Help converting Conductivity units

Hi Guys

I need a bit of help here with my math!

I have to measure the conductivity of caustic in a line. Now what I have been given by the supplier of the fluid is that we need a conductivity target range of 1oz/gal 3.16 mS. However the conductivity meter that has been installed gives an output of 0…999 mS/cm (as a 4-20mA signal).

Now according to Google 1 fluid oz/gallon (US) = 7.8125 × 10^-5 meters / centimetre (0.000078125 mS).

But surely converted this means that we are looking for 0.000078125 * 3.16 = 0.000246875 mS/cm (well outside the range of the meter) and too small to detect.

I am obviously doing something seriously wrong with my math here.

Can anybody point to my basic error?

Thanks

I might be wrong . . .
But I'm guessing that you have been given that 'they' are putting 1 oz of caustic stuff
into one gallon of water . . . to get a conductivity that should be 3.16 mS or 3.16 mS/cm.

Poet.

The units of the target range do not align with the units of the meter. The units oz and gal are both volume measurements and will cancel out if you divide them. I don’t know if perhaps that i supposed to be “specific conductivity“ ??

You need to get the units aligned before you can compare them. The meter gives what looks like the standard conductivity unit form of Siemens over length.

You may be overthinking this a bit. Poet is correct, they are telling you the treat rate to get to 3.16 mS/cm which is the conductivity number you are looking for. You dont need to do any math at all. Just scale your Conductivity Meter to 0-10 mS/cm if you can so you can get more resolution and go.

Thanks for the replies.

However the actual data I have is as follows...

Oz. per Gal Microsiemens Millisiemens
0.50oz/gal 1775.2 uS 1.78 mS
1.00oz/gal 3164.2 uS 3.16 mS

Which is different values based on different volumes. I don't think I can just use 3.16 mS because the two values above Equate to the same conductivity (so why not use 1.78mS instead)

Hope this makes sense



Thanks

twocozz said:

Oz. per Gal Microsiemens Millisiemens
0.50oz/gal 1775.2 uS 1.78 mS
1.00oz/gal 3164.2 uS 3.16 mS

Which is different values based on different volumes.

Click to expand...

It makes ultimate sense. 1.00 ounces has twice as many ions as 0.50 ounces.

Conductivity is measurement of how many conductive ions are available to conduct an electrical current. More ions, higher conductivity. Fewer ions, lower conductivity.

Mix up your solution and test it with your conductivity meter and you should get 4.048mA output [(3/999)*16]+4

The meter is pretty high ranging to measure 3mS. Are you sure it's not 0-999uS (microSiemens), in which case it would overrange at any value greater than 1mS.

yes it's definitely 0-200.

from the manual...

Output 1 (terminals 4 and 5): conductivity 1 with measurement range 0…200 ms



Thanks

I would be mindful that US fluid measures are different fron Imperial fluid measures 1 US fluid pint = 16 US fluid oz, whereas 1 Imperial pint = 20 Imperial fluid oz.

Steve

TwoCozz> However the conductivity meter that has been installed gives an output of 0…999 mS/cm (as a 4-20mA signal).

TwoCozz> yes it's definitely 0-200.

TwoCozz> Oz. per Gal Microsiemens Millisiemens
0.50oz/gal 1775.2 uS 1.78 mS
1.00oz/gal 3164.2 uS 3.16 mS

Which is different values based on different volumes. I don't think I can just use 3.16 mS because the two values above Equate to the same conductivity (so why not use 1.78mS instead)

>>>>

Poet: I'm confused. In your first posting you gave us 0 - 999 . . . and now you say 1-200?

And then you say two values equal the same conductivity - when they clearly do not.
0.50oz/gal 1775.2 uS 1.78 mS
1.00oz/gal 3164.2 uS 3.16 mS
It looks to me like if they put .50 oz into one gallon of clean water one should get a conductivity of 1.78 mS . . . if one doubles the amount of stuff being added to the water one gets 3.16 mS of conductivity. Which, as DanW said, makes perfect sense.

Is this the application? Your PLC is supposed to monitor the process and add more 'stuff' to the water if the conductivity is too low, or put less 'stuff' into the water if the conductivity is too high? (Or add more water if the conductivity is too high?)

If so all you really need is what is the conductivity supposed to be for the application (as read by your sensor) and adjust the doser (machine that adds the 'stuff') to keep the conductivity where it is supposed to be.
Poet.

Does this help?

It assumes a straight line relationship between mS and oz/Gal

Mad_Poet

I'm as confused as you. The unit actually has two outputs 0-999 and a separate 0-200 mS/cm as 4-20mA signal which we are feeding into a ControlLogix. Yes we are batching caustic into a tank and adjusting the amount based on the value coming back - Then cleaning much of the factory with it. It’s not a complicated job to be honest - Just nobody seems to know what mS/cm values I need to use so I can adjust the strength accordingly. I have asked the rep from the chemical company and he has just given me the info I posted. I asked him how this equates to what I am getting back from the conductivity meter and said he doesn’t know. I have emailed the company who supplies the chemical(Omegachlor)for advise and currently received no answer . The company where we are installing it simply relies on the chemical company for advice so no use there.



That was when I posted here.

I'm assuming this is for CIP (clean in place), which conductivity meter is it (manufacturer & model), maybe we can help with a little more info given the confusion?

Steve

I think what I would try . . .
I would take a sample of what is currently in the tank, and see how that cleans.
Then take a second sample, dilute it with some water, and see how that cleans.
Then take a third sample, add some more caustic, and see how that cleans.

If what is currently in the tank cleans best, then record the settings from the
sensor and try to maintain that conductivity.

(If the diluted version cleans better, cut back on the caustic or if the concentrated version cleans better add more caustic - once the best solution is found record the conductivity and program the PLC to get and keep that value.)

There will also be the cost versus performance issue - as in a stronger solution might clean better but the bean counters won't want to pay as much . . . Or a stronger solution might clean better . . . but safety concerns might rule out using it.

Good Luck.
Poet.

Would like to add a quick note that many conductivity meters show a temperature adjusted conductivity. So it shows the conductivity at x degrees.

twocozz said:

Mad_Poet

I'm as confused as you. The unit actually has two outputs 0-999 and a separate 0-200 mS/cm as 4-20mA signal which we are feeding into a ControlLogix. Yes we are batching caustic into a tank and adjusting the amount based on the value coming back - Then cleaning much of the factory with it. It’s not a complicated job to be honest - Just nobody seems to know what mS/cm values I need to use so I can adjust the strength accordingly. I have asked the rep from the chemical company and he has just given me the info I posted. I asked him how this equates to what I am getting back from the conductivity meter and said he doesn’t know. I have emailed the company who supplies the chemical(Omegachlor)for advise and currently received no answer . The company where we are installing it simply relies on the chemical company for advice so no use there.



That was when I posted here.

Click to expand...

I believe there is confusion in the communication between you and the chemical company. Are they perhaps telling you to test a bucket with a gallon of water combined with 1 oz of caustic. Use the meter to measure this and the value your meter shows will be the ideal setpoint. The meter may have a display showing this setpoint. Then you will need to scale the 4-20 mA signal in your program to 0-200. This should give you the same value on the meter display and in the program assuming they are both in the mS/cm unit range.

Hope this is on the correct path and helps.