Enter has no effect on form text field

[hariskar]

I made a small script for a formula calculation and what I have not managed to do yet, is make the Enter act as pressing the button.
Any help please?
Here is the form:

<form id=hba1c action="" method="post" name="HbA1c_form">
Hba1c% <input type="text" id="userValue" size="15"><input type="button" value="Υπολογισμός" onclick="calcResult();">
</form>

<table id="table_hba1c" style="display:none;">
<tr>
<td width="460"></td>
<td width="200">Αποτέλεσμα</td>
</tr>
<tr>
<td>Εκτιμώμενη μέση τιμή σακχάρου</td>
<td id="resultl"></td>
</tr>
</table> 

<script type="text/javascript">
function calcResult() {
n=document.getElementById('userValue').value
n=parseFloat(n);
var userValue=document.forms["hba1c"]["userValue"].value;

if (userValue==null || userValue=="")
{
alert("Παρακαλώ καραχωρήστε την τιμή της HbA1c!");
return false;
}

resultEl=(n*33.3-86).toFixed(1);
m='mg/dl';
d='Εκτιμώμενη μέση τιμή σακχάρου:&nbsp ';
document.getElementById('resultl').innerHTML=resultEl + " " + m;
document.getElementById('table_hba1c').style.display="table";
}

</script>

[hariskar]

Actually on hitting Enter I don't want the form to be submited.
I would like these to start:

document.getElementById('resultl').innerHTML=resultEl + " " + m;
document.getElementById('table_hba1c').style.display="table";