How To Measure The Resistivity of Distilled Water?

The water will be very nonconductive.

The different ohm meters may well be applying different voltages to the probes and will also have different impedences.

DC can also polarise the electrodes and.or the liquid. Conductivity meters usually use AC or pulsed DC.

You can buy a cheap conductivity meter from companies like Hanna.

(Conductivity is the inverse of ohms, measured in (milli)Seimens).

PS Why are you measuring it?

Del

Normally conductivity is measured there are instruments available to do the job and expressed as Mho which is reverse of resistivity Distilled water is non conductor of current.

crm

<Distilled water is non conductor of current.>

'Kohlrausch water' is theoretically pure water, not achievable in practice, and has a calculated resistivity of 18.8MΩcm at 25degC.

The semiconductor fabrication industry uses water typically at 18.2MΩcm at 25degC, and vast quantities of it. One plant, in East Kilbride/Scotland, runs at 80m³/h completely automatically, and distillation is not part of the 17 stages of processing involved in getting there from the local Town Mains inlet quality. 18.2MΩcm implies impurity levels in the fractions-of-a-part-per-billion region.

Thornton produces instruments capable of measuring these sort of resistivities consistently <usual disclaimer>. One is not likely to achieve the same using a £8GBP pocket DMM from Screwfix!

Can't really answer the question, but I will make a couple of observations that may help somewhat.

The assumption that the water should be very pure is not necessarily valid, as any contamination in the air (dust, pollen, atmospheric pollutants) will be picked up in the water when it condenses.

Also, you say the result should be the same regardless of the seperation of the electrodes. This I would disagreee with. Think for a second if you replaced the water with copper wire having a finite resistance per inch. Double the seperation, and the resistance doubles. I think the same will hold true for the water situation.

What kind of resistance values are you reading?

Thank's to all of you for answering.

Ok for contamination. This why I was trying to estimate this contamination by measuring the conductivity. Ok to do it with AC.

As far as the distance between electrodes, the exemple of a copper wire is not valid. If I am using 2 square containers with same depth and puting the electrodes on opposite sides. When the size increase, so does the length but also does the width and I think the result should not vary.

"The assumption that the water should be very pure is not necessarily valid, as any contamination in the air (dust, pollen, atmospheric pollutants) will be picked up in the water when it condenses."

I wonder whether the above may be more to the point. Moisture in the air is very susceptible to picking up any atmospheric pollutants which will condense out with the moisture when passing over the cooling coils. Once used a series of dehumidifiers to demonstrate just this point at a steel mill near here. Hydrochloric acid fumes from a nearby pickling line caused significant rust generation in the mill. The fume levels were undetectable by virtually any other cost effective approach and they could readily be tracked.

There is no need to measure the resistance of distilled water, it is infinite. As one poster mentioned, you are very unlikely to get distilled water from an air conditioner. You would be more accurate to call your water a condensate. Unless you are running clean room grade air through it there will always be contaminants. Pure water is an electrical insulator. Seems counter intuitive but it's true!

The resistance of distilled water is not infinite. The highest resistance found in water is generally accepted to be a bit above 18 Mohm-cm. This would be obtained by deionization, not distillation. Distilled water would generally have a much lower resistance than that, typically 1 Mohm-cm. As soon as water is exposed to air, it begins to pick up CO2 rather quickly, and it's resistivity drops quickly.

So, I agree, water from AC condensate would not likely be distilled water quality.

Tad

Indeed it is clear I was in error. Another case of CR4 clarifying what I know and what I only thought I knew. The water thing was based on something an instructor told me or perhaps my misinterpretation of what he told me, many many years ago. It is great to have such a vast pool of knowledge CR4!

There is a device called conductometer to check the conductivity of water. It's similar to an Ω-meter but it uses AC in order to prevent the galvanic effects. The readout is in mS and µS (milliSiemens and microSiemens). There are special glass-covered bell-electrodes and you have to dip it into the water you want to check. The measuring frequency depends on the range: the higher the conductivity the higher the measuring frequency should be. The higher applicable frequency is some kHz, the lower one is some 100 Hz.

