This is probably going to be a long post, but here's the TLDR version... I have purchased three different brands of 53mS calibration solution and made my own solution and after calibrating with one of the solutions I got four vastly different salinity readings with my conductivity probe... 35PPT with the one I used to calibrate it with... 38PPT with another solution, 33PPT with the third solution, and 32.4PPT with the solution that I made. Is there a better source for a calibration solution I can trust?
Anyway, here's the long version... I recently got back into reefing after being out for a number of years. About a month ago I set up a 75G tank and I bought a (cheap) refractometer and an Apex with a conductivity probe. When I first got the Apex I had a bit of trouble calibrating the conductivity probe due to the small amount of solution and the fact that I was trying to do it out of the tank and put both the temperature probe and the conductivity probe in the solution. So, the Apex had consistently been measuring my water at about 38PPT, but I didn't trust it at all because of my issues trying to calibrate it. I trusted the refractometer because I had calibrated that with RO water (yes, I know now), so I was basically using the Apex to monitor for stability, but I was trusting the readings on the refractometer.
After having the apex up for a few weeks and realizing that the reading was stable, if high, I decided that I wanted to try to calibrate it again, so that hopefully it would read correctly. (At least I was encouraged that it was stable). As I was doing a bit of research on calibrating the conductivity probe I happened to find the long thread on here that talked about calibrating refractometers with a calibration standard instead of fresh water. So I decided to go all out with my calibration and I ordered a 35PPT refractive index standard solution for my refractometer and the pinpoint 53mS calibration solution for the conductivity probe.
The refractive solution came first and I calibrated my refractometer with it. It was definitely off a bit from my RO water calibration and after calibration my water was reading exactly 35PPT, so I was pretty happy about that. (Before the calibration it was showing about 33PPT and I had been hoping to slowly bring it up to 35PPT anyway).
So I was feeling pretty good when the pinpoint solution came the next day and I set out calibrating my conductivity probe. This time I put the solution in a plastic baggie and kept it in the tank water so I knew the temp was correct and it went much better. I went through the calibration process, then stuck the probe back in the tank and it read 32PPM
This is the same water that reads 35PPT with my newly calibrated refractometer. I went ahead an tried to calibrate the conductivity probe a second time and got the same results... the probe read about 35PPT in the solution and 32PPM in the tank.
So now I'm thinking that maybe the refractometer is calibrated wrong and I figured I'd try to re-calibrate that. I grabbed the refractive calibration solution and noticed that in addition to saying "35PPT refractive index standard solution" it also said "53.0mS". So I went ahead and put some of that solution in a baggie, dropped it in the tank for a few minutes to bring it up to temp and put the conductivity probe in it and low and behold it read 38PPT. So at this point I have two 53mS 'standards' that are off by 3PPT and I have no idea which one to believe.
I did a bit more research and found Randy's recipe to make a 53mS solution with a two liter bottle and morton's iodized salt. I went out and bought the salt and used a 2 liter diet coke bottle and made my own solution. Since I wasn't able to measure the components by weight I wasn't expecting this solution to be exactly the same as one of the others, but I was hoping it would be pretty close and then I'd know which one was right. Instead, my homemade solution read 32.4 when I put the conductivity probe in it. So now I have three different solutions with three different readings.
I discovered that one of my LFS's had the Neptune brand 53mS calibration solution on the shelf, so I drove out and got their last packet. I brought it home... brought it up to temp, stuck the probe in and... 0.0PPT. It didn't even register. I'm not sure what happened there, but apparently it was a bad batch of solution (or possibly just purified water). I talked to Neptune and they sent out a replacement packet which just arrived in the mail today.
I just tested with that packet, expecting it to be close to one of the other two standards that I had used, but instead I got a reading of 33.0PPT... it was actually closest to my homemade solution. So now I really have no idea what to believe. I didn't put a lot of faith in the homemade solution because I wasn't able to measure the components by weight, but maybe that and the Neptune ones are the most accurate... at least now I've got two standards that are within about 2% of each other.
