henrik242/Scuba tank size and buoyancy calculator.md

## Scuba tank size and buoyancy calculator.md

      
    Raw
  

              Scuba tank size and buoyancy calculator.md
            
          
    Scuba tank size and buoyancy calculator, hosted at https://henrik.synth.no/scuba/tanks.html

  
## tanks.html
<html lang="en">
  <head>
    <title>Scuba tank size and buoyancy calculator</title>
    <meta charset="utf-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <script type="text/javascript">
      "use strict";

      let toDec = function (num, fourDigits) {
        if (isNaN(num) || !isFinite(num)) return 0.0;
        if (fourDigits) return parseFloat(num.toFixed(4));
        if (num.toFixed(1).slice(-1) === "0") return parseFloat(num.toFixed(0));
        return parseFloat(num.toFixed(1));
      };

      const BAR_PER_PSI = 0.06895;
      const PSI_PER_ATM = 14.6959;
      const LBS_PER_KG = 2.20462;
      const LITERS_PER_CUFT = 28.31685;
      const KG_LITER_IN_LBS_CUFT = LBS_PER_KG * LITERS_PER_CUFT;
      const STEEL_DENSITY = 7.9; // kg/liter
      const STEEL_DENSITY_IMP = toDec(STEEL_DENSITY * KG_LITER_IN_LBS_CUFT, true); // lbs/cuft
      const ALU_DENSITY = 2.699; // kg/liter
      const ALU_DENSITY_IMP = toDec(ALU_DENSITY * KG_LITER_IN_LBS_CUFT, true); // lbs/cuft
      const AIR_DENSITY = 0.001225; // kg/liter
      const AIR_DENSITY_IMP = toDec(AIR_DENSITY * KG_LITER_IN_LBS_CUFT, true); // lbs/cuft
      const SALT_DENSITY = 1.024; // kg/liter
      const SALT_DENSITY_IMP = toDec(SALT_DENSITY * KG_LITER_IN_LBS_CUFT, true); // lbs/cuft
      const FRESH_DENSITY = 1.0; // kg/liter
      const FRESH_DENSITY_IMP = toDec(FRESH_DENSITY * KG_LITER_IN_LBS_CUFT, true); // lbs/cuft
      const VALVE_WEIGHT = 0.8; // kg

      const METRIC = "metric";
      const IMPERIAL = "imperial";

      let lastunit = METRIC;

      let elem = function (id) {
        return document.getElementById(id);
      };

      let valid = function (val) {
        return !(val.value.slice(-1) === "\." || val.value.slice(-1) === "," || val.value.length === 0);
      };

      let parse = function (elem) {
        let parsed = parseFloat(elem.value.replace(",", ".").replace(/[^0-9.]/, ""));
        if (isNaN(parsed) || parsed.length === 0) return 0;
        return parsed;
      };

      let setval = function (elem, val) {
        if (elem.value !== toDec(val)) {
          elem.value = toDec(val);
        }
      };

      let update = function (unit, message) {
        if (unit) {
          lastunit = unit;
        } else {
          unit = lastunit;
        }
        if (message) {
          elem("message").style.display = "block";
        } else {
          elem("message").style.display = "none";
          elem("predefined").value = "";
        }
        let cuftElem = elem("cuft");
        let psiElem = elem("psi");
        let barElem = elem("bar");
        let litersElem = elem("liters");
        let kgElem = elem("kg");
        let lbsElem = elem("lbs");

        if (!(valid(cuftElem) && valid(psiElem) && valid(barElem) && valid(litersElem) && valid(kgElem) && valid(lbsElem))) {
          return;
        }

        let cuft = parse(cuftElem);
        let psi = parse(psiElem);
        let bar = parse(barElem);
        let liters = parse(litersElem);
        let kg = parse(kgElem);
        let lbs = parse(lbsElem);

        if (unit === METRIC) {
          cuft = (liters / LITERS_PER_CUFT) * bar;
          psi = bar / BAR_PER_PSI;
          lbs = kg * LBS_PER_KG;
        } else {
          liters = (cuft / (psi * BAR_PER_PSI)) * LITERS_PER_CUFT;
          bar = psi * BAR_PER_PSI;
          kg = lbs / LBS_PER_KG;
        }

        setval(cuftElem, cuft);
        setval(psiElem, psi);
        setval(barElem, bar);
        setval(litersElem, liters);
        setval(kgElem, kg);
        setval(lbsElem, lbs);

        if (unit === METRIC) {
          window.location.hash = subst("#kg={0}&liters={1}&bar={2}&salt={3}&alu={4}&valve={5}&doubles={6}", [
            kg,
            liters,
            bar,
            elem("salt").checked,
            elem("alu").checked,
            elem("valve").checked,
            elem("doubles").checked,
          ]);

          buoyancyMetric(kg, liters, bar);
        } else {
          window.location.hash = subst("#lbs={0}&cuft={1}&psi={2}&salt={3}&alu={4}&valve={5}&doubles={6}", [
            lbs,
            cuft,
            psi,
            elem("salt").checked,
            elem("alu").checked,
            elem("valve").checked,
            elem("doubles").checked,
          ]);

          buoyancyImperial(lbs, cuft, psi);
        }
      };

