diff --git a/src/reaction_modules/asm3_class Koch.js b/src/reaction_modules/asm3_class Koch.js
index e5e98b3..0742afa 100644
--- a/src/reaction_modules/asm3_class Koch.js	
+++ b/src/reaction_modules/asm3_class Koch.js	
@@ -1,4 +1,9 @@
-const math = require('mathjs')
+const math = require('mathjs');
+
+const ASM_CONSTANTS = {
+  S_O_INDEX: 0,
+  NUM_SPECIES: 13
+};
 
 /**
  * ASM3 class for the Activated Sludge Model No. 3 (ASM3). Using Koch et al. 2000 parameters.
@@ -208,4 +213,4 @@ class ASM3 {
   }
 }
 
-module.exports = ASM3;
\ No newline at end of file
+module.exports = { ASM3, ASM_CONSTANTS };
\ No newline at end of file
diff --git a/src/reaction_modules/asm3_class.js b/src/reaction_modules/asm3_class.js
index d228619..dc519db 100644
--- a/src/reaction_modules/asm3_class.js
+++ b/src/reaction_modules/asm3_class.js
@@ -1,4 +1,9 @@
-const math = require('mathjs')
+const math = require('mathjs');
+
+const ASM_CONSTANTS = {
+  S_O_INDEX: 0,
+  NUM_SPECIES: 13
+};
 
 /**
  * ASM3 class for the Activated Sludge Model No. 3 (ASM3).
@@ -208,4 +213,4 @@ class ASM3 {
   }
 }
 
-module.exports = ASM3;
\ No newline at end of file
+module.exports = { ASM3, ASM_CONSTANTS };
\ No newline at end of file
diff --git a/src/specificClass.js b/src/specificClass.js
index 18ac54a..3f368e1 100644
--- a/src/specificClass.js
+++ b/src/specificClass.js
@@ -1,4 +1,4 @@
-const ASM3 = require('./reaction_modules/asm3_class.js');
+const { ASM3, ASM_CONSTANTS } = require('./reaction_modules/asm3_class.js');
 const { create, all, isArray, i } = require('mathjs');
 const { assertNoNaN } = require('./utils.js');
 const { childRegistrationUtils, logger, MeasurementContainer } = require('generalFunctions');
@@ -10,8 +10,6 @@ const mathConfig = {
 
 const math = create(all, mathConfig);
 
-const S_O_INDEX = 0;
-const NUM_SPECIES = 13;
 const BC_PADDING = 2;
 const DEBUG = false;
 const DAY2MS = 1000 * 60 * 60 * 24;
@@ -39,7 +37,7 @@ class Reactor {
     this.volume = config.volume; // fluid volume reactor [m3]
 
     this.Fs = [0]; // fluid debits per inlet [m3 d-1]
-    this.Cs_in = [Array(NUM_SPECIES).fill(0)]; // composition influents
+    this.Cs_in = [Array(ASM_CONSTANTS.NUM_SPECIES).fill(0)]; // composition influents
     this.OTR = 0.0; // oxygen transfer rate [g O2 d-1 m-3]
     this.temperature = 20; // temperature [C]
 
@@ -59,7 +57,7 @@ class Reactor {
     if (this.Fs.length <= i_in) {
       this.logger.debug(`Adding new inlet index ${i_in}.`);
       this.Fs.push(0);
-      this.Cs_in.push(Array(NUM_SPECIES).fill(0));
+      this.Cs_in.push(Array(ASM_CONSTANTS.NUM_SPECIES).fill(0));
       this.setInfluent = input;
     }
     this.Fs[i_in] = input.payload.F;
@@ -243,8 +241,8 @@ class Reactor_CSTR extends Reactor {
     const inflow = math.multiply(math.divide([this.Fs], this.volume), this.Cs_in)[0];
     const outflow = math.multiply(-1 * math.sum(this.Fs) / this.volume, this.state);
     const reaction = this.asm.compute_dC(this.state, this.temperature);
-    const transfer = Array(NUM_SPECIES).fill(0.0);
-    transfer[S_O_INDEX] = isNaN(this.kla) ? this.OTR : this._calcOTR(this.state[S_O_INDEX], this.temperature); // calculate OTR if kla is not NaN, otherwise use externaly calculated OTR
+    const transfer = Array(ASM_CONSTANTS.NUM_SPECIES).fill(0.0);
+    transfer[ASM_CONSTANTS.S_O_INDEX] = isNaN(this.kla) ? this.OTR : this._calcOTR(this.state[S_O_INDEX], this.temperature); // calculate OTR if kla is not NaN, otherwise use externaly calculated OTR
 
     const dC_total = math.multiply(math.add(inflow, outflow, reaction, transfer), time_step)
     this.state = this._arrayClip2Zero(math.add(this.state, dC_total)); // clip value element-wise to avoid negative concentrations
@@ -273,7 +271,7 @@ class Reactor_PFR extends Reactor {
     this.alpha = config.alpha;
 
     this.state = Array.from(Array(this.n_x), () => config.initialState.slice());
-    this.extendedState = Array.from(Array(this.n_x + 2*BC_PADDING), () => new Array(NUM_SPECIES).fill(0));
+    this.extendedState = Array.from(Array(this.n_x + 2*BC_PADDING), () => new Array(ASM_CONSTANTS.NUM_SPECIES).fill(0));
 
     // initialise extended state
     this.state.forEach((row, i) => this.extendedState[i+BC_PADDING] = row);
@@ -324,15 +322,15 @@ class Reactor_PFR extends Reactor {
     const dispersion = math.multiply(this.D / (this.d_x*this.d_x), this.D2_op, this.extendedState);
     const advection = math.multiply(-1 * math.sum(this.Fs) / (this.A*this.d_x), this.D_op, this.extendedState);
     const reaction = this.extendedState.map((state_slice) => this.asm.compute_dC(state_slice, this.temperature));
-    const transfer = Array.from(Array(this.n_x+2*BC_PADDING), () => new Array(NUM_SPECIES).fill(0));
+    const transfer = Array.from(Array(this.n_x+2*BC_PADDING), () => new Array(ASM_CONSTANTS.NUM_SPECIES).fill(0));
 
     if (isNaN(this.kla)) { // calculate OTR if kla is not NaN, otherwise use externally calculated OTR
       for (let i = BC_PADDING+1; i < BC_PADDING+this.n_x - 1; i++) {
-        transfer[i][S_O_INDEX] = this.OTR * this.n_x/(this.n_x-2);
+        transfer[i][ASM_CONSTANTS.S_O_INDEX] = this.OTR * this.n_x/(this.n_x-2);
       }
     } else {
     for (let i = BC_PADDING+1; i < BC_PADDING+this.n_x - 1; i++) {
-        transfer[i][S_O_INDEX] = this._calcOTR(this.extendedState[i][S_O_INDEX], this.temperature)  * this.n_x/(this.n_x-2);
+        transfer[i][ASM_CONSTANTS.S_O_INDEX] = this._calcOTR(this.extendedState[i][ASM_CONSTANTS.S_O_INDEX], this.temperature)  * this.n_x/(this.n_x-2);
       }
     }
 
@@ -357,7 +355,7 @@ class Reactor_PFR extends Reactor {
     switch(measurementType) {
       case "quantity (oxygen)":
         let grid_pos = Math.round(context.distance / this.config.length * this.n_x);
-        this.state[grid_pos][S_O_INDEX] = value; // naive approach for reconciling measurements and simulation
+        this.state[grid_pos][0] = value; // naive approach for reconciling measurements and simulation
         break;
       default:
         super._updateMeasurement(measurementType, value, position, context);