function econv(form,field,c){
    //form: form object, field: fieldname, c: conversion array
    //determine number of fields
    var nf=form.elements.length;
    
    var ich=-1;
    for (var i=0; i<nf; i++){
        if (form.elements[i].name == field){
            ich=i;
            break;
        }
    }
    
    if (ich<0){
        document.write("Error, field not found <br>");
    }
    else{
        
        var v0=form.elements[ich].value/c[ich];
        
        for(var i=0; i<nf; i++){
            if(i!=ich){
                var num=v0*c[i];
                form.elements[i].value=num.toPrecision(6);
            }
        }
    }
    
}