when you add captcha to a page the captcha code will look like this:
<img id="capt1" src="webassist/captcha/wavt_captchasecurityimages.php?width=200&height=50&field=Security_Code_1&bgcolor=FFFFFF&transparent=0&bgimage=&gridfreq=20&gridcolor=000000&gridorder=behind&noisefreq=20&noisecolor=000000&noiseorder=behind&characters=5&charheight=&font=fonts/MOM_T___.TTF&textcolor=000000" alt="security code" width="200" height="50" /><br />
<input id="Security_Code_1" name="Security_Code_1" type="text" value="" />
one of the querystring variable for the image source is named "field", this defines the name of the captcha session variable, the naming convention for the capthcha session variable is:
capthcha_<field querystring variable>
in the above code, the field variable:
field=Security_Code_1
is Security_Code_1, so the captcha session to use in the validation is captcha_Security_Code_1