      let buoyancyMetric = function (kg, liters, bar) {
        let metal = elem("alu").checked ? ALU_DENSITY : STEEL_DENSITY;
        let water = elem("salt").checked ? SALT_DENSITY : FRESH_DENSITY;
        let valve = elem("valve").checked ? VALVE_WEIGHT : 0;
        if (elem("doubles").checked) {
          valve = valve * 2;
          liters = liters * 2;
          kg = kg * 2;
        }
        let volMetal = kg / metal;
        let volValve = valve / STEEL_DENSITY;
        let volume = (volMetal + volValve + liters) * water;
        let air = AIR_DENSITY * bar * liters;
        let empty = volume - kg - valve;
        let full = volume - kg - valve - air;

        elem("fullempty").value = toDec(empty) + "/" + toDec(full);
        elem("fullemptylbs").value = toDec(empty * LBS_PER_KG) + "/" + toDec(full * LBS_PER_KG);

        let txt = subst("<br><br>Steel has a density of {0} kg/liter", [STEEL_DENSITY, true]);
        if (metal !== STEEL_DENSITY) {
          txt += subst(", and aluminium is {0} kg/liter", [ALU_DENSITY]);
        }
        txt += subst("<br>The volume of the tank metal is {0} kg / {1} = <b>{2} liters</b><br>", [kg, metal, toDec(volMetal, 1)]);
        let plusValve = "";
        let minusValve = "";
        if (valve !== 0) {
          txt += subst("The volume of the valve is {0} kg / {1} = <b>{2} liters</b><br>", [
            toDec(valve, 1),
            STEEL_DENSITY,
            toDec(volValve, 1),
          ]);
          plusValve = " + " + toDec(volValve, 1);
          minusValve = " - " + toDec(valve, 1);
        }
        if (water === SALT_DENSITY) {
          txt += subst("The density of salt water is {0} kg/liter<br>", [SALT_DENSITY]);
        } else {
          txt += subst("The density of fresh water is {0} kg/liter<br>", [FRESH_DENSITY]);
        }
        txt +=
          subst("Total weight in water: ({0} + {1} {2}) x {3} = <b>{4} kg</b><br>", [
            liters,
            toDec(volMetal, 1),
            plusValve,
            water,
            toDec(volume, 1),
          ]) +
          subst("Air has a density of {0} kg/liter. <br>The air in a full tank weighs {1} x {2} liters x {3} bar = <b>{4} kg</b><br>", [
            AIR_DENSITY,
            AIR_DENSITY,
            liters,
            bar,
            toDec(air, 1),
          ]) +
          subst("Tank buoyancy when empty: {0} - {1} {2} = <b>{3} kg</b><br>", [toDec(volume, 1), kg, minusValve, toDec(empty)]) +
          subst("Tank buoyancy when full: {0} - {1} {2} - {3} = <b>{4} kg</b><br>", [
            toDec(volume, 1),
            kg,
            minusValve,
            toDec(air, 1),
            toDec(full),
          ]);

        elem("calculation").innerHTML = txt;
      };

      let buoyancyImperial = function (lbs, cuft, psi) {
        let metal = elem("alu").checked ? ALU_DENSITY_IMP : STEEL_DENSITY_IMP;
        let water = elem("salt").checked ? SALT_DENSITY_IMP : FRESH_DENSITY_IMP;
        let valve = elem("valve").checked ? VALVE_WEIGHT * LBS_PER_KG : 0;

        if (elem("doubles").checked) {
          valve = valve * 2;
          cuft = cuft * 2;
          lbs = lbs * 2;
        }
        let volInner = (cuft / psi) * PSI_PER_ATM;
        let volMetal = lbs / metal;
        let volValve = valve / STEEL_DENSITY_IMP;
        let volume = (volInner + volMetal + volValve) * water;
        let air = AIR_DENSITY_IMP * cuft;
        let empty = volume - lbs - valve;
        let full = volume - lbs - valve - air;

        elem("fullempty").value = toDec(empty / LBS_PER_KG) + "/" + toDec(full / LBS_PER_KG);
        elem("fullemptylbs").value = toDec(empty) + "/" + toDec(full);

        let txt =
          subst("<br><br>Air has a pressure of {0} psi at 1 ATM.<br> Tank inner volume is {1} cuft / {2} psi x {3} = <b>{4} cuft</b><br>", [
            PSI_PER_ATM,
            cuft,
            psi,
            PSI_PER_ATM,
            toDec(volInner, 1),
          ]) + subst("Steel has a density of {0} lbs/cuft", [STEEL_DENSITY_IMP]);
        if (metal !== STEEL_DENSITY_IMP) {
          txt += subst(", and aluminium is {0} lbs/cuft", [ALU_DENSITY_IMP]);
        }
        txt += subst("<br>The volume of the tank metal is {0} lbs / {1} = <b>{2} cuft</b><br>", [lbs, metal, toDec(volMetal, 1)]);
        let plusValve = "";
        let minusValve = "";
        if (valve !== 0) {
          txt += subst("The volume of the valve is {0} lbs / {1} = <b>{2} cuft</b><br>", [
            toDec(valve, 1),
            STEEL_DENSITY_IMP,
            toDec(volValve, 1),
          ]);
          plusValve = " + " + toDec(volValve, 1);
          minusValve = " - " + toDec(valve, 1);
        }
        if (water === SALT_DENSITY_IMP) {
          txt += subst("The density of salt water is {0} lbs/cuft<br>", [SALT_DENSITY_IMP]);
        } else {
          txt += subst("The density of fresh water is {0} lbs/cuft<br>", [FRESH_DENSITY_IMP]);
        }
        txt +=
          subst("Total weight in water: ({0} + {1} {2}) x {3} = <b>{4} lbs</b><br>", [
            toDec(volInner, 1),
            toDec(volMetal, 1),
            plusValve,
            water,
            toDec(volume, 1),
          ]) +
          subst("Air has a density of {0} lbs/cuft. <br>The air in a full tank weighs {1} x {2} cuft = <b>{3} lbs</b><br>", [
            AIR_DENSITY_IMP,
            AIR_DENSITY_IMP,
            cuft,
            toDec(air, 1),
          ]) +
          subst("Tank buoyancy when empty: {0} - {1} {2} = <b>{3} lbs</b><br>", [toDec(volume, 1), lbs, minusValve, toDec(empty)]) +
          subst("Tank buoyancy when full: {0} - {1} {2} - {3} = <b>{4} lbs</b><br>", [
            toDec(volume, 1),
            lbs,
            minusValve,
            toDec(air, 1),
            toDec(full),
          ]);

