HM Digital COM-100 is my primary with a HM AP-1 as a back up. I have two of the HM DM-1 dual inlines but they lack the accuracy or versatility of a good handheld. They are not truly temperature compensated so can be significantly off unless your air and water temperature are EXACTLY the same. With the handhelds you can also test your ATO reservoir, LFS water, your tap water, your buddies water, bottled water etc. The inlines are edeicated to two spots and cannot be used portable since the probe relies on flow inside the tee for any level of accuracy. A good handheld can be had for $20-$25 and the COM-100 which is 10x more sensitive and accurate can be had for around $50-$60.