# Specific Garvity of IPA **URL:** \ **Category:** General Discussion\ **Created:** [April 17, 2021, 11:42pm UTC](https://forum.formlabs.com/t/specific-garvity-of-ipa/30844 "2021-04-17T23:42:37Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![dstern](https://avatars.discourse-cdn.com/v4/letter/d/94ad74/32.png) [@dstern](https://forum.formlabs.com/u/dstern)\ **Post date:** [April 17, 2021, 11:42pm UTC](https://forum.formlabs.com/t/specific-garvity-of-ipa/30844/1 "2021-04-17T23:42:37Z")
I don’t have a form wash and therefore no Formlabs hydrometer. What is the specific gravity of spent IPA? ---
**Author:** ![Randy\_Cohen](https://sea1.discourse-cdn.com/flex025/user_avatar/forum.formlabs.com/randy_cohen/32/1500_2.png) [@Randy\_Cohen](https://forum.formlabs.com/u/Randy_Cohen)\ **Post date:** [April 18, 2021, 5:03pm UTC](https://forum.formlabs.com/t/specific-garvity-of-ipa/30844/2 "2021-04-18T17:03:04Z")
It depends on how “spent” it is. ---
**Author:** ![dstern](https://avatars.discourse-cdn.com/v4/letter/d/94ad74/32.png) [@dstern](https://forum.formlabs.com/u/dstern)\ **Post date:** [April 18, 2021, 6:09pm UTC](https://forum.formlabs.com/t/specific-garvity-of-ipa/30844/3 "2021-04-18T18:09:40Z")
Understood. I want to know the threshold SG when it’s time to change it… ---
**Author:** ![Randy\_Cohen](https://sea1.discourse-cdn.com/flex025/user_avatar/forum.formlabs.com/randy_cohen/32/1500_2.png) [@Randy\_Cohen](https://forum.formlabs.com/u/Randy_Cohen)\ **Post date:** [April 18, 2021, 7:18pm UTC](https://forum.formlabs.com/t/specific-garvity-of-ipa/30844/4 "2021-04-18T19:18:38Z")
If you stir it up and you can’t see the model because the IPA is so cloudy, it’s time to change it. But the resin settles to the bottom of the IPA if left for a few days. You can decant the clear stuff and then dispose of the sediment and small remaining amount of IPA. Or filter it through a coffee filter. And exposing the IPA to UV cures the resin that’s mixed in to the IPA, making it easier to filter and/or usable for longer without filtering it. ---
**Author:** ![dstern](https://avatars.discourse-cdn.com/v4/letter/d/94ad74/32.png) [@dstern](https://forum.formlabs.com/u/dstern)\ **Post date:** [April 18, 2021, 8:28pm UTC](https://forum.formlabs.com/t/specific-garvity-of-ipa/30844/5 "2021-04-18T20:28:17Z")
Thanks for the info. That said, Formlabs offers a hydrometer so somebody should know the SP of spent IPA. ---
**Author:** ![Randy\_Cohen](https://sea1.discourse-cdn.com/flex025/user_avatar/forum.formlabs.com/randy_cohen/32/1500_2.png) [@Randy\_Cohen](https://forum.formlabs.com/u/Randy_Cohen)\ **Post date:** [April 18, 2021, 9:13pm UTC](https://forum.formlabs.com/t/specific-garvity-of-ipa/30844/6 "2021-04-18T21:13:00Z")
I’m sure they do, but in \>5 years of running Formlabs printers, the way I describe has always worked great. In fact, I have a new trick. I put a paper towel down on the wash bucket basket after I’ve finished rinsing a print. Over the course of a couple of days the resin settles out and the IPA is crystal clear again. Then I slowly pull out the basket and throw the paper towel, with all the sedimentary resin deposited on it, away. I haven’t actually tossed a bucket full if IPA in as long as I can remember… ---
**Author:** ![Eks](https://avatars.discourse-cdn.com/v4/letter/e/958977/32.png) [@Eks](https://forum.formlabs.com/u/Eks)\ **Post date:** [April 18, 2021, 11:22pm UTC](https://forum.formlabs.com/t/specific-garvity-of-ipa/30844/7 "2021-04-18T23:22:33Z")
Hi Randy\_Cohen That’s actually a cool trick. Mind sharing how exactly you put the paper towel? Do you lay it down flat on top of the bottom of the basket? Also, does that really suck up the resin after a few days? ---
