Why the change to: Charge by weight not psi?

[jamyers]

I've done my own automotive A/c work for the last 30 years, converted several R12 systems to R134a, replaced/repaired everything in a POA, TXV, and CC/orifice tube system over the years - so while I'm no professional or expert, I do have a fair bit of experience - and was wondering...

Why is it that within the last couple of years, the professional/expert opinions have shifted (seemingly dramatically) from charging a system using temperature/pressure to the latest advice of "reclaim, weigh, recharge by weight"?

It't to the point that I've had people who 5 years ago told me to charge my '71 Buick with POA and R134a using Temp/PSI is now telling me that the ONLY way I can get a good running system is to reclaim, weigh, and recharge by weight. Why was one method good for so long, and now it's completely unacceptable?

What's behind the change?

[Dougflas]

Well for one reason is the systems today use small amounts of refrigerant. Some use less than 2 lbs. So if your charge is 4 oz off because you used PSI and temps,your % of error is higher. Also, the older systems had driers that stored extra refrigerant for TXV's. Systems today use orfices and flooded evaps. Systems today also use control valves and variable capacity compressors, & thermistors that use computer signals & electronic blend door motors, It is a whole new ball game.

[GM Tech]

I've done cars that with 1 ounce overcharge will cut out on high pressure- some systems take only 14 ounces or a pound-- very small room for error.

[Cusser]

I've done cars that with 1 ounce overcharge will cut out on high pressure- some systems take only 14 ounces or a pound-- very small room for error.

And with modern equipment: adding refrigerant by weight is much more accurate, and faster too.

[goosemaster]

jamyer; In my opinion you're asking a VERY good question. I think I see what the difference is. There's a whole LOT more money to be made if you reclaim, weigh, and recharge by weight versus adding some freon with your gauge set. I know I'll step on a lot of toes here but I'm with you in your questioning; what's wrong with doing it the way you've done it for 30 yrs. (MONEY)

[wptski]

I've done cars that with 1 ounce overcharge will cut out on high pressure- some systems take only 14 ounces or a pound-- very small room for error.

Many scales might not have the accuracy for that. I have a Mastercool 98210-A and it's supposed to be +/- 0.5oz but I'm not sure if it really is. I weigh the refrigerant tank before use, fill the lines(2 or 3 oz.), TARE or zero it, charge the required amount, weigh the tank again and it rarely adds up within that +/- 0.5oz.

If I had it to do over again, I'd get a different brand.

[Tim]

You're also working with a refrigerant that has different properties. Much less tolerant for an over or under charged system when guessing at refrigerant level. Add in the fact that system's take less refrigerant these days. Just a smart move to know you have a proper refrigerant level.

I've seen this it's all about money phrase before. Doing a proper procedure! Seems more of a wise move than a incentive for profit.

[jamyers]

Okay, I completely get it when dealing with a newer (smaller) system with a ton of sensors, variable displacement hoorah, etc, etc.
And while I've heard the money angle, I do believe that in automotive a/c, the smart money is spent on doing the job right the first time.

But how about older systems, like my '71 Buick w/ r134a, or like when I'm about to add an underdash a/c to a '69 Chevy pickup? Or my 2000 Silverado that I've charged by temp/psi and it blows ice cold air? At what point does it become truly mandatory to go by weight?

I guess it would be less frustrating to see some leeway allowed for vehicles that don't have all the latest fancy gear, especially given that temps/psi were fine on many vehicles until (relatively) recently. In my non-expert opinion, the blanket "recover, weigh, recharge by weight" isn't going to cover every troubleshooting situation or repair.

[Tim]

Your not going to find OEM specs on a 71 Chevelle with R134a. So obviously your going to have to guess at refrigerant levels. There is no specific pressures one can use in this situation. As your Ford Faclon will run a different pressure than your Chevelle on R134a.

I would say 90% of the time we suggest charging by weight, is on a OEM R134a system.