White and blue Bluelab pH Pen seen from the front, screen lit

pH Pen Bluelab held in the hand, probe immersed in the water of a tank

pH Pen probe rinsed under the tap after a reading

Measuring pH in a bucket of nutrient solution with the Bluelab pen, gloved hand

Bluelab pH pen held above the water, screen facing the camera

Waterproof floating pH pen with a double-junction probe

Name: Waterproof floating pH pen with a double-junction probe
Brand: Bluelab
SKU: TK-BLUELAB-PHPEN
Price: 15000.0 RSD
Availability: InStock

15,000.00 RSD

5,000.00 RSD × 3 with no fees Klarna.

This pen measures pH to one decimal place and water temperature, compensates automatically for the effect of heat, and floats if it falls into a tank. Its double junction probe is stored in the potassium chloride solution supplied, along with the pH 7 and pH 4 calibration sachets.

Ref. TK-BLUELAB-PHPEN·Electronic tester

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The summary

What's included: the pen, its AAA battery, pH 7 and pH 4 sachets, a storage solution and a guide.
Measurements: pH from 0.0 to 14.0 to the nearest tenth, and temperature from 0 to 50 °C.
Before the first measurement: 24 hours of soaking the probe in the storage solution.
Calibration: in two points, pH 7 then pH 4, using the sachets supplied.
At the water's edge: a fully waterproof pen that floats and switches off by itself.
In aquaponics: the fish water kept between 6.4 and 7.5, measured every day at start-up.

Fully waterproof pocket pH meter, supplied ready to calibrate and store

The Bluelab pH Pen is a waterproof, floating pen that reads pH from 0 to 14 to one decimal place, with an accuracy of ±0.1, and also reads temperature. It is calibrated at two points, pH 7 and then pH 4, and lasts about 350 hours on one AAA battery. Its probe, which cannot be replaced, measures the water in a fish tank just as well as a nutrient solution.

The pH Pen is a pH measuring pen designed for soilless growing and nutrient water. It displays the pH to one decimal place and the water temperature, and its automatic compensation corrects the effect of heat on the reading. The casing is fully waterproof and floats: a pen that falls into a tank of nutrient solution can be retrieved from the surface and carries on working. It runs on a single AAA battery, which you can find anywhere, for about 350 hours of measurement, and it switches itself off when you forget it. Its probe needs water of at least 0.2 mS/cm, that is 200 µS/cm, which a nutrient solution or the water of an aquaponic system exceeds, whereas pure osmosis water would give an unstable reading.

🖊️ A double-junction probe

The pH Pen's probe has a double junction: the reference is separated from the sample by two successive barriers instead of one. Nutrient solutions contain salts and organic matter that eventually contaminate the reference of a standard electrode, and this double barrier slows that contamination, which keeps the reading stable over time. The probe cannot be replaced on this model, so its upkeep directly determines the lifespan of the pen. A probe kept moist and calibrated regularly stays accurate for a long time.

Correct a pH that is too high in small doses

The pen tells you where your water stands, and it is then a matter of bringing it back into the right range. The liquid pH- pH down solution is dosed drop by drop into a hydroponics nutrient solution, never into the fish water, and the pH Pen shows you the effect of each addition a few minutes later. You can therefore approach the target without overshooting it, which spares the plants an abrupt swing.

🧴 The first soak and storage

Before its first reading, the probe must soak for 24 hours in the potassium chloride storage solution supplied in the box. This soaking hydrates the sensitive glass and brings the reference into equilibrium, which gives a stable reading from the very first calibration. Afterwards, between readings, a few drops of the same solution in the cap keep the probe moist. You calibrate at two points, with the pH 7 sachet and then the pH 4 one, rinsing the probe between the two, and the pen reminds you to recalibrate after 30 days.

Waterproof and floating

The pen withstands immersion and floats back to the surface if it falls into a tank.

AAA battery

A single standard alkaline battery, with an indicator that warns you before it runs out.

Automatic shut-off

The pen switches itself off after a period of inactivity, which saves the battery.

pH and temperature

Both values are displayed, with the temperature in °C or °F.

A dried-out probe is hard to repair

A pH probe that dries out responds slowly, drifts and eventually can no longer be calibrated. Never leave the cap empty, and do not replace the storage solution with pure water, which depletes the probe. If the pen has been left dry for a few days, soak it again in the storage solution before any calibration. Poor probe maintenance can also void the warranty.

🌱 The pen at the edge of the tank

In an aquaponic system, the pH is a compromise between three groups of living things: plants absorb their nutrients best below 6.5, nitrifying bacteria work best above 7.5 and slow down markedly towards 6, while fish tolerate a wide range. The pH is therefore kept between 6.4 and 7.5, and it is its trend from one week to the next that decides whether a correction is needed. Nitrification acidifies the water continuously, so the pH drops gradually, at a rate that depends on the water's carbonate reserve.

This is why pH is measured every day when a system is starting up, and often still afterwards, whereas the other parameters move to a weekly check once the system has settled. A waterproof floating pen has its place next to the tank, where you feed the fish, which makes this daily measurement as quick as a glance. Note the value and the temperature in a notebook: it is the trend over several days, more than a single reading, that tells you when to step in. The article on pH regulation in aquaponics explains the causes of these variations in detail.

