You probably mean the Hach HQ40D multi-meter. We use several of these units throughout our various facilities and find them to be accurate. Check that you have the correct sensor/probe for your application (i.e. drinking water). I have also found the Hach technical staff to be quite helpful if you contact them for assistance and advice.
We had issues with out water being a low ionic water. Change probes and haven't had an issue since. Also need to make sure your fluid is full and store water is a neutral of what you will be testing.