Reverse osmosis net driving pressure
With a feed osmotic pressure of 25 bar and a pump at 55 bar, the net driving pressure is 30 bar, so reverse osmosis can proceed. Enter both pressures in the same unit to see the margin and its status. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Feed osmotic pressure
- 25
- Applied
- 55
- Pi feed
- 25
- Osmotic pressure
- 25
- Pressure
- 55
- P
- 55
A reverse osmosis net driving pressure calculator turns feed osmotic pressure and applied hydraulic pressure into the simplified net driving pressure that decides whether water can be forced through a semi-permeable membrane against osmosis. Enter the feed-side osmotic pressure π_feed and the applied pressure P in the same unit—bar, psi, or atmospheres—and the tool returns NDP equals P minus π_feed, a status label for positive, zero, or insufficient driving force, a formula string, and step-by-step substitution you can paste into design notes, lab notebooks, or automated tests.
What net driving pressure means in reverse osmosis
Reverse osmosis forces solvent from a high-solute feed across a membrane that largely blocks dissolved salts. The simplest engineering check subtracts feed osmotic pressure from the applied feed pressure to obtain a net driving pressure, NDP equals P minus π_feed, in consistent units.
When NDP is positive, the hydraulic head exceeds the osmotic barrier on the feed side and reverse osmosis can proceed under this idealised model; when NDP is zero the system is at equilibrium with no net solvent flux; when NDP is negative the feed osmotic pressure still wins and the simplified model predicts no RO driving force.
The check path recomputes P minus π_feed with the same fixed rounding used for published fields, so a successful response always ships with steps you can assert in tests, design worksheets, or reverse-osmosis training modules about minimum pressure and net driving force.
Practical tips, limits, and related chemistry searches
Keep both pressures in the same unit before calling the tool—mixing bar with psi without conversion will invent a meaningless NDP. Convert or measure feed osmotic pressure first if you only know concentration and temperature; pair this capability with an osmotic-pressure calculator when you start from molarity and van 't Hoff data rather than a known π_feed. Prefer sending numbers rather than pre-formatted strings when you control the client; string numbers are accepted only when they match a strict finite numeric pattern.
Prefer this capability when you already have π_feed and a candidate pump set-point and only need the simplified NDP; if you need permeate-side corrections or concentration polarisation factors, apply those outside this pure arithmetic step.
What you can do with it
Homework checks for RO net driving pressure
Students enter textbook feed osmotic pressure and applied pump pressure, then compare hand-written NDP and status against a deterministic result before submitting water-treatment problems.
First-pass RO pump set-point screening
Quickly screen whether a candidate applied pressure exceeds feed osmotic pressure under the simplified NDP model before running a full membrane design package.
FAQ
What is net driving pressure in reverse osmosis?
In this simplified model, NDP = applied pressure P minus feed osmotic pressure π_feed. A positive value means hydraulic pressure exceeds the feed osmotic barrier.
What happens if applied pressure is negative?
Negative applied pressure is not a valid reverse-osmosis driving force in this calculator.
Does this include permeate pressure or π_permeate?
No. It uses only feed osmotic pressure and applied pressure. Apply permeate-side corrections outside this pure difference if your design needs them.
Is the reverse osmosis pressure calculator free?
The browser tool is free and runs locally.
What if the applied pressure is below the osmotic pressure?
The net driving pressure comes out negative and is labelled as insufficient pressure. It still shows how far the pump set-point sits below the osmotic barrier, so you can see how much more pressure is needed.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/chem/reverse-osmosis-pressure \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"feed_osmotic_pressure":25,"applied_pressure":55}'const res = await fetch("https://api.kit.forhosting.com/chem/reverse-osmosis-pressure", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"feed_osmotic_pressure": 25,
"applied_pressure": 55
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/reverse-osmosis-pressure",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"feed_osmotic_pressure": 25,
"applied_pressure": 55
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/reverse-osmosis-pressure", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"feed_osmotic_pressure":25,"applied_pressure":55}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"feed_osmotic_pressure":25,"applied_pressure":55}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/reverse-osmosis-pressure", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"feed_osmotic_pressure": 25,
"applied_pressure": 55
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.reverse_osmosis_pressure",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Limits
max_abs | 1000000000000 |
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
Applied pressure must be non-negative: a negative P is rejected with a clear invalid_input error because suction-style values are not a valid RO driving force in this model. When NDP is negative the response still succeeds and reports insufficient_pressure so you can see how far the pump set-point sits below the osmotic barrier.
What net driving pressure means in reverse osmosis
Nature opposes that flow with osmotic pressure: pure water would spontaneously dilute the feed until the hydrostatic head balances the chemical potential difference. Real plants refine the picture with permeate pressure, permeate osmotic pressure, concentration polarisation, and friction losses, but homework, first-pass design, and API smoke tests often start from this two-number difference. Teachers and automated graders care about rejecting negative applied pressure as much as the success path: a learner who confuses gauge suction with a valid pump set-point should see an explicit invalid_input message rather than a silent absurd result.
How to call the API and what comes back
Applied pressure must be non-negative; values strictly below zero are rejected. Feed osmotic pressure may be any finite value inside the absolute limit, though non-negative aqueous feeds are the physical norm. Send a JSON body with two required finite numbers: feed_osmotic_pressure (aliases pi_feed, osmotic_pressure, pi, feed_pi, and pi_f) and applied_pressure (aliases P, pressure, feed_pressure, p_applied, and applied_p). Absolute values must stay within the published limit of one trillion so intermediate differences remain finite in IEEE double precision on the Worker and in the browser. On success the response echoes rounded feed_osmotic_pressure and applied_pressure, mirrors them as pi_feed, osmotic_pressure, pressure, and P, sets net_driving_pressure, ndp, and delta_p_net to the same rounded NDP, formula to the string NDP equals P minus π_feed, equation to a filled substitution string, status to one of positive_driving_force, zero_driving_force, or insufficient_pressure, message to a short English summary, and steps to an ordered array of calculation lines. Failed validation—missing fields, non-finite numbers, values outside absolute limits, or negative applied pressure—returns invalid_input and is never charged. Pricing is a flat $0.002 per successful request with unit item and zero marginal unit price.
Practical tips, limits, and related chemistry searches
When teaching process chemistry or water treatment, pair examples: fifty-five bar applied against twenty-five bar feed osmotic pressure yields a positive thirty bar NDP; equal pressures yield zero driving force; twenty bar against thirty bar osmotic pressure yields a negative NDP with status insufficient_pressure; and deliberately send a negative applied pressure to demonstrate the hard invalid_input path. This landing targets people searching for a reverse osmosis minimum pressure calculator, net driving pressure tool, or applied pressure minus feed osmotic pressure homework help online; it is not a full membrane flux model, not a multi-stage energy recovery simulator, and not a unit converter between psi and bar. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when learners deliberately send bad pressures. Round-trip checks are easy: recompute applied pressure minus feed osmotic pressure by hand and confirm it equals the published net_driving_pressure within the same twelve-decimal rounding.
Integration use cases
Unit tests for student membrane calculators
Generate known-good positive, zero, and negative NDP cases plus deliberate negative applied-pressure rejections for coding kata expected values in process-engineering tools.
Technical questions
Is a negative NDP an error?
No. Negative NDP is a successful result with status insufficient_pressure, meaning P is below π_feed under this model.