**Author:** ![Randy\_Cohen](https://sea1.discourse-cdn.com/flex025/user_avatar/forum.formlabs.com/randy_cohen/32/1500_2.png) [@Randy\_Cohen](https://forum.formlabs.com/u/Randy_Cohen)\ **Post date:** [April 19, 2021, 3:21am UTC](https://forum.formlabs.com/t/specific-garvity-of-ipa/30844/8 "2021-04-19T03:21:00Z")
I just lay it down, torn to size, in the bottom of the basket. Let it sit for a day or two, slowly draw out the basket, dispose of the paper towel. It’s not perfect, because it’s hard to go slow enough to keep stuff from leaking over the edges. But I would guess I’m pulling out 80% of what’s collected on the towel. I don’t print a huge amount. I think I’ve owned the Form3 for about 300 days and I have barely used 2L of resin, so the IPA buckets sit around for days, undisturbed, which is what gave me the idea. The first-wash bucket would be so cloudy I could not see the print once things got stirred up, but after a couple of days it’d be clear as glass except for a layer of sediment on the bottom. Can’t reach in and vacuum the sediment out with a straw, unfortunately, so this seemed a good alternative. ---
**Author:** ![Eks](https://avatars.discourse-cdn.com/v4/letter/e/958977/32.png) [@Eks](https://forum.formlabs.com/u/Eks)\ **Post date:** [April 19, 2021, 10:08am UTC](https://forum.formlabs.com/t/specific-garvity-of-ipa/30844/9 "2021-04-19T10:08:18Z")
Thank you Randy\_Cohen, I appreciate the explanation. It’s a good thing for me to try out since the alternatives are somewhat messy and / or annoying. ---
**Author:** ![billb](https://sea1.discourse-cdn.com/flex025/user_avatar/forum.formlabs.com/billb/32/5587_2.png) [@billb](https://forum.formlabs.com/u/billb)\ **Post date:** [April 19, 2021, 10:27am UTC](https://forum.formlabs.com/t/specific-garvity-of-ipa/30844/10 "2021-04-19T10:27:00Z")
If the spent IPA is put back into the containers it came in and is kept in a dark cupboard for a couple of weeks there will be about 5-10mm of grey sludge in the bottom and the rest will be clear. Putting the container into the sun for a couple of days will turn the whole of the contents into a solid gel, showing that the active ingredients of the resin, or at least the monomer, and the crosslinker, are **dissolved** in the IPA. The reason why the ‘trick’ of putting spent IPA in a glass jar to clean it appears to work is that glass is partially opaque\* to UV, so not enough UV is transmitted to trigger the curing of the dissolved resin constituents. The only way of removing these chemicals is by distillation. \*There is not enough UV in sunlight transmitted through glass windows, for instance, to give people tans. ---
**Author:** ![mh1974](https://avatars.discourse-cdn.com/v4/letter/m/87869e/32.png) [@mh1974](https://forum.formlabs.com/u/mh1974)\ **Post date:** [April 20, 2021, 10:54pm UTC](https://forum.formlabs.com/t/specific-garvity-of-ipa/30844/11 "2021-04-20T22:54:06Z")
I haven’t tried it myself, but appropriately sized bag filter socks might work better than a paper towel though the cost would certainly be higher. > **[McMaster-Carr](https://www.mcmaster.com/bag-filters)** > > McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day. ---
**Author:** ![Randy\_Cohen](https://sea1.discourse-cdn.com/flex025/user_avatar/forum.formlabs.com/randy_cohen/32/1500_2.png) [@Randy\_Cohen](https://forum.formlabs.com/u/Randy_Cohen)\ **Post date:** [April 20, 2021, 11:59pm UTC](https://forum.formlabs.com/t/specific-garvity-of-ipa/30844/12 "2021-04-20T23:59:49Z")
> [@billb](#): > > The reason why the ‘trick’ of putting spent IPA in a glass jar to clean it appears to work is that glass is partially opaque\* to UV, so not enough UV is transmitted to trigger the curing of the dissolved resin constituents. The only way of removing these chemicals is by distillation. While window glass does a great job of blocking UV B and UV C wavelengths, which are all much shorter than the wavelengths required to cure Formlabs resins, UV A passes through untreated window glass just fine. The UV A band goes up to 400nm. 405nm is the frequency of UV required to cure resin. That’s close enough to 400nm to not matter much. Sunlight through a window will do a fine job of curing resin. Yeah, maybe only 50% of the UV gets through. But that’s 50% of a gagillion-watt light source, so it’s still plenty.