Changing chlorine cylinders on time
A chlorine cylinder’s gauge cannot tell you it is nearly empty, and it runs out on its own schedule rather than the plant’s. Here is how to read what is left in one, how long it will last, how much gas it can give, and how to plan changeovers around the days nobody is there to make them. Every figure and rule on this page was checked against its source on October 17, 2026.
Weigh a cylinder to know what is in it, divide by what the plant draws from it a day to know when it runs out, and change it on the last working day before then. The rest of this page is the detail behind each of those, for 100 lb and 150 lb cylinders feeding gas to a vacuum chlorinator, with ton containers where they differ.
It is a summary for plant staff, not a procedure. Your chlorinator’s manual and your plant’s written procedures are the authority on how a cylinder is changed, and your state’s rules on how the chlorine room is built. If something here is wrong or out of date, tell us and we will check it.
1. The short answer
- What is left
- The scale reading minus the tare weight stamped on the cylinder. The pressure gauge cannot tell you.
- How long it lasts
- What is left ÷ the pounds a day drawn from it. Cylinders drawing together share the feed.
- What one can give
- For a 150 lb cylinder at 70°F, from 40 to 105 lb a day depending on the source. Frost on the lines means it is being pulled too hard.
- Before a day off
- A cylinder that would run out when nobody is there is changed on the last working day before.
- Spares
- Order while the full spares on hand still cover what the plant uses before the delivery arrives.
2. Cylinder sizes and weights
Chlorine cylinders hold up to 150 lb, and the 100 lb and 150 lb sizes are the common ones. A ton container holds 2,000 lb. Each cylinder carries its own empty weight, its tare, stamped near the neck ring with the date of its last hydrostatic test, which the Department of Transportation requires every five years. Ton containers are stamped with the tare too.
| Cylinder | Tare, lb | Full, lb |
|---|---|---|
| 100 lb | 63–115 | 163–215 |
| 150 lb | 85–140 | 235–290 |
| Ton | 1,300–1,650 | 3,300–3,650 |
A 150 lb cylinder is 10.25 to 10.75 in across and 53 to 56 in tall to the top of its valve hood. A ton container lies on its side, 30 in across and 79.75 to 82.5 in long. Empty weights for one size vary by as much as 55 lb, so only the figure stamped on a cylinder will do for that cylinder. Keep cylinders upright and chained or strapped, and full ones apart from empties.
Sources: the Chlorine Institute’s Water and Wastewater Operators Chlorine Handbook (Pamphlet 155, edition 2, 2008; PDF), sections 3.2 and 3.3 and table 4; full weights from Hydro Instruments’ Chlorine Handling Manual (revised July 2025; PDF), table 2, which also advises storing full and empty containers apart.
3. What is left in a cylinder
Chlorine is shipped as a liquid under its own pressure, and that pressure follows the temperature of the liquid, not how much of it is left. The Chlorine Institute is plain that the only accurate way to know what is in a container is to weigh it. So every cylinder in service sits on a scale:
A cylinder on its scale
A 150 lb cylinder stamped with a tare of 112 lb reads 182 lb.
A scale that can be zeroed to each cylinder’s own tare reads the chlorine directly, which is why Hydro Instruments suggests a scale for each cylinder in service, set to that cylinder. Weigh at the same point in each round and write down the time: two weights and the hours between them are the cylinder’s real rate, which the next sections need.
Sources: the Chlorine Institute’s Chlorine Basics (Pamphlet 1, edition 8, 2014; PDF), section 3.8.4, and Pamphlet 155, section 6.1.1, which recommends load cells or scales to follow a cylinder’s contents; Hydro Instruments’ manual, section V.
4. How much gas a cylinder can give
Gas leaves a cylinder by boiling off the liquid inside, and boiling takes heat, which the cylinder draws from the air around it. Pull gas faster than the room can warm the cylinder and the liquid cools, its pressure falls, and the feed rate falls with it. Kept up, the lines sweat and frost, and ice can form on the cylinder itself. So each cylinder can give only so much a day, and the figure depends on whose you use:
- The Chlorine Institute: chlorinator makers work on 1 to 1.5 lb a day for each °F of room temperature, or 70 to 105 lb a day at 70°F, with more for short periods. From a ton container, 6 to 8 lb a day per °F, or 420 to 560 lb a day at 70°F.
- Hydro Instruments: around 1 lb a day per °F for a 100 lb or 150 lb cylinder, and 8 lb a day per °F for a ton container.
- Illinois’ design table for wastewater plants: 47.5 lb a day from a 150 lb cylinder at 70°F, and 536 lb from a ton container.
- Oklahoma’s design limit for wastewater plants: 40 lb a day per cylinder up to 150 lb, and 400 lb a day per ton container.
