Slightly OT - Conductivity & Resistance

[BryanG]

Hi All

A chemical company my customers buy from have developed a new paint that gives a much better applied finish. Unfortunately it is causing major problems with gear type flowmeters, it has small bits that jam the gears. If we use a filter fine enough to stop the problem it blocks after a very short time and so filtering is no good. I am looking at alternate non-contact flowmeters and have seen electromagnetic versions that would do the job, assuming the material has a low enough conductivity. I have asked the chemical company for the conductivity but am not holding my breath, I asked them for a specific heat capacity three years ago and am still waiting for the answer. I have done research on conductivity and everything says that it is simply the inverse of resistance. So my question is if I attached a couple of bits of copper wire to a multimeter and dip them into the paint 1cm apart and get a reading of 500ohms does that mean the conductivity would be 1/500 siemens or 2 millisiemens. This doesn't have to be accurate, it is just so that I can know it is above the minimum conductivity required by the flowmeter.

Thanks

Bryan

 

[Mark Buskell]

You could go with a coreolis type meter such as as Micro-Motion or an E & H. They are a lot more expensive than gear types but have no moving parts and can detect any material that flows through them.

 

[John Soltesz]

I would have your customer contact the chemical company for that information. I would think it could be part of their material spec, but maybe not. I would think their lab would be able to do that. Another choice would be a coriolis type. A bit pricey but will do the job and it doen't care about conductivity and some are straight through for easy cleaning. Check with Endress +Hauser and others.
John

 

[pouch]

Resistivity is the inverse of conductivity.

Regards

Pouch

 

[jdbrandt]

Resistivity is the inverse of conductivity.

Regards

Pouch

Resistance is measured in OHM
Conductivity is measured in MHO (commonly said "ohm spelled backward",)

MHO = 1 / OHM

 

[bernie_carlton]

'MHO' is a coined name. The SI unit is indeed Siemens

 

[BryanG]

Thanks All

So it is as simple as 500 ohms would be 2 millisiemens/centimtre (2 millimhos/centimetre), though I do realise that the resistance would depend on how far into the paint I put the copper wire and how accurate the 1 cm gap is.

I will have to look at prices for the coriolis type meter, each normal machine needs two meters and a new version we have designed would need four! Reliability is important but not if you can't sell any machines because no-one can afford them.

Bryan

 

[mellis]

I am looking at alternate non-contact flowmeters and have seen electromagnetic versions that would do the job, assuming the material has a low enough conductivity.

No one else has pointed this out yet, so I will...

Mag flow meters operate well on liquids WITH high conductivity. They have a problem with non-conductive liquids. Mag meters are quite a bit cheaper than coriolis meters. Mag meters are usually a straight tube so they drain well too.

My recommendation: Since you are going to be buying many of these meters, contact the vendors and supply them with some sample material. Or, have them loan you a meter for testing. Find out for sure if a particular meter will work.

 

[BryanG]

Hi Mellis

Low or High - me, confused, never!

You should have seen my first attempt at the PID settings manual for a machine that used a S7-224 PLC. The P effect is higher with a higher value, the I effect is lower with a higher value, the D effect is higher with a lower value and the output smoothing is higher with a higher value. Except that the loop is reverse acting so for some of the values higher and lower were inverted!

Bryan

 

[L D[AR2P#0.0]]

Clarification of terms used:

Resistance measured in ohms, Conductance is 1/Resistance measured in siemens and applies to defined objects in an electrical circuit.

Resistivity measured in ohm meters, Conductivity is 1/Resistivity measured in siemens per meter and is the property of a material.

So a resistance measurement of 500 ohms cannot be used to calculate the conductivity - the units are not consistent

 

[danw]

Water based paint (latex) is likely to work through a mag, which likes high conductivity (not low conductivity).

Solvent based paints are much less likely to work in a mag, because the solvents are hydrocarbon based and have low conductivities.

 

[rsdoran]

I would be curious to what causes the filtration problem. It depends on how it is being applied but in many situations this could create further issues later on.

It could be a situation where the paint needs to be "mixed" prior to use or if it is using something like metal flake then add that later in the process. There could be bonding or drying agents that have a tendency to make it clump, these could possibly be mixed separately.

I am just throwing ideas out there. I would attempt to get more information on the paint, especially if they used a paint before that did not have these problems.

 

[BryanG]

Hi L D
Thanks for the clarification.

Hi danw
I would guess that it is solvent based, though I don't remember a strong solventy smell from either of the two parts of it.

Hi rsdoran
Paint is a generic term used by the chemical companies. In this process it is usually a 2 part chemical that is mixed at or just before the application head, which can be up to 200 Metres away from the pumping system. In the past they have used epoxy or polyeurathane, one part of this new stuff looks more like non-drip gloss except that is matt. The chemical companies in this particular industry just tend to make the paint and then say 'OK it's your problem to pump and apply it'. As I put in my original post, I am still waiting for information I asked for three years ago.

I will ask the guys on site to do the copper wire and multimeter test, I can give the flowmeter peopel a call with the result and see if the mag meter stands a chance.

Thanks All

Bryan