🔬 Correcting the system's pH

When the pH drops below 6.4, it is raised back towards 7.0 with a base, added slowly over several days rather than all at once. The usual practice is to alternate calcium hydroxide and potassium hydroxide in equal parts, which also supplies the calcium and potassium that fish feed provides too little of; potassium bicarbonate plays the same role more gently. Also keep an eye on alkalinity, held at 100 mg/L of calcium carbonate at least, because it is this reserve that cushions the drop between two corrections.

Always measure at the same spot, in flowing water rather than in a stagnant corner, then wait for the reading to settle on the screen before noting it down. After each correction, let the water circulate for a good while, then measure again: this shows you the real effect of the dose and stops you from repeating it too soon. In hydroponics, where no fish share the water, correction is done with a liquid pH adjuster, always in small, controlled doses checked with the pen, and the calibration kit keeps the probe accurate from one season to the next.

Technical specifications

The reading

pH range: 0.0 to 14.0Resolution of 0.1 pH
Accuracy: ±0.1 pH at 25 °C
Temperature: 0 to 50 °CResolution of 1 °C, accuracy of ±1 °C, in °C or °F
Compensation: Automatic
Probe: Double junction, non-replaceable
Housing: Fully waterproof, floatingWater of at least 0.2 mS/cm for a stable reading

Calibration and maintenance

Calibration: 2 points, pH 7 then pH 4Calibration reminder after 30 days
First use: 24-hour soakIn the storage solution supplied
Storage: A few drops of KCl in the cap
Feeding: 1 AAA alkaline batteryApproximately 350 hours, low battery indicator

Warranty

Duration: 1 year
Condition: Probe maintained as per the guide

To allow for in addition

Calibration buffer solutions: pH 7 and pH 4 buffer solutionsWhen the two sachets supplied are empty
Storage solution: Potassium chlorideTo refill the cap

6 further specifications are available in the detailed view.

In the box

1 Bluelab pH Pen
1 AAA battery
1 18 ml sachet of pH 7.0 calibration solution
1 18 ml sachet of pH 4.0 calibration solution
1 18 ml sachet of potassium chloride storage solution
1 starter guide

Manufacturer's values (technical data sheet and help pages for the pH Pen). Aquaponic benchmarks from New Mexico State University and the FAO manual.

Manufacturer and product safety

Manufacturer: Bluelab Corporation
Website: bluelab.com

Information provided under Regulation (EU) 2023/988 on general product safety.

The pH Pen and its two neighbours

A floating pen, a tester with a replaceable probe, a pH meter with a cloth junction.

Is the pH Pen the right choice? It floats and withstands heavily loaded solutions. The other two have their own strengths.

	Replaceable probe	Double junction	Renewable fabric junction	Floats if it falls in the water	Battery life (h)	Recognised buffers	pH resolution	Price
Waterproof pocket pH meter withYour choice	No	No	Yes	No	800 h	4.01, 7.01 and 10.01	0,1	6,600.00 RSD
IP65 pH meter with replaceable electrode, pH and temperatureYour choice	Yes	Yes	No	No	300 h	4.01, 7.01, 10.01 or 6.86 and 9.18	0,1	8,100.00 RSD
Recommended Waterproof floating pH pen with a double-junction probeYour choice	No	Yes	No	Yes	350 h	7.0 and 4.0	0,1	15,000.00 RSD

Waterproof pocket pH meter with

The probe

Replaceable probeNo

Double junctionNo

Renewable fabric junctionYes

The housing

Floats if it falls in the waterNo

Battery life (h)800 h

Recognised buffers4.01, 7.01 and 10.01

pH resolution0,1

IP65 pH meter with replaceable electrode, pH and temperature

The probe

Replaceable probeYes

Double junctionYes

Renewable fabric junctionNo

The housing

Floats if it falls in the waterNo

Battery life (h)300 h

Recognised buffers4.01, 7.01, 10.01 or 6.86 and 9.18

pH resolution0,1

Waterproof floating pH pen with a double-junction probeRecommended

The probe

Replaceable probeNo

Double junctionYes

Renewable fabric junctionNo

The housing

Floats if it falls in the waterYes

Battery life (h)350 h

Recognised buffers7.0 and 4.0

pH resolution0,1

Figures taken from each manufacturer's documents.

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delivery time

The exact shipping cost is calculated at checkout, based on the total weight of your order and the destination. Lead times start from dispatch and remain estimates. For a destination not listed here, write to us at contact@univers-aquaponie.com.

Frequently asked questions

10 questions

6 key questions, 10 in the detailed view

Why soak the probe for 24 hours before the first measurement?

What pH should you aim for in an aquaponic system?

Can the probe be replaced?

How often should it be calibrated?

Do you need to measure the pH every day?

Does the pen really float?

Which battery does it use?

Is it suitable for a hydroponics nutrient solution?

Yes, that is the use it was designed for. The double-junction probe copes better with the salts and organic matter in nutrient solutions than a single-junction electrode does. It needs water of at least 0.2 mS/cm: a nutrient solution or the water from a fish tank exceeds this, pure reverse-osmosis water does not. Rinse the tip with clean water after each measurement, then replace the cap filled with storage solution.

What does the warranty cover?

Does the pen replace a drop pH test?

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Waterproof floating pH pen with a double-junction probe

The summary