Is it Possible to Put in 500 ppm TDS but run off is O PPM?

Twice during the last week I measured TDS before and after. Friday I measured 1100 ppm with CalMag and all nutrients. Run off was a few ppt. Today was CalMag at 500 ppm and runoff was again low ppt. Is this possible?

1 Like

What kind of soil or medium is it. How old. Here
COPY/PASTE the below list into your forum post.

Answer these simple questions the best you can.
If you do not know, or do not use something; Just say so; Or post
NA (non applicable)

-What strain, Seed bank, or bag seed (photo or auto)
-Age of plant/what week of flower
-Method: Soil w/salt, Organic soil, Hydroponics, Aquaponics, KNF
-Vessels: Pots, Grow beds, Buckets, fabric pots
-PH and TDS of Water, Solution, runoff (if Applicable)
-PPM/TDS or EC of nutrient solution if applicable
-Method used to measure PH and TDS
-Indoor or Outdoor if indoor, size of grow space
-Light system LED, MH/HPS/CMH/Fluorescents, or other
-Actual wattage draw of lights
-Current Light Schedule
-Temps; Day, Night
-Humidity; Day, Night
-Ventilation system; Yes, No, Size
-AC, Humidifier, De-humidifier,
-Co2; Yes, No

If growing Hydro some additional questions:

-DWC? RDWC? Autopots? Ebb and Flow? Other?
-Distance of liquid below net pot (DWC)
-Temperature of reservoir
-TDS of nutrient solution
-Amount of air to solution

Always try to upload a clear picture in white light of any issues you may have to allow the community to assist you.

Add anything else you feel would help us give you a most informed answer should be included. Feel free to elaborate, but short and to the point questions and facts will help us help you in a more efficient manner :slight_smile:


I would check and calibrate your meter. It’s very unlikely that 0 PPM runoff is coming out from a soil grow.


Thank you. I am on a PC now so it is easy to cut and paste.
What strain, ILGM Gorilla Glue AF
-Age of plant 42 days from in ground
-Method: FFHF
-Vessels: 2 gallon Fabric pots
-PH and TDS of Water, PH every the last few weeks has been 6.5-6.8
-PPM/TDS or EC i have been using 1/4 strength fox farm trio in RO water w/ Cal mag ~900 ppm
-Method used to measure pH and TDS Apera EC&pH 60 2 seperate units
-Indoor or Outdoor if indoor, indoor 2.3x2.3 mars hydro
-Light system LED, SF-2000@65% power
-Actual wattage draw of lights 125 from wall
-Current Light Schedule 20 on/4off
-Temps; Day, Night day averages 70-74
-Humidity; Day, Night average 50-60.
-Ventilation system; Yes, Ac infinityT4 will be adding S4 intake tonight to pull more dry air from room so i can maintain closer to 50%
-AC, Humidifier, basement room is always ~70F big dehumidifier has room at 45%RH constant to help tent
-Co2; , No

Friday i watered to drain off. input was pH 6.4 run off was 5.9 TDS was 488 PPM run off 1.8 PPT
Today recalibrated meter pH was 6.49 run off was 5.98 TDS was 700 PPM run off was 3.9 PPT

It does not make sense to me such low run off numbers.

Any ideas?

Uh your not showing 0ppm runoff.

Makes the question weird. LoL sorry and its confusing the way you wrote tds and ppt and ppm.
What is PPT ? Your comparing ppm to ppt ?
I would just use ppm and ph. Make sure your pen is set to 500 scale as well.

Why is it set for parts per million in input and parts per trillion on runoff? Or is it parts per thousand?

If it’s parts per thousand, that 1.8ppt is is 1800 ppm and the 3.9 ppt is 3900 ppm

1 Like

The display scale changes automatically from ppm to ppt. I will measure the EC next time to see what it reads and do the conversion manually. I always thought PPt was trillions but you would never get that accuracy from a 60 buck unit. Thank you

1 Like

Ok so abbreviation is Not in the manual but I googled it and got the following:


When used in reference to TDS (Total Dissolved Solids) testers, PPT stands for parts per thousand.
Thank you.

1 Like

Again thank you. I found what I posted below on the Apera website. It changes scale from millions to thousands automatically.


Why is my TDS measurement showing PPT instead of PPM? Print

Modified on: Wed, May 20, 2020 at 2:58 AM

If you are in an industry that requires you to measure the TDS (total dissolved solids) in a water solution, you may sometimes get a measurement that uses parts-per-thousand (PPT) rather than parts-per-million (PPM) on your Apera tester.

It is important to know that 1000 PPM = 1.0 PPT. They are the same measurement made different by only 3 decimal places.

So for example, if you get a reading of 1.86 PPT, that would be 1860 PPM.

It is similar to if you are measuring distance in feet. Eventually, once you have thousands and thousands of feet, you can instead measure in miles. It is the same principle here. Large numbers are transformed into smaller more manageable numbers yet still mean the same thing; 10,560 feet is 2 miles. 2000 PPM is 2 PPT.

Most Apera testers will switch from PPM to PPT at the point it reaches 1000 PPM. It does this because even the most inexpensive of our meters are capable of testing up to 10,000 PPM. That number is too large for the 4 digit display on our instruments, so we must convert these measures into smaller numbers for ease and convenience. That is why we include the PPT measurement unit.

Here are some more examples to help clarify the relationship between these 2 measures:

1500 PPM = 1.5 PPT

1750 PPM = 1.75 PPT

2.5 PPT = 2500 PPM

3.95 PPT = 3950 PPM

By remembering this simple math conversion, you will never be lost when reading measurements with PPT.


Oh ok, makes sense. There isn’t enough space on the meter to read higher than 1000ppm. Was wondering why it would change automatically. So at 3.9ppt (3900 ppm) you can skip a feed and do water only next water day.

1 Like

Good info there. It all makes sense now !!

Looks like your good to go now.

Thank you all so much. I was freaking out my soil had no nuts.
Should I also skip my CalMag?

The TDS setting is also on the 700 scale. From what I understand it should be changes to 500 as that is the standard on this forum. Thanks again

1 Like

If you can change it that’s fine. If not, EC is universal so you can always use that.


I’m new to this, but it would appear that your run off TDS is greater than the TDS of the water going in? 488ppm in and 1800ppm out? Shouldn’t it be the other way around?

“TDS was 488 PPM run off 1.8 PPT” (or 1800ppm)

The soil I used had extra nutrients in it. So the water was coming out higher because some of the nutrients dissolved into the water from the nutrient rich soil. Measuring the run off or slurry let’s you know if you need to fertilize more or less. There will be let’s of info here in the forum if you search run off.

1 Like

Copy that, makes perfect sense. Thanks Bullpup!