The first two come from the industry and an equipment maker; the last two are what a plant in those states has to be designed for, and they are lower. Where your state sets a figure, your plant was designed to it; otherwise, use your chlorinator maker’s. Illinois’ table, in pounds a day:
| Room | 100 lb | 150 lb | Ton |
|---|---|---|---|
| 40°F | 6 | 9 | 100 |
| 50°F | 14 | 21 | 240 |
| 60°F | 23.7 | 35.5 | 385 |
| 70°F | 32 | 47.5 | 536 |
| 80°F | 41.2 | 62 | 700 |
The rule adds that some vacuum chlorinators deliver more than these rates under the same conditions, and asks for the maker’s figures and test results where a design counts on it.
How many cylinders to draw from at once
A plant feeds 90 lb a day from 150 lb cylinders, in a room kept at 60°F, using the Illinois table.
When one cylinder cannot give enough:
- Draw from several at once through a manifold, all at the same temperature. Gas moves from a warmer cylinder on the same manifold into a cooler one.
- Move the air. A fan blowing past the cylinders is the most effective way to raise what each one gives.
- Never heat a cylinder. The fusible plug in its valve melts at 158–165°F and lets the chlorine out, and Texas prohibits heating blankets on chlorine cylinders outright.
- Let a moved cylinder settle first. One brought from storage into the feed room should be given time to come to the room’s temperature before it goes on line.
- Above 150 lb a day, Oklahoma requires ton containers, and Illinois says they should be considered.
Sources: Pamphlet 155, section 6.3; Hydro Instruments’ manual, sections IV.4 and V; Pamphlet 1, sections 3.3.1 and 3.8.2; and the rules in section 10.
5. How long a cylinder lasts
Cylinders drawing together share the plant’s feed: three on a 90 lb a day feed give 30 lb each. Cylinders that went on line together, full, draw down together and come due together.
The cylinder on the scale
The cylinder from section 3, with 70 lb left, at a plant that feeds 30 lb a day from one cylinder at a time.
The daily figure has to be the plant’s real one. A rate from weigh-ins a few days apart is steadier than one from two weights an hour apart, and the chlorinator’s set point says what it is set to feed, not what left the cylinder.
6. Automatic switchover
An automatic switchover such as Archer Instruments’ SR9 pairs a cylinder on line with one on standby, each with its own vacuum regulator. The standby feeds nothing until the cylinder on line runs out, then takes over by itself, so the feed does not stop when a cylinder empties. Virginia requires one at every waterworks feeding chlorine gas, and Utah wherever disinfection has to be continuous.
Two things follow from it:
- After a switchover, nothing is behind the cylinder on line. Change the empty and set the new cylinder as the standby (on the SR9, with its reset knob), or the next empty stops the feed.
- A cylinder can look empty when it is not. The Chlorine Institute warns that a gauge can read zero while liquid chlorine is still inside, when that liquid is cold: from a cold room, or from gas pulled out too fast. Weigh a cylinder before sending it back as empty.
Sources: Archer Instruments’ Model SR9 Automatic Switchover manual (PDF); Pamphlet 155, section 6.2; and the rules in section 10.
7. Days nobody changes cylinders
A cylinder runs out when its chlorine does, whatever day that is. If it falls on a day nobody is there to change cylinders, change it on the last working day before, with chlorine still in it. Left to run out over the weekend, it hands the feed to the standby with nothing behind it, or, where there is no standby, stops the feed until somebody comes in.
The chlorine left in a cylinder changed early goes back with it. Weigh it as it comes off and keep the figure with your records, or they will count it as used, and the plant’s daily use will read high by that much.
A Friday afternoon
A plant changes cylinders Monday to Friday and feeds 25 lb a day. At 3 p.m. on Friday the cylinder on line has 40 lb left.
8. Changing a cylinder
Follow your chlorinator’s manual and your plant’s written procedure: regulators differ, and what follows is only what the sources below agree on. Before connecting or disconnecting a cylinder, make sure your safety and emergency equipment is at hand and working, and wear the protective equipment your procedure calls for. Use the cylinders that have been on site longest first.
Taking the empty off
- Close the cylinder valve.
- Let the system draw the chlorine out of the line before anything is disconnected. With a vacuum regulator mounted on the cylinder, close the regulator’s own valve and take it off as its maker describes.
- Put the valve outlet cap and the valve hood back on, mark the cylinder empty, and store it with the empties.
Putting the full one on
- Chain or strap it upright on its scale, and note its weight and stamped tare.
- Check that the valve is closed and its packing nut at least hand tight before the outlet cap comes off. A loose packing nut is a call to your supplier.
- Use a new gasket for every connection. Never reuse one.
