Comparing tv value to decimal

SELECT `id`, `contentid`, value FROM `wood_site_tmplvar_contentvalues` WHERE (`tmplvarid` = 18 AND CAST(value AS  DECIMAL(5,2)) >= 7.5 )

<?php
$c = $modx->newQuery('modTemplateVarResource');
$c->select('id, contentid, value, CAST(value AS DECIMAL(5,2)) AS cvalue');
$c->where(array(
    'tmplvarid' => 18,
    'cvalue:>=' => 7.5,
    ));
$collection = $modx->getIterator('modTemplateVarResource',$c);
foreach ($collection as $object) {
    $output .= 'ContentId: '.$object->get('contentid').' Value: '.$object->get('value') . '<br>';
}
return $output;

<?php
$c = $modx->newQuery('modTemplateVarResource');
$c->select('id, contentid, value');
$c->where(array(
    'tmplvarid' => 18,
    'CAST(value AS DECIMAL(5,2)):>=' => 7.5,
    ));
$collection = $modx->getIterator('modTemplateVarResource',$c);
foreach ($collection as $object) {
    $output .= 'ContentId: '.$object->get('contentid').' Value: '.$object->get('value') . '<br>';
}
return $output;

SELECT `id`, `contentid`, `value`, CAST(value AS DECIMAL(5, 2)) AS cvalue FROM `wood_site_tmplvar_contentvalues` AS `modTemplateVarResource` WHERE ( `modTemplateVarResource`.`tmplvarid` = 18 AND `modTemplateVarResource`.`cvalue` >= '7.5' )

SELECT `id`, `contentid`, `value` FROM `wood_site_tmplvar_contentvalues` AS `modTemplateVarResource` WHERE ( `modTemplateVarResource`.`tmplvarid` = 18 AND `modTemplateVarResource`.`CAST(value AS DECIMAL(5,2))` >= '7.5' )

$c->where("CAST(value AS  DECIMAL(5,2)) >= 7.5", xPDOQuery::SQL_AND);