Has anyone used a cheap manometer to test their pressure?

[Sleep2Snore]

Was it accurate? 

I bought my machine from a company in the States (I'm in the UK) who suggested I double check if the pressure is really inaccurate (as it feels) before sending another replacement. He told me that in the States you can just go to any CPAP outlet and have them test your machine for free in literally 2 mins. 

This doesn't seem to case over here. I contacted Philips Respironics UK who wouldn't do it, one of the major cpap distrbutors over here would also wouldn't do it and a Sleep Apnea Association who initially wanted to charge me £180 for a consultation in order to do it although eventually offered to do it for free when I said it was free 2 min job in the States...but kind of reluctantly and the office is a fair bit away from me. 

I think it would be simpler to just buy a cheap manometer on Amazon to do it but reviews are mixed, thoughts?

I took my machine to the local university when they refused to believe my machine dropped in pressure.
I told the guy there what the problem what I thought the machine was doing, he got a class to check the readings and low and behold he had a class doing the checking instead of theory and they found it was indeed dropping pressure and gave me a letter to hand in to the Sleep Clinic with a load of signatures on it.  One from a professor, a lecturer and around 25 students.
They didn't argue and gave me a new machine on the spot (but kept my letter) and when another machine started to do the same, they just said they would have a new one waiting.
The lecturer said something about it not just being pressure, but it was to do with flow rates and some other jargon, I stopped listening at some point.
You can't just block the end and see what the pressure is apparently!  But then he appeared to know what the machine was for as soon as he saw it.
However, he was the man in the know and not me.  I must have looked blank at one point as he stopped speaking and said "Anyway, we will test it for you, come back around 4pm".
Nice one guys, as it was coming up for Christmas they had a huge box of chocolate covered sweets/candy dropped off for their help.
The guy said the students had shown more interest in working things out that day that had for a month