As they come from the factory, these (at least the two I got) seem to only be good for desert plants (cacti & succulents) that need to be totally bone dry before watering. First let me debunk a couple of reviews I read that said these were ''crap'' because when the...
As they come from the factory, these (at least the two I got) seem to only be good for desert plants (cacti & succulents) that need to be totally bone dry before watering.
First let me debunk a couple of reviews I read that said these were ''crap'' because when the customer put them in a glass of water they read ''dry''. They are NOT made to read the presence of PURE water! They only take a reading on water that has impurities. I got some clean pure water, stuck the probe in it and it read ''dry''. I then boiled the water and made a cup of strong black coffee, and a cup of strong black ''Earl Gray'' tea;stuck the probe in each one of those and the meter pegged over to a 10++ reading. If you get a pot of good soil, and soak it well with water until it''s dripping out of the bottom of the pot (waiting until all water dripping stops first) and then stick it in the dirt, it also reads 10++. Makes sense, the water in the pot is also impure due to the soil.
OK, so now we know what these are designed to read, and how the work. Now I''ll discuss what they are good for as they come, new from the factory; they are VERY sensitive to moisture. Almost all cacti & succulents need to have the soil get bone dry between waterings. For those types of plants, you really don''t need these meters so long as you are willing to invest in a large supply of disposable tongue depressors or pop-sickle sticks. The way to check the dirt in your cacti is to insert the dry stick; if it comes out even slightly discolored (indicating some moisture in the soil has impregnated into the wood) then you need to withhold water longer. Only when the stick comes out just as dry and the same color as before you inserted it, do you add water to your cacti (and even then, it''s usually best to withhold water an additional 2 weeks in most climates except for low desert southern Arizona where it''s 110+ degrees). The way this meter comes from the factory it''s perfectly calibrated for this type of water monitoring.
However, if you are trying to keep sensitive water-loving tropical plants alive (such as papaya, guava, pineapple, passion fruit, etc.), especially in a hot dry place like low desert southern Arizona in summer time, these meters WILL kill your plants if you simply go by the readings they give rather than by common sense and how you wilty plants are actually looking! If you wait until these meters read in the 4-7 (moist) range before you water these types of plants, your water loving tropicals will wilt and start to die (even in the Arizona winter when it''s only in the low 70''s outside). But that''s ''OK'' because I''ve got a fix for you that you can implement.
I took the back off of one of the meters and took it into my electronics lab to see what makes it tick. There are no markings on the meter to indicate it''s full scale deflection; I''m going by my 45 year''s experience as an electronics tech to go out on a limb and say they must be either 100uA or 500uA meters (uA means micro-Amperes for you non-technical types - a very small amount of current that''s non-lethal even to a baby newborn mouse). One micro-Ampere is one millionth of an amp. Remember above I said the probes are very sensitive? OK, so I carefully pried the back off of one of the meters - they are held on by the press-fit of 6 little plastic pins protruding from the back of the case, that fit into 6 little sockets in the main case of the instrument. None of mine were actually glues, but still they are a tight fir and you have to VERY carefully pry off the back a little at a time. I eventually got mine apart, and even though they are not glues, I still broke off one of the 6 little pins (no big deal).
I then got some banana-to-minigrabber test leads and hooked them up to the output of the probe (right on the little soldered tabs on the back of the meter). I connected the other end of the test leads to a calibrated Hewlett-Packard model 34401A bench-top DMM, set to read DC millivolts. Sticking the probe into the cup of black coffee (or the black Earl Gray tea) gave a reading of 485 millivolts (or or just a tad under 1/2 volt for non-technical types). That was a lot more than I anticipated it would read, considering there is no battery in these things. Then I desoldered the red wire going to the meter and read the output of the probe without the meter movement''s internal resistance to load it down; back into the cup of tea and it read 5512 millivolts (just a tad over 1/2 volt). Then I dried off the probe with a paper wipe and a little application of gentle heat from a heat gun. Next experiment was to get my hungover coworker with the obnoxiously strong stale old alcohol breath to breath some humid exhaled air over the probe tip - that gave an open circuit reading of 4.3 millivolts! So yeas, this probe is sensitive indeed!
