sol <[email protected]> wrote: >I have a PWT from Silvergen. It has been a reliable meter for me. But if >I was buying a meter today, I'd get the COM 100 from Silverpuppy >(Silverwell in AU also sells it I think). >sol
Hi sol, I agree completely. I have a Hanna PWT but the COM100 is next on my shopping list. I really like the temperature display, and I understand from David that the probe is a bit smaller than the Hanna which will make it easier to get readings from a bit of cs in a shot glass prior to sublingual absorption. I should also mention that my Hanna arrived several years ago in perfect calibration, and has held the original calibration extremely well. I have verified it numerous times with precision measurements using the Faraday calculation, and it has always agreed within 1uS up to about 15uS. The readings tend to depart around there due to AgOH formation. So I am confident my unit is functioning well. Whenever I have an instrument that has a calibration adjustment, I rarely use the adjustment to change the reading during calibration. Doing so makes it difficult to track long-term drift, or short-term changes due to environmental factors. For example, changes in ambient temperature or humidity can affect the calibration. If you keep adjusting it, you may be unable to detect these changes. So instead of twisting the pot, I merely record the actual reading and calculate an offset. I should also mention this procedure is often used in precision metrology labs that have to maintain accurate records to track instrument performance. My faith in Hanna plummeted recently when I recommended a friend get one. She got two, with the calibration solution. When she brought them over to compare calibration with mine, we discovered that one unit was so far out of calibration that the adjustment screw could not bring it into calibration. The second unit was fine, but it was out of calibration. We discovered her calibration solution read high, about 96uS. So I recommended she send them back and get a refund. It is very difficult to recommend what to do to verify the manufacturer's products in a case like this. If you can do the Faraday calculation, it can help up to about 15uS. The salt test is useful, but it takes practise to judge the readings, and they won't be closer than about 5uS. So it can only catch gross calibration errors. Diluting a saturated salt solution takes some skill, and the resulting solution may not be very stable. It can also become contaminated from the container the solution is stored in. Another method might be to find what the local water department uses to check the calibration on their meters. They may use TDS for routine work due to the higher conductances they have to measure. But if you're lucky, someone in the lab may have a PWT and can do a quick compare with yours as a favor. Or there may be a standards lab or school in the vicinity that might be willing to help. If none of these are available, pehaps the only real way of verifying the calibration may be to buy several units from different manufacturers. If they all agree, then very likely they are telling the truth. Best Wishes, Mike M. -- The Silver List is a moderated forum for discussing Colloidal Silver. Instructions for unsubscribing are posted at: http://silverlist.org To post, address your message to: [email protected] Address Off-Topic messages to: [email protected] The Silver List and Off Topic List archives are currently down... List maintainer: Mike Devour <[email protected]>