        elem("calculation").innerHTML = txt;
      };

      let subst = function (str, arr) {
        let i,
          pattern,
          re,
          n = arr.length;
        for (i = 0; i < n; i++) {
          pattern = "\\{" + i + "\\}";
          re = new RegExp(pattern, "g");
          str = str.replace(re, arr[i]);
        }
        return str;
      };

      let showHide = function () {
        if (elem("calculation").style.display === "inline") {
          elem("calculation").style.display = "none";
          elem("showhide").innerHTML = "Show calculation";
        } else {
          elem("calculation").style.display = "inline";
          elem("showhide").innerHTML = "Hide calculation";
        }
        elem("showhide").href = window.location.href;
      };

      let showHelp = function () {
        if (elem("help").style.display === "inline") {
          elem("help").style.display = "none";
        } else {
          elem("help").style.display = "inline";
        }
      };

      let metric = function (liters, bar, kg, alu, doubles) {
        elem("bar").value = bar;
        elem("liters").value = liters;
        elem("kg").value = kg;
        elem("alu").checked = alu === "1";
        elem("doubles").checked = doubles === "1";
        update(METRIC, liters !== "0" || bar !== "0" || kg !== "0" || alu !== "0" || doubles !== "0");
      };

      let imperial = function (cuft, psi, lbs, alu, doubles) {
        elem("cuft").value = cuft;
        elem("psi").value = psi;
        elem("lbs").value = lbs;
        elem("alu").checked = alu === "1";
        elem("doubles").checked = doubles === "1";
        update(IMPERIAL, cuft !== "0" || psi !== "0" || lbs !== "0" || alu !== "0" || doubles !== "0");
      };

      let selectTank = function () {
        let p = elem("predefined").value.split(";");
        switch (p[0]) {
          case METRIC:
            return metric(p[1], p[2], p[3], p[4], p[5]);
          case IMPERIAL:
            return imperial(p[1], p[2], p[3], p[4], p[5]);
          default:
            update();
        }
      };

      let parseHash = function (unit) {
        let parsed = window.location.hash.match(new RegExp(unit + "=([0-9.]+)"));
        if (parsed && parsed.length > 0 && !isNaN(parsed[1]) && isFinite(parsed[1])) {
          elem(unit).value = parsed[1];
        }
      };

      let parseHashCheckbox = function (unit) {
        let parsed = window.location.hash.match(new RegExp(unit + "=([a-z]+)"));
        if (parsed && parsed.length > 0) {
          elem(unit).checked = parsed[1] === "true";
        }
      };