Obviously what was needed is a way to reduce the sensitivity of the thing so that is would better read the correct water level in the soil of water-loving tropical plants. I found an extra sealed mini-potentiometer laying around (well, actually I had 6 different values to choose from) so I chose a 1,000 ohm trimpot. I didn''t think one of a larger ohmic value would be needed, and the size of the potentiometer was such that it easily fit inside the instrument''s case. It''s the type of potentiometer that has a 1/8" slotted shaft for adjustment with a small screwdriver, and it has a 5/16" nut that you can lock down the shaft after making the adjustment so''s it won''t change afterwards. It''s a sealed military grade 1/2W linear taper potentiometer, part number RV6NAYSD102; these are available at digikey.com or mouser.com (and maybe Amazon though I didn''t check). There was plenty of extra red wire inside the instrument already, so no additional wire was needed. I put the potentiometer in series with the red wire going to the meter, and I wired it so that rotating the slotted shaft will result in decreasing the instrument''s sensitivity.
Next experiment was to insert the probe back into the coffee and the Earl Gray tea (no potted plants in the lab to check it in soil); I rotated the shaft of the potentiometer so that instead of the meter reading 10++ it was now reading 9.5. I then hooked up the Hewlett-Packard 34401A DMM across the potentiometer and I found that I had set it up so that only 178 ohms out of the 1000 ohms available were used in order to get that reading of 9.5. So I considered that a good outcome for my experiment - I still had plenty of adjustment range in case I needed to make it even less sensitive to moisture level in actual soil.
Then I used a ''P-Touch'' label maker to make two 1/2" high labels (white letters on black tape); one label says "Desert Plants" (for the un-modified instrument) and "Tropical Plants" (for the modified instrument). You can see the results of this in the attached photos.
After work I checked out my work in actual soil. I used the "Desert" one to check out the pots of cacti, and verified (by checking it against the pop-sickle stick method) that my wife has been over watering those plants because the meter was reading between 8 and 10++ depending on the pot I checked - I counseled her to stop watering those cacti & succulents for a month or so, then check them again. After that I looked at several potted tropical plants with the "Tropical" meter - these pots had all been watered yesterday until water came running out the hoeld in the bottoms of the pots. I decided to reduce sensitivity a bit more on the modified meter, based on how the soil looked and felt in the fist of my hand (it had drip-dried out a bit more since watering yesterday).
So now, overall, I''m satisfied with my purchase and these meters. It''s good that they came in a pair, that way I have a perfectly calibrated meter for each type of plant that I''m raising in pots.
I was going to give a 4-star review, because the meters aren''t perfect. But after my lab experiments, I decided to up the review to 5-stars; reason being is that I think all of the meters like this that you can buy (different brands) are all going to be the same basic thing (regardless of price and outward appearance). No one meter would be the perfect meter for all kinds of plants! Unless the meter came with a toggle or rotary switch and two (or more) trimpots, so that you could self-calibrate them to the sensitivity you need for what ever type of plants you are raising (in the case of wanting to use one meter for multiple types of plants). Or at the minimum the meters should have at least one trimpot in them so that you can self calibrate it to one given setting at least. And indeed I DID find such a meter on Amazon - it had a much stiffer 3/8" diameter stainless steel probe, and the box at the top of the probe was heavy duty metal and it had a trimpot in it for calibrating it. I suppose THAT meter would be the perfect one, but then again it''s priced like a perfect meter and I didn''t want to pay that much. So I''m happy with this inexpensive purchase of a pair of meters, and using the un-modded one for plants that want dry soil, and the modded one for my water loving plants.