If TDS is increasing, that means you're using more water than nutrient. It is also compounded if you're not flushing the system between solution changes as nutrient salts tend to build up in the medium.
If it were me, I would add water to the solution each time a reading was greater than where I started until I brought the reading back down to its original level.
When the TDS reading starts to read lower than at its original reading, add nutrient or change out and start with fresh depending on the age of the nutrient. Something like one month would probably be a good time to change it.

Thanks much, grizzman. Flushing the system between feedings sounds like a good idea to me, and it also sounds like you believe TDS readings do have value. Incidentally, I checked out your Summer Systems photos. Fantastic.

As Grizzman says, there is certainly a lot of value for a TDS reading since it is the only comparative cheap method that can be used to know what has happened to a nutrient solution. Generally speaking, adding water when conductivity increases and flushing the system between feedings are goods ideas generally practiced by almost all hobby and commercial hydroponic growers. However care must be taken to make EC readings at similar pH levels since movements in pH can also greatly affect conductivity since the conductivity of the pH determining ions H3O+ and OH- is several hundreds of times higher than that of other ions.

I think that grizzman's advice is pretty good. Water the conductivity down until it starts to go down by itself. I personally let it go down 20% after that before changing my solution. Also, just doing a change every 3 weeks if your reservoir holds at least 1 gallon per plant seems to be the standard practice on many commercial crops. I hope this helps :o)

Daniel

Here is a link that might be useful: Everything Hydroponics

@nemo2009,

Could it be that you use an unconventionally low quantity of nutrient solution with your setup or system? Anyway, if so or not the loss through evaporation and transpiration would only justify the increase in concentration if it is proportional. Means, if your ppm reading is nearly doubling while the nutrient level is going about half down as well - it's likely to be the cause. If these parameters are disproportional, there may be another cause like Ph fluctuations (as described by Daniel). Are you controlling pH and can you deduce something from pH changes in relation to the increasing ppm?

If you have a lot of nutrient loss due to evaporation and plant transpiration, you would simply top off the level with water every now and then (depending on size of the reservoir and daily consumption). But here you need to be aware of possible pH fluctuations with toping off with notable quantities of water in between nutrient replacements (the base pH of your water would matter here as well) .

If the ppm is increasing, one would normally recommend to switch to a lower nutrient concentration. But as 320 ppm is already low (with either conversion rate;-) that doesn't seem to be the solution. Topping off with water and not letting go your ppm even close to the double, seems to be the way to go until you detect the cause.

One more thing nemo2009,
Does your instrument (TDS-meter I suppose) have automatic temperature compensation - respectively if not, are you aware of different readings of ppm with changing temperatures? Perhaps I am wrongly anticipating an error source here, but better be safe than sorry. To have a correct reading with both, you should either have temperature compensation with your instrument or need to measuring starting nutrient (often cooler) and "used" and running nutrient always at 25°C.

Many thanks to Daniel and Lucas. I appreciate the caution regarding the influence of pH on the TDS-meter readings. I was unaware of that. And, not having a pH meter at present, while I may not do anything about it right now, you can be sure I will WORRY about it! Lucas, I believe I am making up my nutrient solution in accord with the manufacturer's instructions, with no conscious intention of being "unconventional". And, by my rough estimation, my nutrient volume loss did seem approximately proportional to the increase in ppm. So I'm reasonably satisfied that the responsible mechanism is water loss through evapotranspiration (We had some exceptionally and uncharacteristically warm, breezy, and sunny days here in the Pacific Northwest of the US when I was taking those readings). Fortunately, my meter IS temperature-compensated, so that's one worry I don't have. Daniel, if I read you correctly, you indicate that at some point the conductivity will begin to go down by itself. Would that be due to increasingly rapid uptake by an increasing plant mass? And thank you so much for directing me to the Everything Hydroponics website. It looks like great evening reading.