- Open the valve briefly to put chlorine against the connection, close it, and test for leaks with ammonia vapor: a white cloud shows where the leak is. Fix any leak before going on.
- Open the valve. The Chlorine Institute says one full turn gives a cylinder its full flow; some regulator makers say less, and Archer’s manual calls a third of a turn fully open. Use a wrench no longer than 8 in, never a cheater bar, and leave it on the valve so the valve can be closed quickly.
- Check for leaks again, and set the cylinder as the standby if it is going onto an automatic switchover.
A leak that closing the valve does not stop is a matter for your plant’s emergency plan, the Chlorine Institute’s Emergency Kit A for cylinders, and your supplier.
Sources: Pamphlet 155, sections 6.1.3.1 to 6.1.3.3; Pamphlet 1, sections 3.8.1 and 3.8.5 to 3.8.8 and 5.4.2; Archer Instruments’ SR9 manual; Xylem’s changing gas containers on the V10k; Hydro Instruments’ manual, section V.
9. How many spares to keep
Order while the full cylinders on hand can still cover what the plant uses between the order and the delivery:
A week’s lead time
A plant uses 45 lb a day from 150 lb cylinders, and its supplier delivers 7 days after the order.
A larger order saves deliveries but has to fit what the site is allowed to hold. Local rules can limit it, and past federal threshold quantities a site comes under rules such as EPA’s Risk Management Program and OSHA’s process safety management standard.
Sources: Pamphlet 1, section 3.1.1; Hydro Instruments’ manual, section V.
10. What the rules require
Design rules for water and wastewater plants say how a chlorine room is built and equipped. These are the ones we read for this page, quoted from each. Several states adopt the Ten States Standards instead of writing their own; the storage rules by state say which.
Illinois
“Scales shall be provided for weighing cylinders and containers at all plants using chlorine gas.” “All upright chlorine cylinders shall be strapped securely to prevent tipping.” “The use of 1-ton containers should be considered where the average daily chlorine gas consumption is over 150 pounds.” The table of delivery rates in section 4 is the rule’s own. These are Illinois’ standards for sewage works.
Source: 35 Ill. Adm. Code 370.1020(c), (f)(1) and (f)(3)(A).
Oklahoma
“Provide corrosion-resistant scales to weigh chlorine gas cylinders.” “One-ton containers or larger are required if more than 150 pounds of chlorine per day is needed. Limit the withdrawal rate to 40 pounds per day per cylinder for cylinders up to 150 pounds, and to 400 pounds per day for one-ton cylinders.” “Gaseous chlorine cylinders may be connected to a manifold, only when all cylinders are maintained at the same temperature or the system is designed for gas transfer from a warm container to a cooler one.” These are Oklahoma’s construction standards for wastewater plants.
Source: OAC 252:656-21-2(e), (f) and (h).
Texas
“Heating blankets on chlorine gas cylinders are prohibited.” This is in Texas’ design criteria for domestic wastewater systems, which also work out each cylinder’s withdrawal rate from the temperature it is kept at: for cylinders outside a temperature-controlled room, the coldest seven days in a row over ten years of local weather records.
Source: 30 TAC §217.273(a)(1)(A) and (B).
Utah
“Automatic Switch over of chlorine cylinders shall be provided if continuous disinfection is required.” “Scales shall be accurate enough to indicate loss of weight to the nearest one pound for 150 pound cylinders and to the nearest 10 pounds for one ton cylinders.” These are Utah’s rules for drinking water plants.
Source: Utah Admin. Code R309-520-7(2)(a) and (2)(i).
Virginia
“Scales for weighing cylinders shall be provided at all waterworks using chlorine gas.” “Automatic switch-over of chlorine cylinders shall be provided to assure continuous disinfection.”
Source: 12VAC5-590-1001 B 1 b and d.
11. Keeping track of it
ChemLevel keeps a cylinder rack by weight. On each round you enter the weight of every cylinder in service, and it keeps what is left across the rack and how many full spares are on the floor. A weight more than a full cylinder holds is refused, so a slipped digit cannot read as a swap.
- Changeovers in hours. Once a cylinder has less than a day left, its countdown reads in hours (swap in about 7 hours), and the Dashboard lists the racks due a changeover.
- Changeover days. Set the days someone is there to change cylinders, and a cylinder forecast to run out on any other day is flagged on the last of them before it: Cylinder 1 runs out Sunday — swap it today.
- Gas left in a cylinder changed early. Record a swap on a cylinder that was not empty and the sheet asks what was left in it when it came off, so that chlorine is not counted as used.
- Spares. Set the number of full spares at which it is time to reorder, and the order date follows the spares, not the cylinders on line.
The guide shows each of these on screen, How the numbers work explains the daily rate and the days left, and the demo (opens in a new tab) has a chlorine rack to try them on.