Refractometers and ATC

[Reef.]

I agree there is complexity, but I still don’t understand why not minimize the temp variable.

If I use Randy’s calibration (which I am). Walk into my fish room and calibrate the (saltwater) refractometer to 35ppt. Then test my tanks sample, temps are for our purposes the same and if the tank reads 32ppt then I know I need to work to raise it.

Am I missing some element here? I am not an expert on the subject.

Because refractive index changes with temp.

 

[JNalley]

Great thanks, couldn’t get my head around why Randy had different recipes for each device.

Because they're measuring different things. We have nothing that actually measures ppt. A Hydrometer measures Specific Gravity (or more accurately, density), while a refractometer is measuring the refractive index. Table salt varies in composition from saltwater by a lot, so while the refractive index of 36.5 ppt of Brine might be the same as 35ppt of Seawater, the SG/Density is not the same (even though the scale on the refractometer shows SG it's actually showing the expected SG for that refractive index in saltwater). Does that make sense?

I agree there is complexity, but I still don’t understand why not minimize the temp variable.

If I use Randy’s calibration (which I am). Walk into my fish room and calibrate the (saltwater) refractometer to 35ppt. Then test my tanks sample, temps are for our purposes the same and if the tank reads 32ppt then I know I need to work to raise it.

Am I missing some element here? I am not an expert on the subject.

No, as I said in an earlier post, I tend to follow the directions from the scientists who created the refractometer just because they may know something I don't. Your way (and Randy's way) may be perfectly fine, but if ambient temperature didn't matter at all, I don't know why they would create such a stringent calibration regimen... So I tend to air on the side of caution.

 

[Reef.]

Because they're measuring different things. We have nothing that actually measures ppt. A Hydrometer measures Specific Gravity (or more accurately, density), while a refractometer is measuring the refractive index. Table salt varies in composition from saltwater by a lot, so while the refractive index of 36.5 ppt of Brine might be the same as 35ppt of Seawater, the SG/Density is not the same (even though the scale on the refractometer shows SG it's actually showing the expected SG for that refractive index in saltwater). Does that make sense?




No, as I said in an earlier post, I tend to follow the directions from the scientists who created the refractometer just because they may know something I don't. Your way (and Randy's way) may be perfectly fine, but if ambient temperature didn't matter at all, I don't know why they would create such a stringent calibration regimen... So I tend to air on the side of caution.

I sort of get they are measuring different things, what I can’t get my head around is, if I made Randy’s recipe for a refractometer and it read 1.026/35ppt why would that same solution not also read 1.0264 when testing a hydrometer…wait as I’m typing this think I get it, its reading 1.0264 on the refractometer by way of refractive index but that doesn’t necessarily mean it has the density required to raise the hydrometer to the correct lvl of 1.0264?

 

[JNalley]

wait as I’m typing this think I get it, its reading 1.0264 on the refractometer by way of refractive index but that doesn’t necessarily mean it has the density required to raise the hydrometer to the correct lvl of 1.0264?

Exactly. Because Randy is using table salt, he has to mix two different solutions. One that has the same refractive index of SeaWater, and one that has the same density as SeaWater. If you make your own using SeaWater salt, both things should line up pretty closely.

 

[Reef.]

Exactly. Because Randy is using table salt, he has to mix two different solutions. One that has the same refractive index of SeaWater, and one that has the same density as SeaWater. If you make your own using SeaWater salt, both things should line up pretty closely.

I get things eventually

 

[JNalley]

Just so you know, because they're measuring two different things, they'll still measure slightly differently. Not to add even more complexity to this conversation, but just so we're on the same page.

35 grams of salt in 1000L of water = 35ppt = the average ppt of sea water. The Red Sea for instance has a ppt ranging from 38-43 I believe (southern end is lower, northern end is higher). The Caribbean is all 35ppt, while others vary from 31-38. As a hobby, we arrived at the 35ppt standard based on averages.

Now, take into account that different salt manufacturers all have different ionic compositions, this means that even though you're mixing 35ppt (35 grams of salt for every 1000 grams of water), the refractive index of RedSea Coral Pro will be different from the Refractive index of TM Bio-Actif, their SG's will also be different from one another. So neither is "accurate" as far as PPT is concerned, they are accurate only to what they're measuring.

You follow?


Edit: This is why I always mix saltwater/calibration fluids by weight, because then I know what my testing equipment actually reads with a known solution and can adjust for that when I am making large batches.

 

[ReefHunter006]

Because they're measuring different things. We have nothing that actually measures ppt. A Hydrometer measures Specific Gravity (or more accurately, density), while a refractometer is measuring the refractive index. Table salt varies in composition from saltwater by a lot, so while the refractive index of 36.5 ppt of Brine might be the same as 35ppt of Seawater, the SG/Density is not the same (even though the scale on the refractometer shows SG it's actually showing the expected SG for that refractive index in saltwater). Does that make sense?




No, as I said in an earlier post, I tend to follow the directions from the scientists who created the refractometer just because they may know something I don't. Your way (and Randy's way) may be perfectly fine, but if ambient temperature didn't matter at all, I don't know why they would create such a stringent calibration regimen... So I tend to air on the side of caution.

Agreed temps matter. My question is basically saying if both solutions are measured at the same temperature, then you eliminate that variable. Maybe there is more to that, i don’t know, but my understanding if both are the same temp then there shouldn’t be an impact.

What may be a significant impact is if you did not recalibrate in a different ambient room temperature(basically means recalibrating before every test using this method).

 

[KStatefan]

35 grams of salt in 1000L of water = 35ppt = the average ppt of sea water.

35 grams in 965 grams of water is 35 ppt

35 grams in 1000 grams of water is 33.8 ppt

 

[JNalley]

35 grams in 965 grams of water is 35 ppt

35 grams in 1000 grams of water is 33.8 ppt

Sorry, I meant sea water... 1000 grams of sea water contains 35 grams of salt, in other words


EDIT: I also used L instead of mL, I should really proofread, lol....

But I wasn't saying 35g salt + 1000mL water = 35ppt, I was saying 35g salt IN 1000mL of seawater = 35 ppt.