So, after all that, I'm wondering is there a better source for a standard that can be trusted. I can't believe that three different brands of a standard solution came up with three vastly different readings. Is there somewhere I can purchase a lab grade standard for a reasonable price? Should I put more faith in my homemade solution even though I wasn't able to weigh the components?
Anyway, here's the long version... I recently got back into reefing after being out for a number of years. About a month ago I set up a 75G tank and I bought a (cheap) refractometer and an Apex with a conductivity probe. When I first got the Apex I had a bit of trouble calibrating the conductivity probe due to the small amount of solution and the fact that I was trying to do it out of the tank and put both the temperature probe and the conductivity probe in the solution. So, the Apex had consistently been measuring my water at about 38PPT, but I didn't trust it at all because of my issues trying to calibrate it. I trusted the refractometer because I had calibrated that with RO water (yes, I know now), so I was basically using the Apex to monitor for stability, but I was trusting the readings on the refractometer.
After having the apex up for a few weeks and realizing that the reading was stable, if high, I decided that I wanted to try to calibrate it again, so that hopefully it would read correctly. (At least I was encouraged that it was stable). As I was doing a bit of research on calibrating the conductivity probe I happened to find the long thread on here that talked about calibrating refractometers with a calibration standard instead of fresh water. So I decided to go all out with my calibration and I ordered a 35PPT refractive index standard solution for my refractometer and the pinpoint 53mS calibration solution for the conductivity probe.
The refractive solution came first and I calibrated my refractometer with it. It was definitely off a bit from my RO water calibration and after calibration my water was reading exactly 35PPT, so I was pretty happy about that. (Before the calibration it was showing about 33PPT and I had been hoping to slowly bring it up to 35PPT anyway).
So I was feeling pretty good when the pinpoint solution came the next day and I set out calibrating my conductivity probe. This time I put the solution in a plastic baggie and kept it in the tank water so I knew the temp was correct and it went much better. I went through the calibration process, then stuck the probe back in the tank and it read 32PPM
This is the same water that reads 35PPT with my newly calibrated refractometer. I went ahead an tried to calibrate the conductivity probe a second time and got the same results... the probe read about 35PPT in the solution and 32PPM in the tank. So now I'm thinking that maybe the refractometer is calibrated wrong and I figured I'd try to re-calibrate that. I grabbed the refractive calibration solution and noticed that in addition to saying "35PPT refractive index standard solution" it also said "53.0mS". So I went ahead and put some of that solution in a baggie, dropped it in the tank for a few minutes to bring it up to temp and put the conductivity probe in it and low and behold it read 38PPT. So at this point I have two 53mS 'standards' that are off by 3PPT and I have no idea which one to believe.
I did a bit more research and found Randy's recipe to make a 53mS solution with a two liter bottle and morton's iodized salt. I went out and bought the salt and used a 2 liter diet coke bottle and made my own solution. Since I wasn't able to measure the components by weight I wasn't expecting this solution to be exactly the same as one of the others, but I was hoping it would be pretty close and then I'd know which one was right. Instead, my homemade solution read 32.4 when I put the conductivity probe in it. So now I have three different solutions with three different readings.
I discovered that one of my LFS's had the Neptune brand 53mS calibration solution on the shelf, so I drove out and got their last packet. I brought it home... brought it up to temp, stuck the probe in and... 0.0PPT. It didn't even register. I'm not sure what happened there, but apparently it was a bad batch of solution (or possibly just purified water). I talked to Neptune and they sent out a replacement packet which just arrived in the mail today.
I just tested with that packet, expecting it to be close to one of the other two standards that I had used, but instead I got a reading of 33.0PPT... it was actually closest to my homemade solution. So now I really have no idea what to believe. I didn't put a lot of faith in the homemade solution because I wasn't able to measure the components by weight, but maybe that and the Neptune ones are the most accurate... at least now I've got two standards that are within about 2% of each other.
So, after all that, I'm wondering is there a better source for a standard that can be trusted. I can't believe that three different brands of a standard solution came up with three vastly different readings. Is there somewhere I can purchase a lab grade standard for a reasonable price? Should I put more faith in my homemade solution even though I wasn't able to weigh the components?
