Single vs Double Junction pH Probes: Why Our Doser Only Uses Double Junction
The junction is the part of a pH probe that quietly decides how often you calibrate and how long the probe lasts. Here's what it does, why a single junction probe is fine in your hand but a liability in a reservoir, and why we build the pH Doser around a double junction probe.
Two Kinds of Probe
Every pH probe has a reference junction: the point where its internal reference electrolyte meets the solution you're measuring. Probes come in two kinds, single junction and double junction, and the difference is invisible from the outside. It's also the single biggest factor in how a probe behaves after weeks in a nutrient reservoir.
Understanding it starts with a different question: how much accuracy does hydroponics actually need?
How Much Accuracy Do You Actually Need?
In practical hydroponics, the amount of accuracy that can be effectively utilised is about one tenth of a pH unit — a single decimal place. Measuring to 6.1 vs 6.2 matters. Measuring to 6.14 vs 6.15? Not so much.
Plants don't respond to hundredths of a pH point. Nutrient availability doesn't change meaningfully at that resolution. The extra decimal places look impressive on a display, but they don't translate to better results in your reservoir.
The same applies to temperature compensation. While it exists in high-end lab equipment, it's rarely necessary for hydroponic applications where solution temperatures are relatively stable.
The Handheld pH Meter Test
Most growers have used a handheld pH meter at some point. These typically use single junction probes and read to one decimal place. And as most people discover — they're more than accurate enough for hydroponics. So the case for a double junction probe isn't about resolution. It's about what happens to a probe that lives in the reservoir.
Single Junction: What Handheld Meters Use
A single junction silver nitrate probe is the technology in most quality handheld meters and entry-level dosers — proven, reliable, and effective when it's looked after.
What You Get
- ✓ One decimal place accuracy (0.1 pH)
- ✓ Reliable automated dosing
- ✓ Industrial single junction probe
- ✓ Cheap, easily sourced replacement probes
- ✓ Low cost
Trade-offs
- • More frequent calibration required
- • Shorter probe lifespan
- • Drifts as the reference is contaminated
- • No temperature compensation
In a handheld meter, none of the trade-offs matter much. You dip it, you read it, you rinse it and put the cap back on. A human checks every reading, and if the number looks wrong you recalibrate. A doser doesn't get that.
Double Junction: What We Build
The OMGrow pH Doser uses a double junction probe paired with an electrically isolated, high-impedance amplification circuit with an input impedance above 1 gigaohm. This is laboratory-grade technology.
What Makes Double Junction Better?
The "junction" in a pH probe is where the reference electrolyte meets the solution being measured. In a single junction probe, this reference material is directly exposed to your nutrient solution. Over time, contaminants can seep in, causing drift and reducing accuracy.
A double junction probe adds a second barrier — an intermediate chamber between the reference electrolyte and the solution. This resists contamination for much longer, meaning:
- • Half as many calibrations required
- • Probe lifespan nearly doubled
- • Reduced drift between calibrations
- • Three decimal places of accuracy
Universal Probe Compatibility
The high-impedance amplification circuit can work with any pH probe on the market — from basic industrial probes all the way up to the highest-end laboratory sensors. You're not locked into our ecosystem.
What Comes With It
The probe front end is one part of the doser. It ships with the full OMGrow platform:
- • Remote viewing and programming — check pH levels and adjust settings from anywhere via the web portal or app
- • SMS and phone call alerts — get notified immediately if pH drifts outside your target range
- • Historical data logging — track pH trends over time to identify patterns
- • Independent operation — continues dosing even if your internet connection drops
Privacy and Security First
Despite all this integration, we are highly privacy focused. All software is written by an extremely security-conscious developer. Your data stays yours — we don't sell it, share it, or use it for anything other than running your system.
Probes Are Consumables
Here's something worth remembering: all pH probes are consumables. Even the highest quality double junction probe, perfectly maintained, will not last more than a year or two in continuous use.
The reference electrolyte depletes. The glass membrane ages. The junction clogs. This is inevitable. The question isn't whether you'll replace the probe — it's how often, and at what cost.
Single junction probes are cheaper and replaced more often; double junction probes cost more and last longer. Over time the spend is similar. What differs is how many times a year you're standing over the reservoir with calibration solution, and how far the reading has wandered by the time you get there.
Side by Side
| Feature | Single Junction | Double Junction |
|---|---|---|
| Accuracy | 1 decimal place (0.1 pH) | 3 decimal places (0.001 pH) |
| Calibration frequency | More frequent | Half as often |
| Probe lifespan | Shorter | Nearly 2× longer |
| Probe compatibility | Single junction industrial | Any pH probe |
| Replacement probe cost | Lower | Higher |
Why We Only Build the Double Junction Model
We did offer a single junction doser for a time, and it did the job. The reason we stopped comes back to the difference between a meter and a doser. A meter reports; a doser acts. When a single junction probe drifts between calibrations, a meter shows you a slightly wrong number and you shrug. A doser pumps acid or base into your reservoir on the strength of that number, unattended, for as long as the drift goes unnoticed.
Automation is supposed to remove pH from your list of daily worries. That only holds if the reading the machine is acting on stays trustworthy for the weeks between calibrations, and that is precisely what the second junction buys you. Pairing it with an isolated front end above 1 GΩ means the electronics never become the limiting factor either, whichever probe you fit.
So there is one pH Doser now, and it's the one we'd want in our own reservoir: longer intervals between maintenance, less drift when you get there, and the freedom to use any professional probe on the market.
See the pH Doser
Double junction probe support, isolated 1 GΩ+ front end, on-device touchscreen. One model, $1,700, probe included.
View pH Doser