      window.onload = function () {
        parseHashCheckbox("salt");
        parseHashCheckbox("alu");
        parseHashCheckbox("valve");
        parseHashCheckbox("doubles");
        if (window.location.hash.match(/lbs/)) {
          parseHash("lbs");
          parseHash("psi");
          parseHash("cuft");
          update(IMPERIAL);
        } else {
          parseHash("kg");
          parseHash("bar");
          parseHash("liters");
          update(METRIC);
        }
      };
    </script>
    <style>
      body {
        font-family: Arial, Helvetica, sans-serif;
      }
      span,
      th {
        white-space: nowrap;
      }
      input,
      td {
        text-align: center;
        font-size: 1.2em;
        border-radius: 5px;
      }
      select {
        background-color: #eee;
        font-size: 1em;
        font-weight: normal;
      }
      th,
      .small {
        font-size: 0.8em;
        font-weight: normal;
      }
      table {
        width: 400px;
      }
      b {
        color: blue;
        font-weight: normal;
      }
      #calculation,
      #message {
        display: none;
      }
      #message {
        font-size: 0.7em;
        color: red;
        padding-bottom: 10px;
      }
      #fullempty,
      #fullemptylbs {
        border: none;
        font-size: 1.1em;
      }
      .check {
        background-color: #eee;
        padding: 1px 6px 2px 2px;
        margin: 2px;
        font-size: 0.8em;
        font-weight: normal;
        border-radius: 5px;
      }
      #checkboxes {
        margin: 10px 1px 15px 1px;
      }
      .circle {
        border: 0.1em solid grey;
        border-radius: 100%;
        height: 1.8em;
        width: 1.8em;
        text-align: center;
      }
      .circle p {
        margin-top: 0.1em;
        font-size: 1.3em;
        font-weight: bold;
        font-family: sans-serif;
        color: grey;
      }
      #help {
        display: none;
      }
      .bottom {
        display: flex;
        align-items: center;
        width: 100%;
        margin-top: 10px;
        margin-bottom: 20px;
      }
      .expand {
        flex: 1;
      }
      #predefined {
        margin-bottom: 10px;
      }
    </style>
  </head>
  <body>
    <form>
      <table>
        <tr>
          <td colspan="4">Scuba tank size and buoyancy calculator</td>
        </tr>
        <tr>
          <td><br /></td>
        </tr>
        <tr>
          <th title="Tank size in liters">liters</th>
          <th title="Tank pressure in bar">bar</th>
          <th title="Tank weight in kilograms">kg</th>
          <th title="Tank buoyancy in kilograms">empty/full kg</th>
        </tr>
        <tr>
          <td><input size="5" class="right" id="liters" onKeyUp="update('metric')" value="0" title="Tank size in liters" /></td>
          <td><input size="6" class="right" id="bar" onKeyUp="update('metric')" value="0" title="Tank pressure in bar" /></td>
          <td><input size="4" class="right" id="kg" onKeyUp="update('metric')" value="0" title="Tank weight in kilograms" /></td>
          <td><input size="10" id="fullempty" readonly title="Tank buoyancy in kilograms" value="0/0" /></td>
        </tr>
        <tr>
          <td><input size="5" class="right" id="cuft" onKeyUp="update('imperial')" value="0" title="Tank size in cubic feet" /></td>
          <td><input size="6" class="right" id="psi" onKeyUp="update('imperial')" value="0" title="Tank pressure in psi" /></td>
          <td><input size="4" class="right" id="lbs" onKeyUp="update('imperial')" value="0" title="Tank weight in pounds" /></td>
          <td><input size="10" id="fullemptylbs" readonly title="Tank buoyancy in pounds" value="0/0" /></td>
        </tr>
        <tr>
          <th title="Tank size in cubic feet">cuft</th>
          <th title="Tank pressure in psi">psi</th>
          <th title="Tank weight in lbs">lbs</th>
          <th title="Tank buoyancy in pounds">empty/full lbs</th>
        </tr>
        <tr>
          <td colspan="4">
            <div id="checkboxes">
              <span title="Diving in salt or fresh water" class="check"
                ><input id="salt" onchange="update()" type="checkbox" checked /><label for="salt"> salt</label></span
              >
              <span title="Double tank" class="check"
                ><input id="doubles" onchange="update()" type="checkbox" /><label for="doubles"> doubles</label></span
              >
              <span title="Aluminium or steel tank" class="check"
                ><input id="alu" onchange="update()" type="checkbox" /><label for="alu"> alu</label></span
              >
              <span title="Calculation with valve weight" class="check"
                ><input id="valve" onchange="update()" type="checkbox" checked /><label for="valve"> valve</label></span
              >
            </div>
            <div id="message">
              This predefined tank might be different from your tank! Check the neck of your tank for real weight and capacity.
            </div>
            <label>
              <select id="predefined" onchange="selectTank()">
                <option value="">select predefined tank</option>
                <option value="metric;0;0;0;0;0">reset tank values</option>
                <option value="imperial;77.4;3000;32;1;0">AL80</option>
                <option value="imperial;80;2640;26.5;0;0">LP80</option>
                <option value="imperial;80;3442;30;0;0">HP80</option>
                <option value="imperial;104;2640;46.4;0;0">LP104</option>
                <option value="imperial;100;3442;33;0;0">HP100</option>
                <option value="metric;10;300;15.4;0;0">10L / 300 bar</option>
                <option value="metric;12;200;14;0;0">12L / 200 bar</option>
                <option value="metric;15;232;16;0;0">15L / 232 bar</option>
                <option value="metric;7;300;10.7;0;1">D7 / 300 bar</option>
                <option value="metric;8.5;232;10.3;0;1">D8.5 / 232 bar</option>
                <option value="metric;12;232;13.8;0;1">D12 / 232 bar</option>
              </select>
            </label>
          </td>
        </tr>
        <tr>
          <td colspan="4" class="small">
            <a id="showhide" onclick="showHide()" href="#">Show calculation</a>
            <div id="calculation"></div>
            <div class="bottom">
              <div title="Source code available at GitHub">
                <a href="https://gist.github.com/henrik242/f0c5879b1ea74d00764b506ac90fe86d">
                  <img alt="" src="https://github.githubassets.com/images/modules/site/icons/footer/github-mark.svg" />
                </a>
              </div>
              <div class="expand"></div>
              <div title="This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License">
                <a rel="license" href="https://creativecommons.org/licenses/by-sa/3.0/">
                  <img
                    alt="This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License"
                    style="border-width: 0"
                    src="https://i.creativecommons.org/l/by-sa/3.0/80x15.png"
                  />
                </a>
              </div>
              <div class="expand"></div>
              <div class="circle" onclick="showHelp()" title="Show help">
                <p>?</p>
              </div>
            </div>
            <p id="help">
              This calculator converts between metric and imperial values, and calculates tank buoyancy. Type the values manually or select
              from the pre-defined list of common tanks.
            </p>
          </td>
        </tr>
      </table>
    </form>
  </body>
</html>
	<html lang="en">
	<head>
	<title>Scuba tank size and buoyancy calculator</title>
	<meta charset="utf-8" />
	<meta name="viewport" content="width=device-width, initial-scale=1.0" />
	<script type="text/javascript">
	"use strict";

