diff --git a/advanced-reactor.html b/advanced-reactor.html
index 6db1abe..ff12949 100644
--- a/advanced-reactor.html
+++ b/advanced-reactor.html
@@ -6,6 +6,7 @@
             name: { value: "" },
             volume: { value: 0., required: true},
             n_inlets: { value: 1, required: true},
+            kla: { value: null },
             S_O_init: { value: 0., required: true },
             S_I_init: { value: 30., required: true },
             S_S_init: { value: 100., required: true },
@@ -36,6 +37,10 @@
                 type:"num",
                 types:["num"]
             });
+            $("#node-input-kla").typedInput({
+                type:"num",
+                types:["num"]
+            });
             $(".concentrations").typedInput({
                 type:"num",
                 types:["num"]
@@ -68,6 +73,11 @@
         <label for="node-input-n_inlets"><i class="fa fa-tag"></i> Number of inlets</label>
         <input type="text" id="node-input-n_inlets" placeholder="#">
     </div>
+    <h3> Internal mass transfer calculation (optional) </h3>
+    <div class="form-row">
+        <label for="node-input-kla"><i class="fa fa-tag"></i> kLa [d-1]</label>
+        <input type="text" id="node-input-kla" placeholder="d-1">
+    </div>
     <h2> Dissolved components </h2>
     <div class="form-row">
         <label for="node-input-S_O_init"><i class="fa fa-tag"></i> Initial dissolved oxygen [g O2 m-3]</label>
diff --git a/advanced-reactor.js b/advanced-reactor.js
index ed8edda..edfad14 100644
--- a/advanced-reactor.js
+++ b/advanced-reactor.js
@@ -10,6 +10,7 @@ module.exports = function(RED) {
         const reactor = new Reactor(
             parseFloat(config.volume),
             parseInt(config.n_inlets),
+            parseFloat(config.kla),
             [
                 parseFloat(config.S_O_init),
                 parseFloat(config.S_I_init),
diff --git a/dependencies/reactor_class.js b/dependencies/reactor_class.js
index 0427462..dcc5cfd 100644
--- a/dependencies/reactor_class.js
+++ b/dependencies/reactor_class.js
@@ -3,7 +3,7 @@ const math = require('mathjs')
 
 class Reactor_CSTR {
     
-    constructor(volume, n_inlets, initial_state) {
+    constructor(volume, n_inlets, kla, initial_state) {
         this.state = initial_state;
         console.log(this.state);
         this.asm = new ASM3();
@@ -12,6 +12,8 @@ class Reactor_CSTR {
         this.Fs = Array(n_inlets).fill(0.0); // fluid debits per inlet [m3 d-1]
         this.Cs_in = Array.from(Array(n_inlets), () => new Array(13).fill(0.0)); // composition influents
         this.OTR = 0.0; // oxygen transfer rate [g O2 d-1]
+        
+        this.kla = kla; // if NaN, use external OTR [d-1]
 
         this.currentTime = Date.now(); // milliseconds since epoch [ms]
         this.timeStep = 1/(24*60*15) // time step [d]
@@ -31,6 +33,11 @@ class Reactor_CSTR {
         return {topic: "Fluent", payload: {inlet: 0, F: math.sum(this.Fs), C:this.state}, timestamp: this.currentTime};
     }
 
+    calcOTR(S_O, T=20.0) { // caculate the OTR using basic correlation, default to temperature: 20 C
+        let S_O_sat = 14.652 - 4.1022e-1*T + 7.9910e-3*T*T + 7.7774e-5*T*T*T;
+        return this.kla * (S_O_sat - S_O);
+    }
+
     // expect update with timestamp
     updateState(timestamp) {
         let newTime = timestamp;
@@ -53,10 +60,10 @@ class Reactor_CSTR {
         const r = this.asm.compute_dC(this.state);
         const dC_in = math.multiply(math.divide([this.Fs], this.Vl), this.Cs_in)[0];
         const dC_out = math.multiply(math.sum(this.Fs)/this.Vl, this.state);
-        const T_O = Array(13).fill(0.0);
-        T_O[0] = this.OTR;
+        const t_O = Array(13).fill(0.0);
+        t_O[0] = isNaN(this.kla) ? this.OTR : this.calcOTR(this.state[0]); // calculate OTR if kla is not NaN, otherwise use externaly calculated OTR
 
-        const dC_total = math.multiply(math.add(dC_in, dC_out, r, T_O), time_step);
+        const dC_total = math.multiply(math.add(dC_in, dC_out, r, t_O), time_step);
 
         this.state = math.add(this.state, dC_total);
         return this.state;