Hi again nemo2009,
Looks to me if the increase in ppm can be mainly explained by the simple fact that half quantity equals around double concentration. Actual consumption of nutrients expressed in ppm is neglectable in "standard situations".
Just a little differentiation: when I said "unconventionally low quantity of nutrient" I actually meant a low volume or a relatively small reservoir. Because the smaller the reservoir the more the physical evaporation and evapotranspiration would matter. But as we are at it: you say that you are making up nutrients according to manufacturers instructions. Not that I don't trust you on that, but how comes that you are running at only 320 ppm? That sounds low to me in fact - and here I really have to ask what conversion rate your instrument uses, /500 or /700?

Here is a link that might be useful: People from around the world meet at INTER- Hydroponic Forum

Hello again, Lucas. I'm afraid you embarass me, because I don't know the conversion factor. I checked the little bit of information that came with the meter, and no mention is made of the conversion factor. As to the volumes involved, the 14" pot (which I fill to the brim with each watering)has a volume of about three and a half gallons, and the reservoir contains about 4 gallons.

No embarrassment intended, truly! ;-)
The thing is that for your own purpose, practise and handling it doesn't really matter, but if I (or anyone else) want to make a relation or calculate it back to mS/cm there simply is a difference of 40% between both c. rates. I must sound kinda obsessed with this and earn the nickname of Mister Con-version Rate sooner or later here hahaha, but what can I do- or how would I 'circumnavigate' -that point? ;-)

The nutrient volume, yeah... as suspected and due to your technique it is indeed "unconventionally" small. No offence or embarrassment intended here either, as I come to understand your doing better now. I guess you have got a so called "manually fed E/F system". Well in that case and with such low volume, I would simply top off a bit on a daily basis. Or what about a secondary reservoir, that would feed the main-one (manually or automatically)?

Otherwise, when replacing more frequently (as smaller quantities call for it) , you can simply use the "not really depleted" nutrients as fertiliser for any pot plants or garden. The other diametrically opposite solution would be to actually have a much larger reservoir and thus increase the autonomy by 10 days, 2 weeks and plus. Which one of these alternatives is more suited and will result in best workflow for you. is merely subjective and definitely your call. ;-)

Cheers,
Lucas

Thanks for all your thoughts and suggestions, Lucas. And thanks particularly for giving a name to my set-up. It's a sort of cross between traditional ebb-and-flow and just container gardening, and it certainly is MANUAL! I have 36 pots that I water every other day. I doubt if you're familiar with my medium, a granular calcined clay ("Turface")that has excellent drainage properties, but lots of airspaces.

Interesting indeed... hope you do not mind if I narrate a bit more about your technique here.

I believe that (as a former potter) I can connect some more dots here.
Your "unconventional" media is in fact half baked, technically speaking low fired clay. Compared to traditional clay pebbles or hydroton, it's much finer and has much higher water retaining capabilities, while eventually having the same drainage properties, when fully soaked. Is that correct so far?

This is interesting because you can extend the feeding cycles and thus manual feeding is actually justified. Also, it explains the higher evaporation and the increasement in nutrient concentration of the "left overs".

If there is a downside to it, that would be the potential of this media for salt build-up on the long run. Have you thought of it and have you envisaged flushing your sets with water now and then? In between feeding, to prevent such build-up? Although I believe flushing is often overrated and is actually recommended by default, a bit like "blood letting" was a popular treatment for many illnesses in the dark ages. Because of simple ignorance, many illnesses were thought to be caused by an excess of blood in the body ;-)

However, I guess that the low concentration (of only ±320 ppm) nutrients is/was exactly the right thing to do with this media.

Cheers,
Lucas

Here is a link that might be useful: People from around the world meet at INTER- Hydroponic Forum

What a fantastic coincidence, Lucas, that you should be a former potter! You've described my medium exactly right. And as to the question of flushing the medium, when I fill the pots I "overfill" them to a depth of about 2", so some flushing action takes place. I haven't flushed with just water, however, but I will keep it in mind. Incidentally (and slightly off the topic), as a retired physician, I can tell you that there are circumstances in which a form of "bloodletting" can be useful in an emergency situation. It is called, not "blood-letting", but "rotating tournaquets" (to sequester the excess blood volume).