	let toDec = function (num, fourDigits) {
	if (isNaN(num) \|\| !isFinite(num)) return 0.0;
	if (fourDigits) return parseFloat(num.toFixed(4));
	if (num.toFixed(1).slice(-1) === "0") return parseFloat(num.toFixed(0));
	return parseFloat(num.toFixed(1));
	};

	const BAR_PER_PSI = 0.06895;
	const PSI_PER_ATM = 14.6959;
	const LBS_PER_KG = 2.20462;
	const LITERS_PER_CUFT = 28.31685;
	const KG_LITER_IN_LBS_CUFT = LBS_PER_KG * LITERS_PER_CUFT;
	const STEEL_DENSITY = 7.9; // kg/liter
	const STEEL_DENSITY_IMP = toDec(STEEL_DENSITY * KG_LITER_IN_LBS_CUFT, true); // lbs/cuft
	const ALU_DENSITY = 2.699; // kg/liter
	const ALU_DENSITY_IMP = toDec(ALU_DENSITY * KG_LITER_IN_LBS_CUFT, true); // lbs/cuft
	const AIR_DENSITY = 0.001225; // kg/liter
	const AIR_DENSITY_IMP = toDec(AIR_DENSITY * KG_LITER_IN_LBS_CUFT, true); // lbs/cuft
	const SALT_DENSITY = 1.024; // kg/liter
	const SALT_DENSITY_IMP = toDec(SALT_DENSITY * KG_LITER_IN_LBS_CUFT, true); // lbs/cuft
	const FRESH_DENSITY = 1.0; // kg/liter
	const FRESH_DENSITY_IMP = toDec(FRESH_DENSITY * KG_LITER_IN_LBS_CUFT, true); // lbs/cuft
	const VALVE_WEIGHT = 0.8; // kg

	const METRIC = "metric";
	const IMPERIAL = "imperial";

	let lastunit = METRIC;

	let elem = function (id) {
	return document.getElementById(id);
	};

	let valid = function (val) {
	return !(val.value.slice(-1) === "\." \|\| val.value.slice(-1) === "," \|\| val.value.length === 0);
	};

	let parse = function (elem) {
	let parsed = parseFloat(elem.value.replace(",", ".").replace(/[^0-9.]/, ""));
	if (isNaN(parsed) \|\| parsed.length === 0) return 0;
	return parsed;
	};

	let setval = function (elem, val) {
	if (elem.value !== toDec(val)) {
	elem.value = toDec(val);
	}
	};

	let update = function (unit, message) {
	if (unit) {
	lastunit = unit;
	} else {
	unit = lastunit;
	}
	if (message) {
	elem("message").style.display = "block";
	} else {
	elem("message").style.display = "none";
	elem("predefined").value = "";
	}
	let cuftElem = elem("cuft");
	let psiElem = elem("psi");
	let barElem = elem("bar");
	let litersElem = elem("liters");
	let kgElem = elem("kg");
	let lbsElem = elem("lbs");

	if (!(valid(cuftElem) && valid(psiElem) && valid(barElem) && valid(litersElem) && valid(kgElem) && valid(lbsElem))) {
	return;
	}

	let cuft = parse(cuftElem);
	let psi = parse(psiElem);
	let bar = parse(barElem);
	let liters = parse(litersElem);
	let kg = parse(kgElem);
	let lbs = parse(lbsElem);

	if (unit === METRIC) {
	cuft = (liters / LITERS_PER_CUFT) * bar;
	psi = bar / BAR_PER_PSI;
	lbs = kg * LBS_PER_KG;
	} else {
	liters = (cuft / (psi * BAR_PER_PSI)) * LITERS_PER_CUFT;
	bar = psi * BAR_PER_PSI;
	kg = lbs / LBS_PER_KG;
	}

	setval(cuftElem, cuft);
	setval(psiElem, psi);
	setval(barElem, bar);
	setval(litersElem, liters);
	setval(kgElem, kg);
	setval(lbsElem, lbs);

	if (unit === METRIC) {
	window.location.hash = subst("#kg={0}&liters={1}&bar={2}&salt={3}&alu={4}&valve={5}&doubles={6}", [
	kg,
	liters,
	bar,
	elem("salt").checked,
	elem("alu").checked,
	elem("valve").checked,
	elem("doubles").checked,
	]);

	buoyancyMetric(kg, liters, bar);
	} else {
	window.location.hash = subst("#lbs={0}&cuft={1}&psi={2}&salt={3}&alu={4}&valve={5}&doubles={6}", [
	lbs,
	cuft,
	psi,
	elem("salt").checked,
	elem("alu").checked,
	elem("valve").checked,
	elem("doubles").checked,
	]);

	buoyancyImperial(lbs, cuft, psi);
	}
	};

