Marcel G
Member
Acknowledgements: In writing this article I was inspired by a post by @Bettatail on the PlantedTank.net forum, to whom I would like to thank.
Volume of 1 bubble (BV) = ⅙ * π * d³ = ⅙ * 3.14 * 43 = 0.166 * 3.1416 * 64 = 33.4 mm³
If you want to increase the amount of CO2 [say] from 1 mℓ to 2 mℓ per minute (i.e. from 30 bubbles to 60), then your needle valve must be able to increase the amount of flowing gas by just 1 mℓ per minute [using the smallest possible turn of the control knob (valve handle)].
Assuming that the smallest possible turn of the control knob is 1/20 of a full turn (which is a really tiny turn), you will add about 20 mℓ of gas to the aquarium after one complete turn of the control knob (360°). If the maximum number of turns of the needle valve is [say] 20, then if you open it fully [from 0 to 20, i.e. from closed to fully open], you will achieve a flow of about 400 mℓ (= 0.4 ℓ) of gas per minute.
Thus, if you want to be able to regulate the gas flow smoothly, in increments of say 1 mℓ per minute (i.e. 30 bubbles per minute = ½ bubble per second), then you need a valve with a maximum flow of ~0.5 ℓ/min.
If you have the outlet pressure on the CO2 cylinder (= pressure after the pressure reducing valve) set to 45 psi (3.1 bar), then you need a needle valve with a flow coefficient (Cv) of approximately 0.001. At an outlet pressure of 20 psi (1.4 bar) you need a valve with a flow coefficient of about 0.0024.
Model graph of gas flow vs. valve handle turns (for Hoke Milli-mite 1300 series valve):
At an inlet pressure of 45 psi:
As you can see (in the above picture for 45 psi), each full turn of the handle of this valve represents an increase in flow of about ~0.6 ℓ/min (except for the very first turn). A 1/20 turn (18°) [which corresponds to one part on the vernier scale] represents a change in flow of ~0.03 ℓ/min (= 3 mℓ/min). So turning the handle one tick (part) on the vernier scale changes the flow rate by 3 mℓ/min, which corresponds to 90 bubbles per minute (= 1.5 b/s).
Hoke Milli-mite 1335 needle valve with black vernier scale handle with twenty ticks (marks) ↓
Basic terms
Calculating flow rate
What valve (flow rate) do I need?
At flow rate of 30 bubbles (Ø 4 mm) per minute, you add 1 mℓ of CO2 gas per minute [to your aquarium], i.e. 60 mℓ (= 0.06 ℓ) per hour.Volume of 1 bubble (BV) = ⅙ * π * d³ = ⅙ * 3.14 * 43 = 0.166 * 3.1416 * 64 = 33.4 mm³
d = diameter of 1 bubble (in mm) ← you have to measure or estimate this [using a photo where you capture the bubble near the ruler]
Volume of 30 bubbles = BV * 30 = 33.4 * 30 = 1,002 mm3 = 1 mℓIf you want to increase the amount of CO2 [say] from 1 mℓ to 2 mℓ per minute (i.e. from 30 bubbles to 60), then your needle valve must be able to increase the amount of flowing gas by just 1 mℓ per minute [using the smallest possible turn of the control knob (valve handle)].
Assuming that the smallest possible turn of the control knob is 1/20 of a full turn (which is a really tiny turn), you will add about 20 mℓ of gas to the aquarium after one complete turn of the control knob (360°). If the maximum number of turns of the needle valve is [say] 20, then if you open it fully [from 0 to 20, i.e. from closed to fully open], you will achieve a flow of about 400 mℓ (= 0.4 ℓ) of gas per minute.
Thus, if you want to be able to regulate the gas flow smoothly, in increments of say 1 mℓ per minute (i.e. 30 bubbles per minute = ½ bubble per second), then you need a valve with a maximum flow of ~0.5 ℓ/min.
If you have the outlet pressure on the CO2 cylinder (= pressure after the pressure reducing valve) set to 45 psi (3.1 bar), then you need a needle valve with a flow coefficient (Cv) of approximately 0.001. At an outlet pressure of 20 psi (1.4 bar) you need a valve with a flow coefficient of about 0.0024.
Model graph of gas flow vs. valve handle turns (for Hoke Milli-mite 1300 series valve):
At an inlet pressure of 45 psi:
As you can see (in the above picture for 45 psi), each full turn of the handle of this valve represents an increase in flow of about ~0.6 ℓ/min (except for the very first turn). A 1/20 turn (18°) [which corresponds to one part on the vernier scale] represents a change in flow of ~0.03 ℓ/min (= 3 mℓ/min). So turning the handle one tick (part) on the vernier scale changes the flow rate by 3 mℓ/min, which corresponds to 90 bubbles per minute (= 1.5 b/s).
Hoke Milli-mite 1335 needle valve with black vernier scale handle with twenty ticks (marks) ↓
Legend
Flow (ℓ/min):Basic terms
- Orifice size: the smaller the better (most valves have an orifice of around 1 mm)
- Flow coefficient (Cv):the smaller the better
- flow coefficient of most valves is linear increase proportional to the number of turns to full open
- Flow (Q):ideally below 1 ℓ/min [at full open] (or at least in units of litres per minute)
- Note: All flow rate data for individual valves corresponds to an inlet pressure of 45 psi.
- Turns to full open: the more the better (most valves have 10-20)
Tip
A number of excellent and very good valves can be found on eBay for a decent price. [P.S. Although sometimes vendors offer a valve at an exorbitant price, I recommend bargaining with them offering a lower price (e.g. $50-90) for the valve. Some will accept it without any problem.] The most common valves you can find there are:- Hoke Micromite 1600 series
- Hoke Milli-mite 1300 series
- Whitey 21/22 series
- Swagelok S series
- Aalborg MFV (VM1/VM2)
- Ideal valve 52/54-1
- Ham-Let HXF 1300 series (1°)