The ohmmeters and/or restivity meters can only apply a voltage equal to the battery used to power the meter which in most cases is 9 VDC.

Most standard restivity measuring devices are calibrated to measure at at a distance of 10 centimeters between probes as it definitely "makes a difference" the farther apart the probes are located.

The greater the distance between probes, the greater the magnitude of the resistance/impedance value becomes.

The ohmmeters and/or restivity meters can only apply a voltage equal to the battery used to power the meter which in most cases is 9 VDC.

!!! Falls off chair laughing....

Yeh like it's totally impossible to chop and step up a battery voltage...or drive opposite polarity pulses through capacitors to give twice the voltage swing....

Del

A company caled Fann (now part of Hellaburning via Baroid I think) to manufacture a resistivity meter for use in the drilling fluids industry. This would at least give you a figure accepted in that area of the industry as "standard."

http://www.fann.com/product_info_main.asp?catid=58

Hope this helps.

I agree with the posts in that distilled water should be inherently non-conductive. However there are specific probes that measure conductance (reciprocal of resistance) and are used throughout the water treatment industry. Check out the following website:

www.sensorex.com

cheese

Tad is quite correct. We process deionized water for use in our Fab and it is typically slightly over 18 Megohms/cm. You might see 1 Meg/cm from high quality distilled water on a good day. So, you will get a small current flow in distilled water and an even lower one in DI depending on the applied voltage. DI is also damned corrosive stuff as it tries to replace missing ions from anything it contacts. You DON'T want to drink it.

Del is also correct about the meters not using a simple DC potential to measure the resistivity. While I'm no expert on the probes themselves they are spec'd with a cell constant (i.e. .01/cm) and take into consideration the water temp as well.

Bottom line...two foil electrodes and an ohmmeter ain't gonna cut it.

We used to measure TDS(total dissolved solids) in water in ppm(parts per million) using a special conductivity meter- rain water would measure say 20ppm- distilled water through our own distiller would measure around 5ppm. the statement that deionised water is aggressive means it would not be suitable in lead-acid cells?- Or would it be more suitable as it attracts pollutants to itself from it's lead environement?.

ppm meter? sold at almost every garden/hydroponic store

.........except the conductivities to be measured are off the bottom of the scale of such an instument........

Idle curiosity or is there something I'm missing? A/C condensate usually goes to waste water disposal or a secondary use where the conductivity is of no consequence.

If it is necessary to know the actual value then other posters have provided the answers.

Curiosity of course. But not only.

I wanted to know if it was safe to use part of it to pour in lead automotive batteries.

I'm trying to convert water pH value to resistivity. Some mentioned some conversion table from conductivity to resistivity. But I only able to obtain pH value. Need help. Anyone with suggestions? All reply welcome. Thanks!

It is not possible to <...convert water pH value to resistivity...> without knowing the nature of the dissolved species present. The indicator <...pH...> refers to the acidity or the alkalinity of the water. At hypothetical absolute purity water has a theoretical <...pH...> of 7.0; the figure will not change much with impurities such as, oh, say, sodium nitrate., whereas it will with sodium hydroxide, for example, whereas the <...resistivity...> will be similar for similar concentrations of each of these impurities..

<...pH...> and <...resistivity...> are therefore two indicators that are not interchangeable, and the best way to describe the water is to use both measurements.

I too thought about drawing distilled water from the AC evaporator coils for simple uses, (NOT FOR DRINKING OR AQUARIUM) but rather topping off batteries, or adding to auto anti-freeze etc. I think a major source for contamination, besides air mentioned earlier, would be exposure to the aluminum evaporator coil, which is covered in aluminum corrosion products, perhaps aluminum oxide or aluminum chloride which would certainly skew your conductivity reading.

As mentioned this water quality from your coils is not clean enough for your application. What you need is a low tech high quality color changing mixed bed indicator. Go to www.arieswater.com and look at he Unix system with VP-17-4030 product. As the color goes from blue to amber the resin is being used and resulting in 10 plus megaohmn water quality which is the requirement for your application.

Best of luck

Srock