	let buoyancyMetric = function (kg, liters, bar) {
	let metal = elem("alu").checked ? ALU_DENSITY : STEEL_DENSITY;
	let water = elem("salt").checked ? SALT_DENSITY : FRESH_DENSITY;
	let valve = elem("valve").checked ? VALVE_WEIGHT : 0;
	if (elem("doubles").checked) {
	valve = valve * 2;
	liters = liters * 2;
	kg = kg * 2;
	}
	let volMetal = kg / metal;
	let volValve = valve / STEEL_DENSITY;
	let volume = (volMetal + volValve + liters) * water;
	let air = AIR_DENSITY * bar * liters;
	let empty = volume - kg - valve;
	let full = volume - kg - valve - air;

	elem("fullempty").value = toDec(empty) + "/" + toDec(full);
	elem("fullemptylbs").value = toDec(empty * LBS_PER_KG) + "/" + toDec(full * LBS_PER_KG);

	let txt = subst("<br><br>Steel has a density of {0} kg/liter", [STEEL_DENSITY, true]);
	if (metal !== STEEL_DENSITY) {
	txt += subst(", and aluminium is {0} kg/liter", [ALU_DENSITY]);
	}
	txt += subst("<br>The volume of the tank metal is {0} kg / {1} = <b>{2} liters</b><br>", [kg, metal, toDec(volMetal, 1)]);
	let plusValve = "";
	let minusValve = "";
	if (valve !== 0) {
	txt += subst("The volume of the valve is {0} kg / {1} = <b>{2} liters</b><br>", [
	toDec(valve, 1),
	STEEL_DENSITY,
	toDec(volValve, 1),
	]);
	plusValve = " + " + toDec(volValve, 1);
	minusValve = " - " + toDec(valve, 1);
	}
	if (water === SALT_DENSITY) {
	txt += subst("The density of salt water is {0} kg/liter<br>", [SALT_DENSITY]);
	} else {
	txt += subst("The density of fresh water is {0} kg/liter<br>", [FRESH_DENSITY]);
	}
	txt +=
	subst("Total weight in water: ({0} + {1} {2}) x {3} = <b>{4} kg</b><br>", [
	liters,
	toDec(volMetal, 1),
	plusValve,
	water,
	toDec(volume, 1),
	]) +
	subst("Air has a density of {0} kg/liter. <br>The air in a full tank weighs {1} x {2} liters x {3} bar = <b>{4} kg</b><br>", [
	AIR_DENSITY,
	AIR_DENSITY,
	liters,
	bar,
	toDec(air, 1),
	]) +
	subst("Tank buoyancy when empty: {0} - {1} {2} = <b>{3} kg</b><br>", [toDec(volume, 1), kg, minusValve, toDec(empty)]) +
	subst("Tank buoyancy when full: {0} - {1} {2} - {3} = <b>{4} kg</b><br>", [
	toDec(volume, 1),
	kg,
	minusValve,
	toDec(air, 1),
	toDec(full),
	]);

	elem("calculation").innerHTML = txt;
	};

	let buoyancyImperial = function (lbs, cuft, psi) {
	let metal = elem("alu").checked ? ALU_DENSITY_IMP : STEEL_DENSITY_IMP;
	let water = elem("salt").checked ? SALT_DENSITY_IMP : FRESH_DENSITY_IMP;
	let valve = elem("valve").checked ? VALVE_WEIGHT * LBS_PER_KG : 0;

	if (elem("doubles").checked) {
	valve = valve * 2;
	cuft = cuft * 2;
	lbs = lbs * 2;
	}
	let volInner = (cuft / psi) * PSI_PER_ATM;
	let volMetal = lbs / metal;
	let volValve = valve / STEEL_DENSITY_IMP;
	let volume = (volInner + volMetal + volValve) * water;
	let air = AIR_DENSITY_IMP * cuft;
	let empty = volume - lbs - valve;
	let full = volume - lbs - valve - air;

	elem("fullempty").value = toDec(empty / LBS_PER_KG) + "/" + toDec(full / LBS_PER_KG);
	elem("fullemptylbs").value = toDec(empty) + "/" + toDec(full);

	let txt =
	subst("<br><br>Air has a pressure of {0} psi at 1 ATM.<br> Tank inner volume is {1} cuft / {2} psi x {3} = <b>{4} cuft</b><br>", [
	PSI_PER_ATM,
	cuft,
	psi,
	PSI_PER_ATM,
	toDec(volInner, 1),
	]) + subst("Steel has a density of {0} lbs/cuft", [STEEL_DENSITY_IMP]);
	if (metal !== STEEL_DENSITY_IMP) {
	txt += subst(", and aluminium is {0} lbs/cuft", [ALU_DENSITY_IMP]);
	}
	txt += subst("<br>The volume of the tank metal is {0} lbs / {1} = <b>{2} cuft</b><br>", [lbs, metal, toDec(volMetal, 1)]);
	let plusValve = "";
	let minusValve = "";
	if (valve !== 0) {
	txt += subst("The volume of the valve is {0} lbs / {1} = <b>{2} cuft</b><br>", [
	toDec(valve, 1),
	STEEL_DENSITY_IMP,
	toDec(volValve, 1),
	]);
	plusValve = " + " + toDec(volValve, 1);
	minusValve = " - " + toDec(valve, 1);
	}
	if (water === SALT_DENSITY_IMP) {
	txt += subst("The density of salt water is {0} lbs/cuft<br>", [SALT_DENSITY_IMP]);
	} else {
	txt += subst("The density of fresh water is {0} lbs/cuft<br>", [FRESH_DENSITY_IMP]);
	}
	txt +=
	subst("Total weight in water: ({0} + {1} {2}) x {3} = <b>{4} lbs</b><br>", [
	toDec(volInner, 1),
	toDec(volMetal, 1),
	plusValve,
	water,
	toDec(volume, 1),
	]) +
	subst("Air has a density of {0} lbs/cuft. <br>The air in a full tank weighs {1} x {2} cuft = <b>{3} lbs</b><br>", [
	AIR_DENSITY_IMP,
	AIR_DENSITY_IMP,
	cuft,
	toDec(air, 1),
	]) +
	subst("Tank buoyancy when empty: {0} - {1} {2} = <b>{3} lbs</b><br>", [toDec(volume, 1), lbs, minusValve, toDec(empty)]) +
	subst("Tank buoyancy when full: {0} - {1} {2} - {3} = <b>{4} lbs</b><br>", [
	toDec(volume, 1),
	lbs,
	minusValve,
	toDec(air, 1),
	toDec(full),
	]);

