saw a 4 foot 2x4 spout out a flame after a few seconds with 80KV across it's length. The fire went out as the voltage was turned off. Which did not take long.