ELP Controller described a MVA infraction when vectoring a departure aircraft westbound; indicating possible misunderstanding by the pilot; hot temperature/s and human error as causal factors.

Narrative

Tower requested an IFR release for a C172 from Runway 8L climbing to 100. Due to other aircraft; I released the aircraft heading 220 to take it south of the mountains. At the time I had no other aircraft south of the mountains. As the C172 took off Runway 8L; I had three VFR aircraft call in for VFR services coming from the west over downtown to T27 and ELP. The C172 was slow climbing when I saw him meet the 064 MVA; I turned him direct HANCH on course. I then had him expedite his climb through 7;000 for VFR traffic. I issued traffic to the C172 on the VFR aircraft. I noticed that the C172 was entering an 8;000 MVA at about 6;800 climbing. I asked him if he had the mountains in sight which he responded he did. When the next controller took over he noticed that the C172 was at 7;000 FT level. When the controller asked the aircraft what his assigned altitude was he said 7;000. He was then climbed to 10;000. I believe when he was issued and expedited climb through 7;000 FT he mistook that for an altitude of 7;000. No MVA alarms were noted throughout his climb out. Recommendation; keep slow climbing aircraft during hot spring/summer days eastbound until they reach an appropriate altitude before turning them westbound.