	elem("calculation").innerHTML = txt;
	};

	let subst = function (str, arr) {
	let i,
	pattern,
	re,
	n = arr.length;
	for (i = 0; i < n; i++) {
	pattern = "\\{" + i + "\\}";
	re = new RegExp(pattern, "g");
	str = str.replace(re, arr[i]);
	}
	return str;
	};

	let showHide = function () {
	if (elem("calculation").style.display === "inline") {
	elem("calculation").style.display = "none";
	elem("showhide").innerHTML = "Show calculation";
	} else {
	elem("calculation").style.display = "inline";
	elem("showhide").innerHTML = "Hide calculation";
	}
	elem("showhide").href = window.location.href;
	};

	let showHelp = function () {
	if (elem("help").style.display === "inline") {
	elem("help").style.display = "none";
	} else {
	elem("help").style.display = "inline";
	}
	};

	let metric = function (liters, bar, kg, alu, doubles) {
	elem("bar").value = bar;
	elem("liters").value = liters;
	elem("kg").value = kg;
	elem("alu").checked = alu === "1";
	elem("doubles").checked = doubles === "1";
	update(METRIC, liters !== "0" \|\| bar !== "0" \|\| kg !== "0" \|\| alu !== "0" \|\| doubles !== "0");
	};

	let imperial = function (cuft, psi, lbs, alu, doubles) {
	elem("cuft").value = cuft;
	elem("psi").value = psi;
	elem("lbs").value = lbs;
	elem("alu").checked = alu === "1";
	elem("doubles").checked = doubles === "1";
	update(IMPERIAL, cuft !== "0" \|\| psi !== "0" \|\| lbs !== "0" \|\| alu !== "0" \|\| doubles !== "0");
	};

	let selectTank = function () {
	let p = elem("predefined").value.split(";");
	switch (p[0]) {
	case METRIC:
	return metric(p[1], p[2], p[3], p[4], p[5]);
	case IMPERIAL:
	return imperial(p[1], p[2], p[3], p[4], p[5]);
	default:
	update();
	}
	};

	let parseHash = function (unit) {
	let parsed = window.location.hash.match(new RegExp(unit + "=([0-9.]+)"));
	if (parsed && parsed.length > 0 && !isNaN(parsed[1]) && isFinite(parsed[1])) {
	elem(unit).value = parsed[1];
	}
	};

	let parseHashCheckbox = function (unit) {
	let parsed = window.location.hash.match(new RegExp(unit + "=([a-z]+)"));
	if (parsed && parsed.length > 0) {
	elem(unit).checked = parsed[1] === "true";
	}
	};

	window.onload = function () {
	parseHashCheckbox("salt");
	parseHashCheckbox("alu");
	parseHashCheckbox("valve");
	parseHashCheckbox("doubles");
	if (window.location.hash.match(/lbs/)) {
	parseHash("lbs");
	parseHash("psi");
	parseHash("cuft");
	update(IMPERIAL);
	} else {
	parseHash("kg");
	parseHash("bar");
	parseHash("liters");
	update(METRIC);
	}
	};
	</script>
	<style>
	body {
	font-family: Arial, Helvetica, sans-serif;
	}
	span,
	th {
	white-space: nowrap;
	}
	input,
	td {
	text-align: center;
	font-size: 1.2em;
	border-radius: 5px;
	}
	select {
	background-color: #eee;
	font-size: 1em;
	font-weight: normal;
	}
	th,
	.small {
	font-size: 0.8em;
	font-weight: normal;
	}
	table {
	width: 400px;
	}
	b {
	color: blue;
	font-weight: normal;
	}
	#calculation,
	#message {
	display: none;
	}
	#message {
	font-size: 0.7em;
	color: red;
	padding-bottom: 10px;
	}
	#fullempty,
	#fullemptylbs {
	border: none;
	font-size: 1.1em;
	}
	.check {
	background-color: #eee;
	padding: 1px 6px 2px 2px;
	margin: 2px;
	font-size: 0.8em;
	font-weight: normal;
	border-radius: 5px;
	}
	#checkboxes {
	margin: 10px 1px 15px 1px;
	}
	.circle {
	border: 0.1em solid grey;
	border-radius: 100%;
	height: 1.8em;
	width: 1.8em;
	text-align: center;
	}
	.circle p {
	margin-top: 0.1em;
	font-size: 1.3em;
	font-weight: bold;
	font-family: sans-serif;
	color: grey;
	}
	#help {
	display: none;
	}
	.bottom {
	display: flex;
	align-items: center;
	width: 100%;
	margin-top: 10px;
	margin-bottom: 20px;
	}
	.expand {
	flex: 1;
	}
	#predefined {
	margin-bottom: 10px;
	}
	</style>
	</head>
	<body>
	<form>
	<table>
	<tr>
	<td colspan="4">Scuba tank size and buoyancy calculator</td>
	</tr>
	<tr>
	<td><br /></td>
	</tr>
	<tr>
	<th title="Tank size in liters">liters</th>
	<th title="Tank pressure in bar">bar</th>
	<th title="Tank weight in kilograms">kg</th>
	<th title="Tank buoyancy in kilograms">empty/full kg</th>
	</tr>
	<tr>
	<td><input size="5" class="right" id="liters" onKeyUp="update('metric')" value="0" title="Tank size in liters" /></td>
	<td><input size="6" class="right" id="bar" onKeyUp="update('metric')" value="0" title="Tank pressure in bar" /></td>
	<td><input size="4" class="right" id="kg" onKeyUp="update('metric')" value="0" title="Tank weight in kilograms" /></td>
	<td><input size="10" id="fullempty" readonly title="Tank buoyancy in kilograms" value="0/0" /></td>
	</tr>
	<tr>
	<td><input size="5" class="right" id="cuft" onKeyUp="update('imperial')" value="0" title="Tank size in cubic feet" /></td>
	<td><input size="6" class="right" id="psi" onKeyUp="update('imperial')" value="0" title="Tank pressure in psi" /></td>
	<td><input size="4" class="right" id="lbs" onKeyUp="update('imperial')" value="0" title="Tank weight in pounds" /></td>
	<td><input size="10" id="fullemptylbs" readonly title="Tank buoyancy in pounds" value="0/0" /></td>
	</tr>
	<tr>
	<th title="Tank size in cubic feet">cuft</th>
	<th title="Tank pressure in psi">psi</th>
	<th title="Tank weight in lbs">lbs</th>
	<th title="Tank buoyancy in pounds">empty/full lbs</th>
	</tr>
	<tr>
	<td colspan="4">
	<div id="checkboxes">
	<span title="Diving in salt or fresh water" class="check"
	><input id="salt" onchange="update()" type="checkbox" checked /><label for="salt"> salt</label></span
	>
	<span title="Double tank" class="check"
	><input id="doubles" onchange="update()" type="checkbox" /><label for="doubles"> doubles</label></span
	>
	<span title="Aluminium or steel tank" class="check"
	><input id="alu" onchange="update()" type="checkbox" /><label for="alu"> alu</label></span
	>
	<span title="Calculation with valve weight" class="check"
	><input id="valve" onchange="update()" type="checkbox" checked /><label for="valve"> valve</label></span
	>
	</div>
	<div id="message">
	This predefined tank might be different from your tank! Check the neck of your tank for real weight and capacity.
	</div>
	<label>
	<select id="predefined" onchange="selectTank()">
	<option value="">select predefined tank</option>
	<option value="metric;0;0;0;0;0">reset tank values</option>
	<option value="imperial;77.4;3000;32;1;0">AL80</option>
	<option value="imperial;80;2640;26.5;0;0">LP80</option>
	<option value="imperial;80;3442;30;0;0">HP80</option>
	<option value="imperial;104;2640;46.4;0;0">LP104</option>
	<option value="imperial;100;3442;33;0;0">HP100</option>
	<option value="metric;10;300;15.4;0;0">10L / 300 bar</option>
	<option value="metric;12;200;14;0;0">12L / 200 bar</option>
	<option value="metric;15;232;16;0;0">15L / 232 bar</option>
	<option value="metric;7;300;10.7;0;1">D7 / 300 bar</option>
	<option value="metric;8.5;232;10.3;0;1">D8.5 / 232 bar</option>
	<option value="metric;12;232;13.8;0;1">D12 / 232 bar</option>
	</select>
	</label>
	</td>
	</tr>
	<tr>
	<td colspan="4" class="small">
	<a id="showhide" onclick="showHide()" href="#">Show calculation</a>
	<div id="calculation"></div>
	<div class="bottom">
	<div title="Source code available at GitHub">
	<a href="https://gist.github.com/henrik242/f0c5879b1ea74d00764b506ac90fe86d">
	<img alt="" src="https://github.githubassets.com/images/modules/site/icons/footer/github-mark.svg" />
	</a>
	</div>
	<div class="expand"></div>
	<div title="This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License">
	<a rel="license" href="https://creativecommons.org/licenses/by-sa/3.0/">
	<img
	alt="This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License"
	style="border-width: 0"
	src="https://i.creativecommons.org/l/by-sa/3.0/80x15.png"
	/>
	</a>
	</div>
	<div class="expand"></div>
	<div class="circle" onclick="showHelp()" title="Show help">
	<p>?</p>
	</div>
	</div>
	<p id="help">
	This calculator converts between metric and imperial values, and calculates tank buoyancy. Type the values manually or select
	from the pre-defined list of common tanks.
	</p>
	</td>
	</tr>
	</table>
	</form>
	</body>
	</html>